RV-722 Voltage Divider User and Service Manual
Contents
1. lt p 20 00 e 5 ro j PE iO 291 io 5 5 2268 99 c oi 6 Bl 9 roO o e 2 89 9 2 99 Vall 8 8 _ 9 3588 253 66 91 2 gt 2 3 m oj ui 6 27 28 2 983 5 O81 o L4 o gt veg 9 er e ooo 5 E 0820 BE 61 19 as 5 S R 215 0 2 229 9 8856 oO o og 5 5 IE 8 8 91 mu ED o 6 1 5 fobo 5 sQ 5 2258 ae 9 5 ez 92. OUTPUT ____ 0 a q I e INPUT RESISTANCE 100 000 5 LEAD COMPENSATOR DETECTOR Y STANDARD 901 Figure 3 3A Adjust the input voltage to 100 volts for first decade 10 volts for other decades With both dividers set at zero as shown adjust the lead compensator so the outputs are within 0 1 ppm of each other 100 microvolts ppm for first decade 10 microvolts 1 ppm for other decades With both dividers set at full scale adjust the lead compensator so that the outputs are within 0 1 of each other Measure the linearity difference between the two dividers at nominally equal settings Two microvoltmeter readings are taken at each setting with the exception of the last decade one with all the decades to the right of the first decade set to zero and one with all the decades to the right of the first decade set to their maximum setting sure that the microvoltmeter reads positive difference when the RV 722 ratio is greater than the ratio of the transfer standard Correct the transfer standard readings by subtracting the linearity deviation of the transfer standard from these readings Divide the readings by the appropriate power of ten divide the second decade readings by ten the third decade readings by 100 etc to convert them to linearity deviation contributions expressed as a fraction of full scale for the whole div
3. Idde xeyo pue pue 1deoxe einjosqe 20 90054 jueuno pue umouy HgONVISIS3H LNdNI se owes 5 jndu 948 H3dOHd HO TV INVIW NOILONEISNI 1 8 55 5NILV83dO 8301 33 110 222 AH 17 230 01 00 Specifications RV 722 Chapter 3 INSTALLATION 3 1 Initial Inspection IET instruments receive a careful mechanical and electrical inspection before shipment Upon receipt verify that the contents are intact and as ordered The instrument should then be given a visual and operational inspection If any shipping damage 1 found contact the carrier and IET Labs If any operational problems are en countered contact IET Labs and refer to the warranty at the beginning of this manual Save all original packing material for convenience in case shipping of the instrument should become necessary 3 2 Installation For a rack mounted model installation in a 19 inch rack may be made using the slots in the rack mount ing ears A mounting location that does not expose the unit to excessive heat or temperature variations is recommended It may be necessa
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5. 12 4 7 Operation and Controls nore ioni ede t cedens 12 4 8 Use Applications and Calibration of the 722 13 Chapter 5 Decade Adjustment ceres 14 5 1 Introduction to decade adjustment 14 5 2 Considerations and Overview of Adjustment Procedure of Resistors 14 9 2 Calibratioto ss uem oti eere eet cm sts 15 5 2 2 Review of Measurement 15 5 3 Resistor Adjustment Proced re 4 cese ee eeu 16 5 3 1 Adjusting the first three 6 80065 sse 16 Chapter 6 Verification Calibration and Maintenance 18 iv Table of Contents Table of Contents Figures and Tables Figure 1 1 RV 722 Kelvin Varley Voltage Divider 1 Figure 1 2 Digital Potentiometer Model for Kelvin Varley Voltage Divider 2 Figure 1 3 Th venin Equivalent Circuit of Kelvin Varley Voltage Divider 3 Figure 1 4 Output Resistance as a Function of Dial Setting 3 Figure 1 5 Schematic Diagram of RV 722 Kelvin Varley Divider Circuit 4 Figure 2 1 Sample Label Attached to the Instrument 7 Figure 4 1 Power Distribution Effect of Interpolating Resistors 10 Figure 4 2 Th venin Equivalent Circuit to Show Loading Effects 10 Figure 4 3 Output Resistance
6. og OK 71 8 708 d s 4 4 5 10 3 1 2 22 Er gt 6 1 63 000ppm 5 5 a TO USE a 5 A Enter with R and Find 5 OUTPUT DEVIATION on left scole OUTPUT DEVIATIONS 4 RL 2 00 8 Tronsfer OUTPUT DEVIATION to 0 0 Rour 10 e right scole E a A LINEARITY DEVIATION Sx OUTPUT DEVIATION 2 on right scole ond with Find 2 LINEARITY DEVIATION 10 0 001 10 Figure 4 4 Nomogram for Output Deviation and Linearity Deviation Due to Output Loading Operation 11 RV 722 4 6 Switch Conditioning The main switch wipers employed in this unit are self cleaning and are constructed of solid silver alloy with solid silver alloy contacts Whenever left idle all the wipers and contacts should still be conditioned or re broken in to remove any oxide that develops over time This is standard metrology practice when high accuracy is required This effect 1s of the order of less than 1 so it may be ignored whenever measurements of that magnitude are not important To perform this breaking in simply rotate each switch seven to ten times in each direction 4 7 Operation and Controls AN WARNING Observe all safety precautions if high voltages are used Figure 4 5 shows the front panel and the various controls and connection terminals of the RV 722 divider The input voltage is con
