Respiratory gas analysis instrument having improved volume
Contents
1. m and b Naturally the equations for these line segments may also be stored in other well known forms such as the point slope form or the two point form if desired As pulses per second data for breath is derived from the output signals of turbine 38 and breath switch 40 it is compared with the maximum and minimum pulses per second values of each line segment until the segment along which it lies is identified The pulses per second value may then be substituted into the equation for the line segment which is then solved to provide the pulses per liter value which corresponds to the mea sured pulses per second value Once the pulses per liter value is available volume data for the breath may be provided in one of two forms On the one hand the pulses per liter value may be divided into the total num ber of pulses to yield a volume value in liters Alterna tively the pulse per liter value may be divided into the pulses per second value to produce volume data in the form of a volume rate of flow in liters per second Note that volume rate of flow in liters per second need only be multiplied by the duration of a breath to yield the volume of a breath in liters Since one or both of these determinations may be made for any point on curve C1 it will be seen that the storage and use of curve C1 allows the volume data for a breath to be determined in spite of changes in regard to the rate at which that breath is de2. United States Patent TT 4 448 058 Jaffe et al 45 May 15 1984 54 RESPIRATORY GAS ANALYSIS FOREIGN PATENT DOCUMENTS 75 73 21 22 51 52 58 56 INSTRUMENT HAVING IMPROVED VOLUME CALIBRATION METHOD AND APPARATUS Inventors Michael B Jaffe Charles R Luper both of Anaheim Eric Mabry Westminster Howard J Reid Brea all of Calif Assignee Sensormedics Corporation Anaheim Calif Appl No 394 608 Filed Jul 2 1982 Int Cl eee eit GOIN 31 00 US o MP 73 23 73 1 128 719 364 571 Field of Search 73 23 1 128 719 422 84 436 900 364 571 497 References Cited U S PATENT DOCUMENTS 3 948 604 4 1976 Hoppesch e 73 1 G 4 178 919 12 1979 422 84 4 316 380 2 1982 Heim et al 73 23 PULSES LITER 1000 D bg Fog 2 1 1 Pa carc cauc 5 2950746 6 1981 Fed of Germany 422 84 Primary Examiner Stephen A Kreitman Attorney Agent or Firm Lyon amp Lyon 57 ABSTRACT An improved gas volume calibration method and appa ratus for use in respiratory gas analyzers A control unit monitors the flow of calibration gas through the analy zer by monitoring the electrical signals produced by a gas turbine and a breath switch During calibration a known volume of calibration gas is repeatedly delivered to the analyzer from a calibration syringe at
3. 45 50 55 65 20 flowing through the turbine to the rate of flow of gas therethrough 30 The calibration apparatus of claim 28 in which the turbine produces an output signal comprising a succes sion of pulses and in which the first and second linear approximations relate the pulses per liter values for gas flow through the turbine to respective pulses per second values 31 The calibration apparatus of claim 30 in which the first and second linear approximations are stored by storing the parameters of the eauations for a plurality of linear segments together with the ranges of pulses per second values associated with those linear segments 32 The calibration apparatus of claim 31 in which the parameters of the equations of at least one of the linear segments of the second linear approximation are pro duced by combining the parameters of the correspond ing linear segment of the first linear approximation with a correction factor that is a function of a the total number of pulses produced as said known volume of gas flows through the turbine and b the time interval during which the latter pulses occurred 33 The calibration apparatus of claim 28 in which the first and second linear approximations are stored by storing the parameters of the equations for a plurality of linear segments together with the ranges of flow rates that are associated with those linear segments 34 The calibration apparatus of claim 33 in wh
4. The above described stroke entry sequence is re peated as necessary until the actual number of accept able strokes is equal to the required number of strokes in the present example this number is T 1 since Cs was initially set to 1 in block 210 When the required number is reached comparison block 238 directs CPU 100 to block 244 to clear a counter A which will be described presently Thereafter as called for by block 246 CPU 100 calculates the coefficient of variation Cy of the pulses per second values for the accepted strokes by calculating the standard deviation thereof and divid ing the same by the mean value thereof a calculation that is familiar to those skilled in the art and fetches the maximum acceptable value for Cy Cymax C is greater than the maximum acceptable value therefor the desired flow of the program is inter 20 25 30 35 40 45 50 55 60 65 14 rupted block 248 directs CPU 100 to a block 252 In the latter block CPU 100 increments counter A and then proceeds to a comparison block 254 which com pares the value of A to a test value in this case 2 If the content of counter A is not equal to 2 CPU 100 is di rected to block 256 which calls for the deletion of the pulses per second values for the two strokes that are furthest from the mean value of Py CPU 100 is then returned to block 246 to compute a new value for Cy This in effect gives the instrument a se
5. known volume of gas said syringe having a housing and a piston slidably mounted therein and b a gas turbine for providing a number of output pulses that varies in accordance with the volume and rate of flow of gas therethrough said method including the steps of a establishing a plurality of pairs of maximum and minimum acceptable pulses per second values cor responding to a pluraliy of flow rates for said known volume of gas through the turbine b moving the piston in a succession of strokes to cause said known volume of gas to flow through the turbine at a first estimated flow rate c calculating the pulses per second values associ ated with the strokes of step b d comparing the resulting pulses per second values to a first pair of maximum and minimum acceptable pulses per second values e storing those pulses per second values which compare favorably with said first pair of maximum and minimum acceptable values for use in calibrat ing the instrument and f repeating steps b through e for at least a second estimated flow rate and a second pair of maximum and minimum acceptable pulses per second values 47 method of claim 46 including the step of calculating the average of the pulses per second values for the acceptable strokes performed at each estimated flow rate 48 The method of claim 47 including the step of determining the coefficient of variation of the pulses per second values for the acc
6. bine 38 to operate at different regions of its non linear characteristic Operation at these different regions in turn causes a known volume of calibration gas to pro duce different numbers of turbine output pulses In the 10 15 20 25 30 35 40 45 50 55 60 65 6 absence of a procedure for dealing with the different apparent volumes that are associated with different numbers of turbine output values such differences re sult in volume uncertainties and errors In accordance with the present invention there is provided an improved method and apparatus for cali brating the instrument of FIG 1 at gas flow rates that correspond to a number of different regions of the non linear operating characteristic of its turbine and thereby enabling the instrument to determine the vol umes of gas that are later delivered at any of those flow rates In addition for each of the plurality of flow rates at which the instrument is calibrated the invention imposes criteria for the acceptance or nonacceptance of the turbine data that is produced by the operation of the syringe thereby assuring that only ejection strokes that meet predetermined minimum standards are used in the calibration process Finally the present invention con templates improvements to the calibration syringe itself which improvements eliminate the above mentioned siphoning problem and facilitate the establishment of a number of different calibrati
7. the volume calibration method and apparatus of the invention assures that the instrument always has available to it volume data which reflects the current condition of the turbine Generally speaking the calibration system of the present invention contemplates the storage of a piece wise linear approximation of the nonlinear characteris tic of a typical gas turbine of the type used in the instru ment Each of the linear segments of this characteristic represents a particular range of turbine pulses per unit volume of calibration gas as a function of the rate at which that gas is delivered On the basis of the turbine output data that is gathered as the operator manually operates the syringe the instrument generates a correc tion factor for each piecewise linear segment When all of these correction factors are available the instrument then generates a corrected piecewise linear approxima tion of the characteristic curve of the actual turbine The latter characteristic is then stored for use during the taking of measurements As a result of the availability of this nonlinear approximation of the actual turbine char acteristic the instrument is able to provide accurate concentration readings in spite of changes in the rate at which the test subject breaths 4 448 058 3 DESCRIPTION OF THE DRAWINGS Further objects and advantages of the present inven tion will be apparent from the following description and drawings in which FI
8. 65 12 continued Symbol B Meaning Y B is a variable that represents the number of times an operator has at tempted to produce an acceptable set of strokes at setting N of the syr inge is a variable representing the number of acceptable syringe strokes which have been made by the operator at setting N T is a constant that indicates the total number of acceptable syringe strokes which are necessary to fix the position of one line segment of curve C2 Cp is the total number of pulses pro duced by turbine 38 during an ejec tion stroke of the syringe is a constant representing the minimum number of pulses which must be produced by turbine 38 during a stroke in order for the stroke to consid ered an acceptable one 5 the pulses per second value resulting from an ejection stroke of the syringe at setting N PNmax is a constant indicating the maximum acceptable pulses per second value at setting N this value corres ponds to the upper endpoint of one of the line segments of curves C1 and C2 PNmin is a constant that indicates the minimum acceptable pulses per second value at setting N this value corres ponds to the lower end of one of the line segments of curves C1 and C2 A is a variable representing the num ber of times that a loop has been tra versed C is a variable representing the coefficient of variation of a set of strokes at a particular setting that is the standard deviat
9. admitting ambient air into the region behind the trailing edge of the piston and in which the control means limits the stroke speed of the syringe by limiting the rate at which ambient air can flow through said aperture 42 The gas volume calibration apparatus of claim 41 in which the control means includes a plate having at least one hole which may be rotated into alignment with said aperture 43 The gas volume calibration apparatus of claim 35 40 41 or 42 in which the syringe is provided with a calibration gas inlet and a calibration gas outlet that are separate from one another and in which the arrival of the piston in said second predetermined position posi tively shuts off the flow of calibration gas in at least said outlet 44 The gas volume calibration apparatus of claim 35 including means for communicating to a user the need for faster strokes if the flow rate signal for a stroke is less than said minimum acceptable value and for com municating to a user the need for slower strokes if the flow rate signal for a stroke is greater than said maxi mum acceptable value 45 The gas volume calibration apparatus of claim 35 or 44 including means for informing a user of the num ber of acceptable strokes that he has performed 46 A method for providing volume calibration data to a gas analysis instrument of the type including a a 15 20 25 30 35 40 45 50 55 65 22 calibration syringe for providing
10. each of a number of different flow rates On the basis of the infor mation received from the turbine and the breath switch the control unit generates and stores a piecewise linear approximation of the nonlinear characteristic of the turbine This stored turbine characteristic is then made available during subsequent measurements to eliminate those volume errors which are associated with varia tions in the rate at which the sample gas is delivered thereby affording measurements of improved accuracy 50 Claims 8 Drawing Figures 3000 PULSES SECOND 2000 U S Patent 15 1984 Sheet 1 of 7 4 448 058 52a y 56 59 54 N O N N 6 N N 5 7 25 Er 5 2 cis eem 26 5 ANAL sis 1 30 rin 32 ELECTRONIC UNIT FIG U S Patent 15 1984 Sheet 2 of 7 4 448 058 FIG 2 PULSES LITER P dd a y 525 A p si 1 d 2 yg MgXgtbg bg 475 PBMAX bg 425 375 x nee O 2000 3000 PULSES SECOND n bg Fes 2 Pa PBMEAS i U S Patent 15 1984 Sheet 3 of 7 4 448 058 4 U S Patent 15 1984 Sheet 4 of 7 4 448 058 E T3 loo KEY BOARD ERE C RU e i06 26 25 102 CIS PLAY INTERF DISPLAY NETWORK sA Cs aiae 4 ie 22 CU 104 RAM l ANALY ZER BOARD xd FIG
11. flow of each breath Because measurements of the concentration of the gaseous components of breath are strongly affected by the volume thereof the gas concentration readings which are based on this corrected volume data have an accuracy better than that available prior to the present invention In accordance with another feature of the present invention the instrument is arranged to prompt the operator who calibrates it and thereby lead him through the volume calibration process in a way that assures that the latter is properly performed In the event that any of the steps of the volume calibration process are improperly performed the instrument will reject the resulting faulty data and inform the operator of what he must do to provide acceptable data Once an acceptable set of data is available the instrument auto matically produces a piecewise linear approximation of the non linear characteristic of the turbine and stores the same for use during subsequent measurements In spite of the sophisticated nature of the calibration system of the invention the calibration process itself is from the operator s standpoint quite simply and conve niently performed As a result it is practical to volume calibrate the instrument daily or even before each series of measurements Such frequent calibrations are desir able because the characteristics of a turbine can change with wear and with the accumulation of dirt on the blades or bearings Thus
12. known volume of calibration gas and b a gas turbine for providing an output signal that varies in accordance with the volume and rate of flow of gas therethrough said method including the steps of a storing a first piecewise linear approximation of the output response of a typical turbine of the class of turbine to which said gas turbine belongs b directing said known volume of gas through the turbine at a plurality of rates of flow which corre spond to a plurality of the linear segments of the piecewise linear approximation and storing data indicative of the resulting turbine output signals c combining the approximation of step a with the data stored during step b to produce a second piecewise linear approximation of the output re sponse of the gas turbine and d making the second piecewise linear approxima tion available for use in interpreting measurements made by the instrument 4 448 058 19 21 The method claim 20 in which the first stored approximation gives the volume of gas flowing through the turbine as a function of the rate of flow of gas there through 22 The method of claim 20 or 21 in which the first stored approximation is stored by storing the parame ters of the equations for a plurality of linear segments together with the ranges of flow rates over which those linear segments are applicable 23 method of claim 22 which the second approximation is produced by changin
13. plate 80 which allows an operator to selectably control the rate at which air can flow into syringe through end plate aperture 62 As is most clearly seen in FIG 4 control plate 80 includes sector shaped section 80a which is provided with one or more flow limiting holes such as 82 and a circular section 80b which is centered on shaft 58 In the preferred embodiment control plate 80 is held against the outer surface of end plate 52b by being sandwiched between that end plate and a control mem ber 84 which is rotatably fastened to plate 52b by a suitable retaining washer 86 A pin 88 which fits into control plate 80 and control member 84 assures that these two elements rotate as a unit thereby allowing plate 80 to be conveniently positioned by grasping and turning the end of member 84 When control plate 80 is rotated to its counterclock wise limit which may be defined by the position of a stop pin 90 aperture 62 is not blocked by plate 80 and therefore allows gas to flow into the rear of syringe 50 4 448 058 7 at a high rate Under this condition the syringe be stroked rapidly resulting in the delivery of calibration gas at relatively high rates When on the other hand plate 80 is rotated so that hole 82 is aligned with aper ture 62 the rate at which gas can flow into the rear of syringe 50 is greatly reduced The effect of this flow rate reduction is to slow down the ejection stroke of syringe 50 and thereby re
14. rate signal for use in calibrating the instrument only if it com pares favorably with said maximum and minimum values 36 The gas volume calibration apparatus of claim 35 in which maximum and minimum acceptable values are stored for each of a plurality of stroke speeds and in which the third means compares the actual flow rate signals for a plurality of stroke speeds with respective maximum and minimum acceptable values for those stroke speeds 4 448 058 21 37 gas volume calibration apparatus of claim 36 in which a predetermined number of strokes having acceptable flow rate signals must occur before the in strument is calibrated at the flow rate corresponding to the respective stroke speed 38 The gas volume calibration apparatus of claim 37 in which the flow rate values of sid predetermined num ber of strokes are averaged to produce an average flow rate value for use in calibration 39 The gas volume calibration apparatus of claim 38 in which said average flow rate value together with the volume of the calibration syringe determine the posi tion of a line segment that approximates the response of the turbine between said maximum and minimum ac ceptable values 40 The gas volume calibration apparatus of claim 36 including an improved calibration syringe having con trol means for selectably controlling stroke speed 41 The gas volume calibration apparatus of claim 40 in which the syringe includes an aperture for