7. o hoso o vee Bel 6 o 8 9 ol o 5 PM gc 8068 za oo ro ro ooo 5 is o 81 88 8 ox 4 ei o amp 6 68 2 az 5 ed oo ro Ho 0 oO 5 4 ige 5 8 E 3 20 o o ossy E dis 5 oo lo 81 o 30 8 ooo ex m 04 FOE ol vel s 0869 8068 3 oo ro to oc e 891 o 95 5559 30 epis dwog Decade Adjustment 17 RV 722 18 Chapter 6 VERIFICATION CALIBRATION AND MAINTENANCE The following pages are reproduced directly from esi RV 722 manual The RV 722 is highly stable and accurate divider however to use its short term stability to the fullest advantage it should be recalibrated shortly before being used The conditions under which the divider is calibrated should duplicate the ambient temperature and excitation volt age as closely as is practical This will minimize the effect of tempera ture differences and slow drifts in the divider 3 1 EQUIPMENT REQUIRED 1 A ten step 100k input total resistive divider with provisions for intercomparing the individual resistors ESI Model SR 1010 Resistance Transfer Standard 10k per step 2 dc source capable of delivering 300 volts to 100k load power limited to one watt ESI Model 80
8. PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT RV 722 Voltage Divider User and Service Manual formerly manufactured by esi Copyright O 2009 IET Labs Inc Visit www ietlabs com for manual revision updates RV 722 im December 2009 o t LA B 5 www ietlabs com 534 Main Street Westbury NY 11590 TEL 516 334 5959 800 899 8438 FAX 516 334 5988 PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT 6 a LAB 5 www ietlabs com 534 Main Street Westbury NY 11590 TEL 516 334 5959 800 899 8438 FAX 516 334 5988 RV 722 NOTICE This manual for the IET Labs Model RV 722 is offered as a convenience The IET RV 722 is identical in form and function to the original esi Model RV 722 It has been updated in that it employs sealed switches and hermetically sealed stable resistors The esi unit used fixed resistors that allow factory readjustment only And as esi factory service has not been available the units had to be discarded if out of specifications The IET unit offers the user the option of readjustment of the resistors to bring the unit into initial specifications This manual contains the same original esi manual information along with more tutorials and background as well as instructions for the optional adjustment of the resistors The user may use the original esi manual if preferred RV 722 WARRANTY We warrant that
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10. a total effec tive resistance of 10 for that step The 11 steps become equivalent to 10 steps of 10 each and in this way all the steps are kept equal Each step of the second decade 15 2 The 4 of that decade spanned by the switch contacts are shunted by the 4 effective resistance of the third decade Similarly 800 2 of the third decade are shunted by the 800 total resistance of the fourth decade Note that this pattern reduces the resistor value until an final value of 400 Q is reached and repeated This allows the use of a resistance high enough to avoid contact resistance problems Note that the last decade uses only one switch wiper and can therefore span positions 0 10 With all sig nificant decades set to 9 and the last one set to 10 the output 1 1 0 or equal to the input At the low or zero end a small series resistance is added between the OUTPUT LOW terminal the lower end of the divider string and the INPUT LOW terminal to compensate for contact and wiring resistance thus bringing the voltage the OUTPUT LOW terminal equal to the voltage at the OUTPUT HIGH terminal for all switches set to 0 This provides improved performance for low voltage outputs Dials set at 0 1323296 CASE Figure 1 5 Schematic Diagram of RV 722 Kelvin Varley Divider Circuit Introduction RV 722 Chapter 2 SPECIFICATIONS For convenience to the user the pertinent specifications are given