15. second end plate 52c Slidably mounted within housing 52 is a pis ton 54 which is sealed to the inner surface of cylindrical section 52a by a suitable O ring 56 Piston 54 is driven manually by means of a shaft 58 having a knob shaped handle 59 which is slidably mounted on end plate 526 by a bushing 60 The partial vacuum that tends to arise behind the trailing surface of piston 54 during the ejec tion stroke thereof is relieved by the in flow of ambient air through an aperture 62 in end plate 526 The gas that is ejected from syringe 50 by the for ward movement of piston 54 flows through an outlet nipple 64 which is coupled to a T connector 66 The latter connector includes a check valve 68 which per mits gas to flow inwardly through atmospheric inlet line 70 but not in the reverse direction During the intake stroke of piston 54 check valve 68 opens to admit ambi ent air the calibration gas to syringe 50 Under this condition check valve 41 is closed preventing syringe 50 from drawing gas from mixing chamber 26 During the ejection stroke of syringe 50 check valve 68 closes forcing the ejected air to flow into mixing chamber 26 through lines 34 and 28 Under this condition check valve 41 opens to vent to the atmosphere the gas that is displaced from mixing chamber 26 Thus as piston 54 is repeatedly moved between its first or outermost posi tion and its second or innermost position mixing cham ber 26 is provided with a pulsat
16. 5 erm o ME 152 124 130 44 ES 42 5 52 L OE SE TEM FROM 40 FROM 38 U S Patent 15 1984 Sheet 5 of 7 4 448 058 START VOLUME CALIBRATION FIG SET N 200 AND T28 FETCH N 202 OUTPUT MESSAGE SET SYRINGE SETTING N ANO 204 PUSH ENTER 206 yes 208 2 sst 694 210 CLEAR amp 125 212 214 216 OUTPUT MESSAGE READY FOR STROKE Cs Of T SETTING INPUT STROKES DURATION AND PULSE COUNT Cp U S Patent 15 1984 Sheet 6 of 7 4 448 058 FIG 6 b FETCH MIN PULSE COUNT 222 228 224 OUTPUT MESSAGE INCOMPLETE STROKE SETTING N Yes COMPUTE amp STORE PULSES PER SEC 226 PN MAX 5 P SEC Py 5 Pu MN 230 240 OUTPUT MESSAGE REOC STROKE FASTER 242 OUTPUT MESSAGE RECO STROKE SLOWER INCREMENT STROKE COUNT 2 36 Ss U S Patent 15 1984 Sheet 7 of 7 4 448 058 238 dex FIG 2 yes 244 262 OUTPUT MESSAGE REDO ENTIRE SERIES AT SETTING N DELETE TWO STROKES FURTHEST FROM MEAN COMPUTE COEFF OF VARIATION Cy FETCH Cy max NO 260 INCREMENT COUNTER A yes 248 yes yes 252 258 INCREMENT CO
17. G 1 is a block diagram of a respiratory gas analysis instrument shown with the connections which exist during volume calibration including a known type of calibration syringe FIG 2 is a graph showing a piecewise linear approxi mation of the operating characteristic of the gas turbine of FIG 1 FIG 3 is a cross sectional view of an improved vol ume calibration syringe constructed in accordance with the present invention FIG 4 is a cross sectional view taken along the line 4 4 of FIG 3 FIG 5 is a block diagram of the electronic control unit of FIG 1 and FIGS b and together comprise a flow chart that depicts the sequence of operations that are performed by the control unit of FIG 5 during volume calibration DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG 1 there is shown a block diagram of a respiratory gas analysis instrument 10 which is shown with the connections which exist during volume calibration Instrument 10 includes a gas analysis section 12 which may comprise one or more gas analyzers such as for example nondispersive infrared gas analyzers For measurements on human breath these analyzers will typically include one analyzer that is sensitive to the concentration of oxygen and another that is sensitive to the concentration of carbon dioxide The sample gas that is measured by these analyzers is circulated through analysis section 12 by a pump 14 which drives the sample gas through the p
18. UNTER gt 264 COMPUTE 250 CORRECTION OUTPUT FACTOR MESSAGE USER S MANUAL 270 272 INCREMENT N YES COMPUTE by VALUES 214 e GENERATE 2 76 AND STORE EQUATIONS FOR CURVE 4 448 058 1 RESPIRATORY GAS ANALYSIS INSTRUMENT HAVING IMPROVED VOLUME CALIBRATION METHOD AND APPARATUS BACKGROUND OF THE INVENTION In calibrating respiratory gas analysis instruments it has long been the practice to supply the instrument with a pulsatile flow of calibration gas usually air from a device known as a calibration syringe This syringe typically includes a piston and cylinder arrangement which pumps gas into the instrument through a check valve as the operator moves the piston between first and second positions Because the cylinder has a volume comparable to the volume of gas that is exhaled during a typical human breath and because the ejection stroke of the piston has approximately the same duration as an exhaled breath the calibration syringe allows the instru ment to be calibrated under conditions that simulate those which exist when the instrument is later used with a test subject The use of known volume calibration syringes and procedures has been found to result in sizable errors in the volume of gas delivered during calibration One cause of this error known as siphoning results from the fact that the inertia of the gas flowing through the check valve has a tendency to open the check
19. a piece wise linear approximation of the nonlinear characteris tic of a typical turbine of the class of turbines to which turbine 38 belongs As used herein the term piecewise linear approximation refers to a set of linear segments which together approximate a continuous curve Line segments A E of FIG 2 for example approximate respective curvilinear sections of the continuous curve not shown that characterizes the response of a typical turbine of the class of turbines to which turbine 38 be longs The number of linear segments included in curve C1 may in general have any value For practical rea sons however the number of piecewise linear segments is preferably as small as is possible in view of the desired volume correction accuracy It will therefore be under stood that the five linear segments A E shown in FIG 2 represent a reasonable compromise value which af fords both accuracy and ease of use In FIG 2 the horizontal axis indicates the rate of flow of a breath through the turbine and is scaled in terms of the pulses per second value that is associated with a particular breath The vertical axis indicates the total number of pulses produced by the turbine per unit vol ume of breath at the indicated flow rate Because the volume of the calibration syringe Has a known fixed value the unit of volume of breath will also be fixed during calibration From the shape of typical turbine characteristic curve C1 it is appare
20. a suitable bus interface network is present Among the devices which communicate with CPU 100 over bus 102 is a random access memory RAM 104 which serves as bulk read write storage for unit 20 and a display 25 through which CPU 100 may commu 65 8 nicate information to an operator the display being coupled to bus 102 through a suitable display interface network 106 of a conventional type It will be under stood that the term display is used herein in its broad sense to refer to any device by means of which informa tion may be communicated to an operator in human readable form and includes for example alphanumeric displays of the LED or LCD types cathode ray tubes and printers Also connected to bus 102 is gas analysis section 12 of FIG 1 As shown in FIG 5 analysis section 12 may include one or more separate gas analyzers such as oxygen analyzer 108 and carbon dioxide analyzer 110 which may be coupled to bus 102 through respective interface boards 112 and 114 Because these gas analy zers and interface boards operate in a manner known to those skilled in the art to supply gas concentration infor mation to bus 102 on command the internal structure and operation thereof will not be described in detail herein Also connected to bus 102 are gas flow sensing net works 118 and 128 through which CPU 100 receives turbine and breath switch data over conductors 42 and 44 Of these gas flow sensing networks network 118 i
21. and T and outputs to the operator the message ready for stroke 1 of 8 at setting 1 as indicated by blocks 214 and 216 This informs the opera 4 448 058 13 tor that the instrument is ready for the first stroke of syringe 50 and that eight acceptable strokes are neces sary to gather sufficient data to fix the new position of line segment A The instrument then waits in this con dition as indicated by block 218 until a data ready interrupt signal indicates that a stroke has actually been made When the ejection stroke does occur CPU 100 will input the stroke duration and pulse count Cp as called for by block 220 and then fetch the minimum pulse count per block 222 As indicated by comparison block 224 if the actual pulse count value is greater than or equal to the minimum value Cm CPU will go on to compute and store pulses per second value Py for the just completed stroke as indicated by block 226 If on the other hand actual pulse count is less than the minimum pulse count Cm the operator will be informed of this fact by the message incomplete stroke at setting 1 as called for by block 228 This requires the operator to repeat the stroke in a manner which will produce a higher turbine output pulse count In addition to alert ing an operator of the fact that he may not be operating the syringe between its true innermost and outermost positions blocks 224 and 228 may be used to help iden