11. as a Function of Dial Setting 11 Figure 4 4 Nomogram for Output and Linearity Deviation 11 Figure 4 5 7 722 Front 12 Figure 5 1 Internal Hear Panel of HV 722 e 15 Table 5 1 RV 722 Resistance Adjustment Table 16 Figure 5 2 RV 722 Board Layout ure Era um ce tone 17 RV 722 RV 722 vi This page is intentionally left blank Table of Contents RV 722 Chapter 1 INTRODUCTION 1 1 General Description The new RV 722 Rev E Figure 1 1 is equivalent in form and fit to the esi Model RV 722 It is updated with a number of improved features The RV 722 is a calibration grade voltage divider employing the Kelvin Varley circuit It is a highly accurate stable and linear primary ratio standard suitable for use in many applications requiring known voltage or cur rent ratios In particular the RV 722 15 especially appropriate for use in bridge circuits providing two legs of a bridge with a well known ratio RV 722 KELVIN VARLEY VOLTAGE DIVIDER Figure 1 1 RV 722 Kelvin Varley Voltage Divider A Kelvin Varley voltage divider is designed for use in null balance circuits or as a voltage source for high impedance circuits See References It cannot be used as a simple variable resistor because of the additional resistance in series with the tap see Fi
12. in an OPERATING GUIDE shown in Figure 2 1 affixed to the case of the instrument SPECIFICATIONS Definitions Linearity is the expression of the accuracy of the divider in proportional parts of input Two types of linearity are frequently specified absolute and terminal Absolute linearity is the accuracy relative to the output at the end scale settings of 0 and 1 0 That is the divider is defined as correct at these settings Terminal Linearity 15 the accuracy relative to the input at the divider end terminals The RV 722 has separate input and output common terminals so that the output common may be compensated for small voltage drops in leads and switches making the output zero when the setting is zero Overview Two separate terminal linearity specifications are given Absolute relative to the output common terminal and Terminal Linearity relative to the input common terminal Mid scale linearity and coefficient ratings apply for settings between 0 1 and 0 9 The ratings will typically improve below 0 1 in proportion to the square root of the setting and above 0 9 in proportion to the square root of 1 minus the setting Specifications Absolute Linearity At 23 at low power defined relative to zero and full scale outputs at the output terminals Quantitatively it is V a Vin S where S is the dial setting that is the divider is defined as correct at these end settings See definition above Initia
13. the low end correction 3 Connect the equipment as shown in Figure 3 45 With all of the dials set to zero measure the voltage between the output tap terminal 3 and the output common terminal 4 This reading is the compensated common correction 4 Connect the equipment as shown in Figure 3 4C With all of the dials set to full scale measure the voltage between the output tap terminal 3 and the input terminal 1 This reading is the full scale correction 5 Plot these readings as illustrated on the last graph in Section 2 4 In these measurements a reading of 3 microvolts equals 0 01 ppm 300v e T 700 o OHMS Figure 3 4 Figure 3 48 fg O Figure 3 4C 22 Verification Calibration and Maintenance RV 722 3 5 CHECKING THE POWER COEFFICIENT The power coefficient of the RV 722 is 1 ppm per watt less for low settings no more than 100 volts is applied to the RV 722 0 1 watt or if it is not necessary to utilize the fullest capabilities of the divider then there is no need to be concerned with the power coefficient However if maximum ac curacy is necessary in situations where a significant amount of power is applied to the divider it is necessary to know the power coefficient of the unit The recommended procedure for checking the power coefficient is to com pare the divider at low and hig
14. this product is free from defects in material and workmanship and when properly used will perform in accordance with applicable IET specifications If within one year after original shipment it is found not to meet this standard it will be repaired or at the option of IET replaced at no charge when returned to IET Changes in this product not approved by IET or application of voltages or currents greater than those allowed by the specifications shall void this warranty IET shall not be liable for any indirect special or consequential damages even if notice has been given to the possibility of such damages THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES EXPRESSED OR IMPLIED INCLUDING BUT NOT LIMITED TO ANY IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE RV 722 N WARNING OBSERVE ALL SAFETY RULES WHEN WORKING WITH HIGH VOLTAGES OR LINE VOLTAGES Dangerous voltages may be present inside this instrument Do not open the case Refer servicing to qualified personnel HIGH VOLTAGES MAY BE PRESENT AT THE TERMINALS OF THIS INSTRUMENT WHENEVER HAZARDOUS VOLTAGES 45 V ARE USED TAKE ALL MEASURES TO AVOID ACCIDENTAL CONTACT WITH ANY LIVE COMPONENTS USE MAXIMUM INSULATION AND MINIMIZE THE USE OF BARE CONDUCTORS WHEN USING THIS INSTRUMENT Use extreme caution when working with bare conductors or bus bars WHEN WORKING WITH HIGH VOLTAGES POST WARNING SIGNS AND KEEP UNREQUIRED PERSONNEL SAFELY AWA