22. ath including an inlet line 16 and an outlet line 18 The instrument of FIG 1 also includes an electronic control unit 20 which is connected to analysis section 12 10 15 20 25 30 35 40 through a plurality of electrical conductors 22 to keyboard 23 through a set of electrical conductors 24 and to an operator readable display 25 through a set of electrical conductors 26 Control unit 20 may include electronic circuitry of either the hard wired type or the microcomputer controlled type although the latter are preferred because of their greater cost effectiveness The gas to be measured is supplied to analysis section 12 from a mixing chamber 26 which is provided with an inlet line 28 and an outlet line 29 In order to facilitate the connection of inlet line 28 to either a source of sample gas or a source of calibration gas inlet line 28 preferably terminates at one end of a connector 30 that is mounted on an interface panel 32 The other end of connector 30 is thereby made available for coupling to the desired sample or calibration gas source In FIG 1 connector 30 is connected to a line 34 which leads to a source of calibration gas namely volume calibration syringe 50 In order that control unit 20 may monitor the volume and rate of flow of the gas flowing out of mixing cham ber 26 there is provided a gas turbine 38 which is connected to control unit 20 through a set of electrical conductors 42 and a combin
23. ation check valve and breath switch 40 which is connected to control unit 20 through a set of electrical conductors 44 Gas turbine 38 is preferably of a known type that includes turbine 45 50 55 65 4 blades not shown which arranged to rotate as gas flows thereover These blades are ordinarily arranged to interrupt the beams of light which a plurality of LED s direct against a plurality of respective photo transistors to produce a multi phase set of output pulses on conductors 42 Thus turbine 38 supplies to control unit 20 over conductors 42 a train of pulses the number and frequency of which varies in accordance with the volume and rate of flow of gas flowing therethrough Combination check valve breath switch 40 which may be of a known type preferably includes a check valve 41 which is gently biased in its closed position to assure that gas enters mixing chamber 26 only through inlet line 28 Device 40 also includes a breath switch not shown which assumes a first state when valve 41 is open and a second state when valve 41 is closed Be cause of the cyclic nature of breath the openings and closings of the breath switch mark the beginnings and endings of breaths The state of the breath switch is monitored by control unit 20 through conductors 44 to enable it to interpret the concentration readings from gas analysis section 12 In particular by counting the number of pulses received from the gas turbine dur
24. called for by the first linear approximation at the same pulses per second value and d applying said correction factor to one of the linear segments of the first linear approximation to position the corresponding linear segment of the second linear approximation 28 In a gas volume calibration apparatus for a gas analysis instrument of the typeincluding a a source for supplying a known volume of calibration gas b a gas turbine for providing an output signal that varies in accordance with the volume and rate of flow of gas therethrough and c a control circuit connected to the turbine the improvement characterized by a means in the control circuit for storing a first piecewise linear approximation of the response of a typical turbine of the class of turbine to which the gas turbine belongs b means in the control circuit for receiving the output signal of the gas turbine as said known vol ume of gas is directed therethrough at rates of flow that correspond to at least two different linear segments of the piecewise linear approximation and c means in the control circuit for modifying said first piecewise linear approximation in accordance with the output signals produced by the turbine during calibration to produce a second piecewise linear approximation of the response of the gas turbine 29 The calibration apparatus of claim 28 in which the first linear approximation relates the volume of gas 20 25 30 35 40
25. cond chance to find an acceptable set of pulses per second values among the values produced by the operator at syringe setting N If however the program attempts to reach block 256 a second time i e attempts to delete data for two fur ther strokes it will be prevented from doing so by com parison block 254 which then directs CPU 100 to a further block 258 As a result ofencountering block 258 a counter B is incremented to signal the fact that after two attempts to compute an acceptable Cy value CPU 100 was unable to do so Under the latter condition CPU 100 is directed to comparison block 260 which unless counter B has a value of three reaches block 262 which in turn causes the operator to be informed that it is necessary for him to re enter the entire sequence of strokes at setting N Following the outputting of this information the program re enters the stroke entry sequence at block 210 via branch connector Z In those rare instances in which three complete sets of acceptable strokes at setting N fail to result in an accept able coefficient of variation then comparison block 260 will respond to the 3 in counter B to direct CPU 100 to a block 264 The latter block causes CPU 100 to inform the operator that he should consult the user s manual because of the probability of a failure within the instru ment Once this fault is corrected the operator may then restart the calibration sequence at block 200 and repeat the proces
26. duce the delivery rate of cali bration gas Finally when control plate 80 is rotated to its clockwise limit which may be defined by the posi tion of stop pin 92 aperture 62 is substantially blocked by control plate 80 thereby limiting the rate at which gas can flow into the rear of syringe 50 to the rate at which gas can leak through the clearance space be tween plates 80 and 525 Under this condition strokes can be completed only slowly resulting in a low gas delivery rate It will therefore by seen that for a given amount of stroke force by an operator the syringe of FIG 3 establishes three different delivery rates for calibration gas In the preferred embodiment of the present invention these three delivery rates correspond to segments A B and C of the piecewise linear approximation of the turbine characteristic of FIG 2 This correspondence greatly facilitates the process of providing calibration data to the instrument for each of linear segments A B and C and thereby enabling it to accurately determine the volumes of gas which are delivered to it at the rates that are associated with those linear segments The manner in which turbine data produced at these differ ent delivery rates are used to calibrate the instrument will be described later in connection with operation of electronic control unit 20 i Referring to FIG 5 there is shown a block diagram of the contents of control unit 20 those elements which appear in b
27. eptable strokes performed at each estimated flow rate and rejecting those accept able values if the coefficient of variation therefor is greater than a predetermined maximum coefficient of variation 49 The method of claim 46 including the steps of communicating to a user the need for faster strokes if the the pulses per second value for a stroke is less than the minimum acceptable value therefor and of commu nicating to a user the need for slower strokes if the pulses per second value for a stroke is greater than the maximum acceptable value therefor 50 The method of claim 46 or 49 including the step of counting the number of acceptable strokes and commu nicating that number to the operator after each stroke
28. g the parameters of the equations of the linear segments of the first ap proximation to reflect the datareceived during step b 24 The method of claim 23 in which the data associ ated with a flow of said known volume of gas through the turbine is accepted for use in producing the second piecewise linear approximation only if the flow rate associated therewith is within a predetermined range of acceptable flow rates 25 The method of claim 20 in which said known volume of gas is directed through the turbine a plurality of times at each flow rate and in which said stored data is averaged before being used in producing the second approximation 26 The method of claim 20 in which the turbine produces an output signal comprising a succession of pulses and in which the first linear approximation re lates the number of pulses per unit volume of gas flow to the number of pulses per second of gas flow 27 The method of claim 26 in which the position of a linear segment of the second linear approximation is determined by a measuring the pulses per unit volume that result from the flow of said known volume of gas through the turbine b dividing the total number of pulses that result from the flow of said known volume of gas through the turbine by the duration of said flow to determine a pulses per second value c calculating a correction factor from the measured number of pulses per unit volume and the number of pulses per unit vol ume