15. to calibrate the resistors individu ally they must be electrically independent from other decades This is accomplished by the use of the inter nal CALIBRATE OPERATE switch This switch inside the unit is normally set to the OPERATE position For adjustment of the resistors set the switch to the CALIBRATE position which electrically separates the resistors of the first three decades Access to each resistor is now possible on the Adjustment PC board inside the unit 5 2 2 Review of Measurement Options As discussed above what is required is the capability to measure and adjust and match the resistances of 10 000 2 of the first decade steps at e Accuracy 4 ppm Resolution 0 1 ppm e Repeatability 2 ppm e Matching 1 ppm This is the requirement of the first decade which is the most exacting The requirements for the second and third decades are less exacting RV 722 The assumption here is that the user has the capability and the equipment required to perform this measure ment Therefore it is up to the user to make these ad justments and measurements in a proper metrological fashion IET Labs is available for assistance There are a number of instrumentation options for accomplishing this These include but are not lim 10 to e High performance DMM s such as Fluke Model 8508A This may be used in the ratio mode with a resistance standard attached to the rear of the meter or in the direct mea suremen
16. 1 803 or 820 DC Generators 3 A high impedance 1 megohm or greater microvolt meter with a 100 microvolt full scale range and capable of resolving one half microvolt or less ESI Model 801 or 810 DC Detectors 4 Low resistance lead compensating potentiometers ESI Model LC875B Lead Compensator The dc source and lead compensating potentiometers should be isolated from ground by at least 10 ohms 3 2 TRANSFER STANDARD CALIBRATION Intercompare the resistors of the transfer standard and calculate the linearity deviation by the formula 1 105 E sc where A is the measured deviation of the utt resistor and AAV is the average deviation of the ten resistors Figure 3 2A illustrates an example of such a calculation DIFFERENCE FROM LINEARITY NUMBER THE AVERAGE PPM DEVIATION PPM 1 105 15 2 do Aw 0 04 10 0 01 0 4 02 0 66 0 52 0 45 _ 0 14 9 T LINEARITY CALCULATED FROM RESISTANCE MEASUREMENTS OF AN ESI MODEL SR 1010 DECADE RESISTANCE STANDARD Figure 3 2A Verification Calibration and Maintenance RV 722 3 3 DETERMINATION OF TERMINAL LINEARITY The process for determining the terminal linearity of the divider is as follows 1 Connect the equipment as shown in Figure 3 3A to determine the LEAD COMPENSATOR linearity deviation of the first decade of the RV 722
17. Y DO NOT APPLY ANY VOLTAGES OR CURRENTS THE TERMINALS OF THIS INSTRUMENT IN EXCESS OF THE MAXIMUM LIMITS INDICATED ON THE FRONT PANEL OR THE OPERATING GUIDE LABEL RV 722 Contents NOTICE cm i Chapter 1 Introduction 858558 8855853598 1 1 1 General Description lotion ode ee etre ec 1 1 2 Circuit Description a ED ERU Hd red 2 Circuit Model 2 1 22 Theory Operations soi iie eerte eer err cH Pr 4 Chapter 2 Specifications 5 Specifications usu Us e o e RE E 5 Chapter 3 Installation 8 3 1 Initial Inspection sic niea rede Pda 8 3 2 11030931151001 SS EER 8 3 3 Repackasing for 8 34 SOARS M 8 Chapter 4 9 4 1 ualluspect mand 5 e 9 4 2 General Considerations for Use of a Voltage 9 43 Power a EU RU cta RR 9 4 4 Temperature and Power Effects 10 4 5 Loading Errors eri 10 406 Switch Conditioning nici ftre eie md fe re
18. arm Such a circuit model of the RV 722 may be seen in Figure 1 2 In the case ofthe RV 722 the resistance between the input terminals 1 and LOW 15 100 1 0 HIGH LOW LOW Figure 1 2 Digital Potentiometer Model for Kelvin Varley Voltage Divider An actual digital potentiometer uses decades of re sistor steps each decreasing by a factor of ten The problem with such a digital potentiometer however is that its resolution becomes limited by the ever smaller resistor valus They become difficult to implement as the contact resistance of switches and connections become significant A Kelvin Varley circuit over comes this problem with its special design described later Another way to model the RV 722 1 with the Th venin equivalent circuit shown in Figure 1 3 where 8 is the dial setting Note that if the output is being fed into a very high impedance then the output impedance R may be ignored In general however the effect of load impedance R must be taken into consideration as will be discussed below The approximate value of R is shown in Figure 1 4 It may be seen that the output impedance is maximum at about the dial setting of 0 5 and drops to zero at both ends 1 0 and 0 It is the value of R which will influence the effect of loading Note however that in general bridge applications nominally zero current flows out ofthe divider as the bridge comes into balance and therefore the divider effective
19. cautions The input voltage limitation of 700 V will normally protect the instrument from ex cessive power dissipation However with certain dial settings and connections it is possible to draw exces sive current and permanently damage the instrument if the input voltage is maintained For this reason the source should be limited to 1 W if possible This can be done by inserting a resistor with a value 4 in series with the supply voltage E Another method of protecting the divider is to fuse the output with a 10 mA fuse RV 722 10 4 4 Temperature and Power Effects Figure 4 1 illustrates the division of current and power in the RV 722 resistors The temperature rise in each resistor is almost directly proportional to the power applied to it CURRENT TEMPERATURE RISE Figure 4 1 Power Distribution Effect of Interpolating Resistors Uneven power distribution results from the shunting of two of the eleven resistors in each decade by the next decade refer to Figure 1 5 As a result of this uneven power distribution the shunted resistors do not get as hot as the rest Therefore if the divider is to be run at fairly high power and the first decade 18 switched a short time should be allowed for the redistribution of heat in the resistors 4 5 Loading Errors As may be seen from Figure 4 2 the output voltage of an unloaded Kelvin Varley divider is given by Vour SV DEV where DEV 15 the fractional linearit
20. each ofthe first three decades To measure and adjust these resistors refer to Table 5 and Figure 5 2 and do the following l J10C J11C Measure the value of each resistor by con necting to the points indicated in the Measure Across column As necessary adjust potentiometers identified in the Adjust column to values specified in the Target column After completing the adjustments remeasure to confirm stable settings Once all resistors are within specifications setthe CALIBRATE OPERATE switch to OPERATE Slide the housing back onto the unit and screw all the screws back into their original positions JOA J1A 10 kO less 6 7 ppm J1A J2A through R1A through J10A J11A R10A 00 700 JOB J1B through R1B through 2ko J10B J11B R10B JOC J1C through R1C through 6106 400 60 399 94 Table 5 1 RV 722 Resistance Adjustment Table Decade Adjustment RV 722
21. gure 1 2 The resistance ratios between the taps of a Kelvin Varley divider are not linearly related to the voltage ratios Although many new higher performance DMM s Digital Multimeters claim to have a voltage di vider capability that capability is limited to one or two decades and the Kelvin Varley circuits remains the choice for all precision divider applications to a fraction of a ppm Introduction The RV 722 has a resolution of 0 1 ppm and an absolute linearity of 0 5 ppm Long term linearity is 1 0 ppm but the unit may be brought into initial specifications with internal adjustments see Chapter 5 Temperature coefficient of linearity is 0 2 ppm C and power coefficient of linearity is 1 ppm W The RV 722 incorporates a number of advanced fea tures for convenience and high performance These include internal readjustment capability e state of the art very low temperature coef ficient hermetically sealed resistors e Kel F mounted low emf tellurium copper binding posts convenient operating guide attached to the unit Technical references available on IET s website The RV 722 features an internal OPERATE CALIBRATE switch to allow built in trimming so that the unit may be adjusted with conventional instruments whenever required to bring it to initial performance specifications The RV 722 employs sealed precision resistors which have been aged temperature and power cycled to maximize long term stabi
22. h power to a transformer type divider which will not change linearity even with wide variations in the input voltage Figure 3 5A for details of the setup If the same frequency is used at both low power and high power then the ac dc difference will be the same at both powers and the power coefficient at each setting will be the linearity change divided by the power change Figure 3 5A Verification Calibration and Maintenance 23