29. ich a parameter of at least one linear segment of the second linear approximation is derived from the parameter of the corresponding linear segment of the first linear ap proximation by applying a correction factor that is based on the difference between the stored response of a typical turbine and the measured response of the ac tual turbine 35 In a gas volume calibration apparatus for respira tory gas analysis instruments of the type including a a calibration syringe having a housing and a piston slid ably mounted therein said syringe being adapted to provide a known volume of gas as the piston is moved between predetermined first and second positions and b a gas turbine for providing a number of output pulses that varies in accordance with the rate at which said volume of gas flows therethrough the improvement characterized by a a breath switch connected in series with the tur bine for providing an output signal indicative of the duration of a stroke of the piston first means for receiving the turbine output pulses produced during a stroke of the piston and the output signal of the breath switch and for generat ing an actual flow rate signal indicative of the speed of the stroke c second means for storing maximum and minimum acceptable values for said flow rate signal and d third means for comparing the actual flow rate signal with said maximum and minimum acceptable values and for accepting the actual flow
30. ile flow of calibration gas which is similar in quantity and character to that produced by a test subject It will be understood that ambient air is a desirable calibration gas because of the fact that it is the change which the cardiopulmonary system of the test subject produces on ambient air which is of interest to the user of instrument 10 One source of error that is associated with the use of the calibration syringe of FIG 1 is a volume error that is caused by the inertia of the gas that flows in line 34 during an ejection stroke This gas flow tends to open check valve 68 after piston 54 reaches the end of its stroke This opening of the check valve is known as siphoning and causes the volume of gas flowing through the instrument to exceed the actual volume of the syringe Another operator related source of error is associated with the occurrence of incomplete strokes such as those resulting from failure of an operator to move piston 54 between its true outermost and innermost positions during each ejection stroke Such incomplete strokes naturally cause the volume of gas that is caused to flow through the instrument to be less than the full volume of the syringe The most important operator related error that is associated with the use of the calibration syringe of FIG 1 however is the volume error that results from moves piston 54 during a series of ejection strokes This error occurs because different stroke speeds cause tur
31. ing the information necessary to generate curve C2 is produced during calibration by using syringe 50 to produce turbine output data at flow rates that correspond to at least the most frequently used ones of line segments A E of curve C1 of FIG 2 In the preferred embodiment this turbine output data is produced without the need for a costly mechanical device for driving piston 54 This is accomplished by communicating to an operator through display 25 the information necessary for the operator to manually stroke syringe 50 at the speeds which will result in the desired pulses per second values from turbine 38 More particularly after control unit 20 has been placed in its calibration mode it outputs a message to the operator requesting him to stroke syringe 50 so that it may receive turbine output data for a first line seg ment such as segment A of curve C2 If the stroke is too fast or too slow control unit 20 will reject the resulting data and request the operator to repeat the stroke at a faster or slower rate This process is repeated until suffi cient information for the first linear segment has been received Control unit 20 then requests the operator through display 25 to change the syringe setting and stroke the syringe so that it may receive turbine output data for another line segment such as segment B of curve C2 Again the control unit accepts only data resulting from strokes of the proper speed and informs the opera
32. ing the time that the breath switch is open the volume of each breath is determined so that it may be used in interpreting the concentration readings provided by gas analysis section 12 Because the response of a gas turbine is known to be nonlinear it has been the practice prior to the present invention to maintain a flow of bias gas through a tur bine and thereby maintain the latter within a relatively flat portion of its operating characteristic The provi sion of an accurately regulated source of such bias gas is however relatively costly Moreover because of the nonlinearities associated with the mixing of the sample and bias gases as well as the residual slope of the tur bine characteristic such an approach is relatively inac curate In accordance with one feature of the present invention the use of a bias gas flow is eliminated and the nonlinear response of turbine 38 is dealt with by cali brating instrument 10 at a variety of different gas flow rates and thereby providing it with the ability to derive accurate volume data from turbine 38 in spite of fluctua tions in the rate of gas flow therethrough In this man ner the instrument as a whole is made able to provide measurements of improved accuracy over a wide range of sample flow rates In particular in calibrating instrument 10 in accor dance with the present invention control unit 20 takes a previously stored piecewise linear approximation of the nonlinear characte
33. ion The latter approximation is also stored within control unit 20 preferably but not necessarily in RAM 104 for use in interpreting flow rate signals 10 25 30 40 45 50 55 60 65 10 that are received during subsequent measurements on a test subject In this manner instrument 10 is able to accurately determine the volume of a breath in spite of changes in the rate of flow thereof The manner in which curve C2 is produced will be described pres ently Before doing so however it is helpful to first discuss the form in which a curve such as curve C1 is stored and the manner in which it can be used in mak ing breath volume determinations In order to make the most efficient possible use of memory curve is stored in the ROM of CPU 100 by storing therein the maximum and minimum pulses per second values that are associated with each of line seg ments of FIG 2 such as and for segment B along with the equations of the line seg ments that apply between those values These latter equations are preferably stored by storing the parame ters of these equations as expressed in slope intercept form i e in the form b where y is the vertical axis variable x is the horizontal axis variable m is the slope of the line and b is the y axis intercept This form of storage allows each line segment to be uniquely spec ified in terms of only four stored values namely
34. ion of the pulses per second values of the strokes divided by the mean pulses per second value thereof Few is a correction factor which spe cifies the position of a line segment of curve C2 with respect to the posi tion of the corresponding segment of curve a correction factor will exist for each setting of N PNmax PNmin C Fev The flow chart of FIGS will now be described Upon entering the volume calibration sequence and encountering block 200 CPU 100 sets N 1 to select first line segment such as A i e a first gas flow rate for which to receive turbine data CPU 100 also sets T 8 indicating that 8 acceptable strokes are necessary to fix the position of segment The set value of is used in blocks 202 and 204 to output to the operator on display 25 the message set syringe to setting 1 and push en ter CPU 100 then enters wait loop indicated by block 206 which is exited when the operator makes the requested syringe setting and pushes the enter button on keyboard 23 After CPU 100 exits this wait loop it sets variable B 0 as indicated by block 208 and sets vari able 1 as indicated by block 210 Upon encounter ing block 212 CPU 100 clears flow sensing networks 118 and 128 of FIG 5 to prepare the same for the re ceipt of data from the turbine 38 and breath switch 40 After these networks are cleared CPU 100 fetches the current values for Cs
35. le to the instrument The two approaches to storing the piecewise linear approximation are equivalent how ever since both can be used in the above described manner to interpret the gas concentration measure ments made by gas analysis section 12 The intercepts of piecewise linear segments B and C are in general sufficiently different from one another that it is desirable to have independently determined correction factors for use in fixing the intercepts of line segments and For the steepest piecewise lin ear segments such as D and E however it is possible to use the same correction factor that was determined in connection with line segment C without a significant loss of accuracy This application of the correction factor for line segment C to line segments D and E is desirable because it makes unnecessary either the use of a separate calibration syringe with a smaller volume or the use of extremely slow piston speeds The latter approaches are nevertheless available should they be desirable in particular applications It will therefore be understood that the present invention contemplates both embodiments in which the correction factors for all line segments are determined independently and embodiments in which one or more correction factors are derived from measurements made for other line segments In both cases the instrument is made able to determine the volume of gas delivered during a breath with an accuracy
36. livered This in turn allows the instrument as a whole to correctly interpret the output data from the gas analyzers of gas analysis section 12 Because the manner in which each of the above described compari sons and algebraic manipulations may be performed is well known to programmers the specific steps that are followed by CPU 100 in performing the same will not be described in detail herein While the above described volume determination takes into account the nonlinearity of a typical turbine it does not take into account the differences between a particular turbine and a typical turbine of the same type In accordance with an important feature of the present invention there is provided a calibration method and apparatus whereby stored curve C1 of FIG 2 and the pulses per second data that is received for a plurality of gas flow rates during calibration are combined to pro duce and store a corrected curve C2 that reflects the 4 448 058 11 actual properties of the particular turbine its then current operating condition This corrected curve may then be used in the above described manner to provide gas volume data which the instrument can use to pro vide gas concentration readings having an accuracy which far surpasses that available from respiratory in struments that were available prior to the present inven tion The manner in which corrected curve C2 is pro duced during calibration will now be described Generally speak
37. lses per liter value from said pulses per second value and the stored approximation and means for combining said pulses per liter value with said pulses per second value to determine the liters per second value for a breath 12 In a gas analysis instrument of the type having at least one gas analyzer for measuring the concentration of a compound of interest in human breath and a gas turbine for producing an output signal that varies in accordance with the volume and rate of flow of breath therethrough the improvement comprising 5 10 15 20 25 30 35 40 45 55 65 18 a first means for storing the information necessary to make available a piecewise linear representation of the response of the gas turbine said representa tion relating the volume of breath flowing through the turbine to the rate of flow of that breath there through b a breath switch connected in series with the gas turbine to generate an output signal indicative of the duration of a breath c second means responsive to the output signal of the turbine and the output signal of the breath switch for determining the rate of flow of a breath and d third means responsive to the first means and the second means for determining volume data for a breath 13 The instrument of claim 12 in which the first means stores the piecewise linear representation by storing the parameters of the equations for a plurality of line segments toge
38. means for combin ing the pulses per second signal with the respective linear segment to determine the associated pulses per liter value and c means for determining the volume of a breath from said pulses per liter value 6 The instrument of claim 4 in which the third means includes a means for combining said pulses per second value with the stored approximation to produce a pulses per liter value and b means for combining said pulses per second value with said pulses per liter value to determine the liters per second value for a breath 7 The instrument of claim 1 in which the linear ap proximation gives the number of pulses per liter pro duced by the turbine as a function of the number of pulses per second produced thereby 8 The instrument of claim 7 including a breath switch connected in series with the gas turbine to produce a signal indicative of the duration of a breath 9 instrument of claim 8 in which the second means includes a counter for counting the output pulses produced by the turbine said counter being enabled by the breath duration signal from the breath switch 10 The instrument of claim 9 in which the second means includes means for dividing the number of output pulses produced during a breath by the duration of that breath to produce a pulses per second value for use determining said operating point 11 The instrument of claim 10 in which the third means includes means for determining a pu
39. nt that the number of pulses produced per ejection stroke of syringe 50 can have a number of different values depending upon the flow rate that is associated with that ejection stroke i e depending upon the speed of piston 54 during that ejec tion stroke Prior to the present invention it was the practice to deal with the nonlinear response of a turbine by direct ing a flow of bias gas therethrough This bias gas flow caused all turbine output data to be associated with a relatively horizontal region of curve C1 such as the region corresponding to linear segment A In addition to being a relatively costly way of dealing with th non linear response of a turbine the use of a bias gas flow introduced new inaccuracies as a result of the fact that the chosen region was only approximately horizon tal and the fact that the characteristics of a turbine change with time and the accumulation of dirt In accordance with one feature of the present inven tion the need for a bias gas flow is eliminated by storing a complete piecewise linear approximation C1 of the response of a representative turbine within control unit 20 preferably in ROM in CPU 100 In addition in accordance with another important feature of the pres ent invention this stored characteristic is combined with actual turbine data taken during calibration to produce a second piecewise linear approximation C2 of the response of the actual turbine in its then current condit
40. on gas delivery rates To gether these improvements greatly improve the accu racy of the calibration of the instrument and thereby improve the accuracy of all of the subsequent measure ment that are based thereon To the end that the above mentioned siphoning lem may be eliminated the preferred embodiment of the present invention includes an improved gas volume calibration syringe 50 shown in FIGS 3 and 4 The syringe of FIG 3 is in some respects similar to that of FIG 1 like functioning parts being similarly numbered but differs therefrom in several important respects The first of these is that in the syringe of FIG 3 end plate 52 is modified to provide separate inlet and output nipples 64 and 645 respectively Associated with this difference is the elimination of T connector 66 of FIG 1 and the connection of a check valve 68 in series with line 70 Finally a flat rubber covering 54 is attached to the leading edge of piston 54 Together these modifica tions eliminate the above described siphoning problem This is because when piston 54 reaches the end of its stroke rubber covering 54a makes contact with the inner surface of second end plate 52c thereby suddenly cutting off the flow of gas in lines 70 and 34 As a result of this positive shut off no additional gas can enter chamber 26 after the end of a stroke A second improvement in syringe 50 of FIG 3 sults from the provision therein of a control
41. oth FIGS 1 and 5 being similarly numbered As shown in FIG 5 control unit 20 includes a central processor unit or CPU 100 which may be of any of a number of different commercially available types such as for example an LS 4800 board manufactured by Beckman Instruments Inc Generally speaking CPU 100 includes the usual arithmetic logic unit ALU a program memory preferably stored in read only mem ory ROM a plurality of working registers preferably comprising random access memory RAM and suitable clock drive circuitry Because the present invention may be understood without reference to the internal operation and circuitry of CPU 100 the internal opera tion and circuitry thereof will not be described in detail herein CPU 100 communicates with the various circuit net works with which it operates through a system bus 102 which may also be of a known type such as the well known multi bus This bus carries a number of differ ent signals such as command signals from CPU 100 status signals to CPU 100 and data signals both to and from CPU 100 all such signals being coded in multi bit digital form CPU 100 is also connected to keyboard 23 through conductors 24 and a CPU input output port not shown which is available for that purpose on the abovementioned LS 4800 board Alternatively key board 23 may be connected to CPU 100 through bus 5 15 20 25 30 35 40 45 50 55 60 102 provided that
42. r intercepts together with the stored slopes of the line segments of curve C1 provide the m and b values that are necessary to com 4 448 058 15 plete the slope intercept form equations for the line segments of curve C2 As will be explained in greater detail presently these calculations involve the multipli cation of the intercepts for the line segments of curve C1 by their respective correction factors Block 274 is followed by block 276 which causes CPU 100 to actu ally generate and store the slope intercept firm equa tions which together define piecewise linear curve C2 Once the latter curve is stored the volume calibration sequence is complete CPU 100 is then in condition to proceed with unrelated calibration procedures such as zeroing or with the taking of actual measurements The manner in which the correction factors are cal culated for the line segments of curve C2 will now be described Referring to FIG 2 it will be seen that line segment B of curve C1 comprises that portion of a line which lies between pulses per second values and Pgmin The latter equation is in the previously mentioned slope intercept form in which mg represents the slope of the line and bg represents the intercept of that line on the vertical or Y axis The graphical significance of and bg are shown on the dotted line extension of curve B of FIG 2 The correc tion factor Fcg for segment B is that number which