23. h this equivalent circuit and a known load resistance the effect on output voltage can be easily analyzed Variations in output voltage can be expressed as LINEARITY DEVIATION deviation from nominal output in proportional parts of the in put oras OUTPUT DEVIATION the deviation from nominal output in proportional parts of the output Operation RV 722 5 o Figure 4 3 is a plot of the output resistance of the RV 722 as a function of its dial setting The re sistance values are symmetrical about a setting of 500 000 0 For example the output resistance at a setting of 985 000 0 will be the same as that for a setting of 015 000 0 The resistance corresponding to 255 555 5 can be found in Figure 4 4 With the Kelvin Varley circuit the maximum output resistance Occurs at settings of 455 555 5 and 544 444 4 The 100 MODEL OUTPUT RESISTANCE 1048 MODEL OUTPUT RESISTANCE 107 1075 1075 1074 1073 107 DIAL SETTING 3 Figure 4 3 Output Resistance as a Function of output resistance measurements are made by short Dial Setting ing the input terminals together and measuring the resistance across the output terminals R ga OMEG ec LOAD 8 2 3 RESISTANCE 0 af OUTPUT DEVIATION E SETTING LINEARITY DEVIATION 0 001 5 MEG 5 50 0 001 Rour B 5 M 0 0 100k 10k 8 2 100pem 5 5 4 P d 4 2 5 3 Ik 2 3 2 2 n
24. ider Verification Calibration and Maintenance 19 RV 722 7 Plot the measured deviations as shown in Section 2 4 8 Connect the equipment as shown in Figure 3 315 Tp Nu Ep 0 Figure 3 3B 9 Repeat steps 2 through 7 except in step 5 where the two microvolt meter readings are taken at each setting one with all the decades to the right of the second decade set to zero and one with all the decades to the right of the second decade set to their maximum settings 20 Verification Calibration and Maintenance RV 722 10 Connect the equipment as shown in Figure 3 3C Figure 3 3C 11 Repeat steps 2 through 7 except in step 5 where the two microvolt meter readings are taken at each setting one with all the decades to the right of the third decade set to zero and one with all the decades to the right of the third decade set to their maximum settings 12 The remaining decades may be calibrated in similar manner if desired However normally this data is not used Verification Calibration and Maintenance 21 RV 722 3 4 DETERMINATION OF END CORRECTION The procedure used for determining the end corrections for the RV 722 is as follows 1 Connect the equipment as shown in Figure 3 4A and adjust the input voltage to the RV 722 to 300 volts 2 With all of the dials set to zero measure the voltage between the output tap terminal 3 and the input common terminal 2 This reading is
25. l 0 5 ppm at midscale improving to Zero at end settings Long Term 1 ppm at midscale improving to zero at end settings Certificate Corrected 0 2 ppm at midscale improving to zero at end settings Terminal Linearity Relative to Input Terminals The accuracy at the divider end terminals relative to the input See definition above Same as absolute linearity except for 0 05 ppm end voltage drop Temperature Coefficient of Linearity 0 2 ppm oC at midscale improving to zero at end settings Power Coefficient of Linearity 1 ppm W at midscale improving to zero at end settings Switch Contact and Wiring Resistance Variations Initial 20 004 ppm Long Term 50 008 ppm Calibration conditions 23 C low input power RV 722 Number of Decades Terminals Seven High quality low thermal emf gold plated Resolution tellurium copper binding posts standard 0 1 ppm 0 75 spacing additional binding posts are connected to the case for shielding Terminals are insulated from the case by non moisture absorbing Kel F spacers Input Resistance 100 50 ppm Maximum Output Power Dimensions 2 5 W continuous 5 W intermittent 4825 0m W x Dem H2 L3onD Maximum Input Voltage 19 0 x 5 25 x 8 4 700 Vrms Weight Breakdown Voltage 5 6 kg 12 5 Ib 1000 V peak to case 6 Specifications RV 722 SOZIALE SI ZZ SONI 09 A jueuinajsu oqe ajd
26. lity and matched to mini mize temperature and power coefficients effects The most significant decades are matched and hermetically sealed to minimize temperature coefficient effects RV 722 The switches are made of solid silver alloy contacts for long life minimum contact resistance and no tarnishing The three most significant decades use sealed switches The switches have multiple wiper contacts to provide stable low contact resistance High quality gold plated tellurium copper binding posts minimize the thermal emf effects which would artificially reflect a change in dc resistance measure ments They are mounted to the case on special Kel F washers to assure low leakage All other conductors within the instrument as well as the solder employed contain no metals or junctions that could contribute to thermal emf problems The front panel is clearly labeled showing step size for each decade Maximum voltage and current limits are indicated as well The unit may be mounted in a standard 19 inch rack Applications include linearity determination the mea surement of voltage and resistance and the calibration of other dividers potentiometers or similar devices involving both voltage and current 1 2 Circuit Description 1 2 1 Circuit Model A Kelvin Varley voltage divider may be thought of as being equivalent to a digital potentiometer except that it has an additional but variable resistance in series with the wiper