43. ristic of a representative turbine of the type being used and combines the same with turbine data that is produced as an operator directs an accurately known volume of gas through the turbine at a plurality of different flow rates This gas is supplied through the use of an improved volume calibration syringe that reduces operator related volume errors Based on the data received control unit 20 produces and stores a corrected piecewise linear approximation that reflects the nonlinear response of the actual gas turbine then being used Illustrative ones of these piece wise linear approximations of representative and indi vidual turbine characteristics are shown as curves C1 and C2 respectively of FIG 2 which will be described in detail later The calibration method and apparatus by which this desirable result is accomplished enables the instrument to operate with high accuracy at a variety of sample gas flow rates not only after its initial calibra 4 448 058 5 tion but also after each of any number of subsequent calibrations In this manner the benefits of the invention are preserved in spite of those changes in turbine char acteristics that are associated with wear the accumula tion of dirt and other factors In the upper portion of FIG 1 there is shown a vol ume calibration syringe 50 of a type that is known in the art This syringe includes a housing 52 having a cylin drical section 52 a first end plate 52 and a
44. ry randomly from stroke to stroke In the past this nonlinearity has been dealt with by introducing a flow of a bias gas which causes the rate of gas flow through the turbine to remain in a range of values within which its response is relatively flat This ap proach however only partially solves the problem It does not actually eliminate variations in the turbine output with variations in the rate of flow therethrough In any case the use of bias gas flows is expensive since it requires the establishment of an additional accurately controlled gas flow and since the gas flow paths 20 25 30 35 45 55 60 65 2 through the instrument more complex than they would be in the absence of a bias gas flow SUMMARY OF THE INVENTION In accordance with the present invention there is provided an improved method and apparatus for cali brating a respiratory gas measuring instrument which is not subject to the above described problems and which affords high gas volume calibration accuracy without requiring the use of bias gas flows The present invention provides a method and appara tus by which data received from a gas turbine and a breath switch during calibration are used to produce a nonlinear characteristic curve for the turbine The re sulting characteristic curve is made available for use during the taking of measurements to provide corrected gas volume data that takes into account the duration and rate of
45. s adapted to supply to CPU 100 via bus 102 a number indicative of the total number of pulses produced by turbine 38 between the opening and closing of breath switch 40 To the end that this may be accomplished network 118 includes a suitable input circuit 120 for logically OR ing the multi phase signals produced by the various LED phototransistor pairs within turbine 38 into a single pulse train a counter 122 for counting the number of pulses produced by input circuit 120 and a conventional bus interface network 124 for supplying the contents of counter 122 to CPU 100 upon command The counting activity of counter 122 is coordinated with the state of breath switch 40 by applying the out put of breath switch 40 thereto as an enable signal via conductors 44 After counter 122 communicates to CPU 100 the total number of turbine output pulses it is preferably reset by an appropriate command from CPU 100 Breath duration information from breath switch 40 is provided to CPU 100 by gas flow sensing network 128 which may include a counter 130 a bus interface net work 132 and a fixed frequency clock 134 having a frequency of for example 40 KHz Like counter 122 counter 130 is enabled by the signal on conductors 44 when breath switch 40 is open During the time that it is enabled counter 130 counts the pulses received from clock 134 and stores the same until CPU 100 requests the same through bus interface network 132 After this information ha
46. s been supplied to CPU 100 counter 130 is preferably reset by an appropriate command from CPU 100 in preparation for the next operation of breath switch 40 Because counter 130 counts pulses having a fixed frequency the number stored in counter 130 dur ing a breath is indicative of the duration of that breath In summary flow sensing network 118 supplies to CPU 100 the total number of output pulses produced by turbine 38 during each operation of breath switch 40 During calibration this number is equal to the number of pulses produced during an ejection stroke of syringe 50 In addition flow sensing network 128 supplies to CPU 100 a number indicative of the length of the time period during which breath switch 40 was open During calibration this number is indicative of the duration of an ejection stroke of syringe 50 Together these num 4 448 058 9 bers allow CPU 100 to generate an actual flow rate signal preferably in terms of a pulses per second value that is associated with each stroke of syringe 50 In accordance with the invention the availability of the latter information for a plurality of calibration gas flow rates makes it possible to calibrate the instrument of FIG 1 over substantially the entire operating range of the turbine and thereby enables the instrument to later determine the actual volume of sample gas delivered in spite of changes in the sample gas flow rate Referring to FIG 2 curve C1 represents
47. s to establish an acceptable set of val ues for the coefficient of variation for each syringe setting i Returning to the desired flow of the calibration se quence at block 248 if coefficient of variation Cris less than or equal to Cymax comparison block 248 will cause CPU 100 to proceed to block 250 In the latter block CPU 100 is caused to calculate a correction factor in a manner that will be described presently and thereby determine the position of the segment of interest of curve C2 with respect to the corresponding segment of curve C1 After the calculation of the correction factor Fon for setting N 1 CPU 100 is directed to a compari son block 270 which compares the then current value of N with the desired maximum value thereof If the value of N is not equal to the maximum value in the preferred embodiment 3 CPU 100 is directed to a block 272 which increments N and returns CPU 100 to block 202 via branch connector X This causes CPU 100 to re quest turbine data for another syringe setting so that it may determine the position of a new line segment on curve C2 such as segment B When this occurs the above described stroke entry sequence will be repeated for the new syringe setting When correction factors have been produced for each of the desired number of line segments of curve C2 CPU 100 is directed to block 274 This block causes CPU 100 to calculate the intercept values b y for each line segment of curve C2 The latte
48. that is substantially greater than that exhibited by previously available instruments It will be understood that since the volume calibra tion process described above is carried out on a regular basis the instrument of FIG 1 is regularly provided with a fresh nonlinear turbine characteristic curve such as C2 of FIG 2 This assures that the instrument always has available to it information concerning the then cur rent response of turbine 38 even as that response changes with time wear and the accumulation of dirt Thus the benefits of the invention are available on a continuing basis In view of the foregoing it will be seen that the vol ume calibration method and apparatus of the invention includes both improvements in the apparatus for deliv ering calibration gas the syringe and in the method and apparatus for using that gas to volume calibrate the instrument Together these improvements result in an instrument which with each calibration reflects not only the nonlinear response of a typical turbine but also the nonlinear response of the particular gas turbine in its then current condition As a result the overall accuracy of all measurements which are made with the instru ment are significantly improved What is claimed is 1 In a gas analysis instrument ofthe type having at least one gas analyzer for measuring the concentration of a component of interest in human breath and a gas turbine for producing a number of outp
49. ther with the ranges of flow rates over which those line segments are applicable 14 The instrument of claim 13 in which the third means determines the volume data for a breath by a substituting into one of said equations the rate of flow determined by the second means and b solving said equation for said volume data 15 The instrument of claim 12 or 14 in which the output signal of the turbine comprises a series of pulses and in which the second means includes a a counter for counting the number of pulses produced by the turbine during the time that the breath switch is open and b means for dividing the number in said counter by the time that the breath switch is open to determine the rate of flow of a breath 16 The instrument of claim 12 including means for storing the information necessary to make available a second piecewise linear representation of the response of an average turbine of the class of turbine to which the gas turbine belongs 17 The instrument of claim 16i in which the stored piecewise linear representation is derived from the sec ond piecewise linear representation on the basis of vol ume data taken during calibration 18 The instrument of claim 12 in which the volume data is the volume of a breath 19 The instrument of claim 12 in which the volume data is the volume rate of flow of a breath 20 A method for volume calibrating a gas analysis instrument of the type having a a source for providing a
50. tify circuit failures such as the failure of one of the LED s or phototransistors within turbine 38 Based on the pulses per second value Py that is com puted in block 226 CPU 100 next fetches the maximum and minimum acceptable pulses per second values PNmax Pymin for N 1 as called for by block 230 and compares Pythereto in blocks 232 and 234 to deter mine if it is between them These comparisons together assure that actual pulses per second value Py lies be tween the end points of the line segment for which calibration data is being sought In the event that this dual test is passed stroke count C is incremented in block 236 and then compared against the total number of acceptable strokes that are required at setting N as called for by block 238 If the required number of ac ceptable strokes has not yet been made block 238 di rects CPU 100 back to block 212 via branch connector y to initiate a request for additional strokes Alternatively if the actual pulses per second value for a particular stroke fails to fall within the range called for by blocks 232 and 234 stroke count C is not incremented and CPU 100 is directed to one of blocks 240 and 242 The applicable one of these blocks informs the operator whether the just completed stroke was unacceptable as a result of being too fast or as a result of being too slow In either case CPU 100 is returned to block 212 via branch connector y to call for another stroke
51. tor whether unacceptable strokes are too fast or too slow This process is then repeated for the de sired number of additional line segments Once the operator has entered sufficient turbine data for each of the desired line segments this data is used to generate a set of correction factors which in effect determine new intercepts for the equations of the line segments of curve C1 These new intercepts together with the stored slopes of the line segments of curve C1 define a corrected piecewise linear approximation C2 which reflects the response of the actual turbine being used The latter is then stored for use by control unit 20 in interpreting the volume of gas delivered during all subsequent measurements i e until the next volume calibration The result is an instrument which is accu rately volume calibrated not only in view of the nonlin earity of the gas turbines generally also in view of the individual characteristics of the actual turbine being used The manner in which the present invention operates to accomplish the above described results is most easily understood in connection with the flow chart of FIGS 6a c The meanings of the various symbols used in this flow chart are as follows Symbol N Meaning N is a variable that identifies the linear segment of curve C1 or C2 for which turbine data is being received and the syringe setting position of control plate 80 that is associated with that segment 55
52. uce a scaling factor that allows any point on line segment B of curve C2 to be expressed in terms of the corresponding point on line segment B of curve In operation the calculation called for by equation 2 is carried out by CPU 100 each time that it encoun ters block 250 of FIG 6c Once CPU 100 has encoun tered block 250 once for each line segment for which a correction factor is required i e once all of the correc tion factors are available the latter may be combined with the previously stored intercepts of the respective line segments of curve C1 in accordance with equation 1 as CPU 100 encounters block 274 of FIG 6c Fi nally as CPU 100 encounters block 276 these inter cepts may be combined with the previously stored re spective slope values to produce andstore the parame ters of the equations for curve C2 Alternatively the 20 25 35 40 45 50 55 60 65 16 correction factors alone may be stored for use in gener ating the equations for the line segments of curve C2 from those of curve C1 on an as needed basis In partic ular when an equation for a segment of curve C2 is needed it may be produced by multipying the intercept of the equation for the corresponding segment of Curve C1 by the applicable correction factor The advantage of the latter approach is that fewer memory locations are required to store the information necessary to make the desired piecewise linear approximation availab
53. ut pulses that varies in accordance with the volume and rate of flow 4 448 058 17 of human breath therethrough the improvement prising a first means for storing a piecewise linear approxi mation of the response of the gas turbine b second means responsive to the number of output pulses produced by the turbine for determining the operating point of the turbine on the stored approx imation and c third means responsive to the stored approxima tion and the output of the second means for provid ing volume data for a breath 2 The instrument of claim 1 in which the first means stores the piecewise linear approximation by storing the parameters of the equations for a plurality of linear segments together with the maximum and minimum values between which respective linear segments are applicable 3 The instrument of claim 2 in which the approxima tion gives the number of pulses per liter produced by the turbine as a function of the number of pulses per second produced thereby 4 The instrument of claim 3 in which the second means determines said operating point by dividing the total number of turbine output pulses by the time inter val during which those pulses occurred to produce a pulses per second value 5 The instrument of claim 4 in which the third means includes a means for comparing the pulses per second value with said maximum and minimum values to iden tify the applicable linear segment b
54. valve when the piston reaches the end of its stroke Such siphoning affects the accuracy of the calibration process by causing the actual volume of gas supplied to the instrument to exceed the volume of the calibration sy ringe Another error that is associated with the use of manu ally operated calibration syringes results from the fact that due to operator inattention the piston may not be moved between exactly the same beginning and end positions during each ejection stroke An operator may for example not withdraw the piston to its true outer most position or may not push the piston to its true innermost position Any such deviations from the de sired inner and outer positions affect the volume of gas delivered by the syringe during calibration and there fore the accuracy of all measurements that are based on that calibration Another even larger error that is associated with the use of manually operated calibration syringes is the error that results from the nonlinearity of the response of the gas turbine This nonlinearity can cause the num ber of output pulses that are produced by the turbine during the flow of a known volume of calibration gas to vary substantially depending upon the rate at which the gas is delivered The difficulty is that most operators have difficulty in operating the syringe in a consistent manner As a result the number of turbine output pulses produced during an ejection stroke of the syringe will va
55. when multiplied by intercept bg of curve C1 yields the value of the intercept of line segment of curve C2 This relationship is expressed in equation 1 of FIG 2 The corrected intercept b p together with slope mg which may reasonably be assumed to be the same for curves C1 and C2 is sufficient to specify the equa tion for line segment B of curve C2 between maximum and minimum pulses per second values Pamax and Pann From the foregong it will be seen that correction factor FcB is a number which in effect allows line segment B of curve C1 to be shifted up or down to a new position in which it more nearly reflects the re sponse of the actual turbine in its then current condi tion The correction factors and for line seg ments and C will be understood to operate in a similar manner to shift those line segments to new positions in which they also reflect the operation of the turbine in its then current condition Referring to equation 2 of FIG 2 there is shown the algebraic expression that is used to calculate correc tion factors such as Fcp In equation 2 Pameas is the average pulses per liter value that is associated with an acceptable set of turbine output data taken with the syringe at a setting such as B is the pulses per liter value from curve C1 which is associated with the average of the measured pulses per second values Equation 2 simply combines these two numbers to prod
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