27. ly sees an infinite impedance and the effect due to Ro may be safely ignored Introduction RV 722 pat o DZ s as Tate 22002 C iai im uer C v p 100k MODEL OUTPUT RESISTANCE MODEL OUTPUT RESISTANCE DIAL acne S Figure 1 4 Output Resistance as a Function of Dial Setting Introduction 3 RV 722 1 2 2 Theory of Operation The actual circuit diagram of the RV 722 is shown in Figure 1 5 This circuit is capable of dividing the input into 10 parts i e 0 1 ppm It consists of seven decades each of which divides its input voltage into 10 equal parts The implementation of this division may be seen as follows The input voltage across each decade 15 di vided by 10 equal resistances Placing the resistance ofthe succeeding interpolating decade in parallel with a portion of the upstream decade reduces the effec tive resistance of that portion In particular examine the figure and note that first decade has 11 not 10 resistors The divider wipers from the second decade encompass two resistors totaling 20 This 20 is shunted by 20 the effective total resistance of the interpolating second decade it is convenient to think ofthe second decade as with all other decades as 10 resistors of 10 x 2 or 20 with all the shunting in parallel with it resulting in
28. may be seen from the schematic of a Kelvin Varley voltage divider shown in Figure 1 5 the linearity de pends primarily on the equivalence of the resistance steps Other sources of error are leakage and switch contact resistance stability The user has the ability to measure the leakage of the unit and take action if required The switch contact resistance 15 known to remain high for decades The new IET RV 722 allows user adjustment of the re sistance values of the first three decades Adjustable trimmers in the first three decades allow setting the various resistances with respect to each other or to some resistance standard This can compensate for the effects of aging and temperature and makes it possible to adjust the unit to initial specifications whenever desired If an accredited calibration is required the unit may be returned to IET labs 5 2 Considerations and Overview of Adjustment Procedure of Resistors The goal of this procedure is to adjust the value of the resistors in the first three decades to be e matched to each other within each decade e linear with each other from decade to decade accurate to their absolute value Clearly the third requirement if met completely would be sufficient but it is important to understand that the matching of the individual values becomes important since the true absolute value cannot be practically achieved Decade Adjustment 5 2 1 Calibration Switch In order to be able
29. nected to the 1 0 INPUT Secure connections should be made to the terminals using spade lugs banana plugs or bare wire Caution 1 RV 722 KELVIN VARLEY VOLTAGE DIVIDER INPUT 1 700 V qu should be exercised since most often such connectors are usually made of brass which can add thermal emf Copper or copper alloy connections should be used or bare wire which is tinned copper may be employed Similar connections are made to the OUTPUT terminals N CAUTION N Do not handle Kel F washers with bare hands to maintain low leakage Implement any current limiting protection as de scribed in the power considerations discussion of section 4 3 The dials may be set and read directly on the front panel Some decades have blank positions to be used only in the calibration mode The least significant decade has a 10 setting which can be used to provide an output of 1 0 i e 100 of the input End errors should be considered in any application Either the specified values should be used or the end errors may measured as described in the Verification Calibration and Maintenance section below 10 10 107 OUTPUT INPUT RESISTANCE 100 000 Q Figure 4 5 RV 722 Front Panel Operation 4 8 Use Applications and Calibration of the RV 722 There is a broad spectrum of applications for a Kelvin Varley divider such as calibrating other voltage di viders measuring unkn
30. own voltages and measuring unknown resistances The best source of information on the use applica tions and calibration of the RV 722 are found in the following references Calibration of a Kelvin Varley Standard Divider M L Morgan and J C Riley Kelvin Varley Resistive Voltage Divider esi Instruction Manual DC and Low Frequency AC Ratio Measurements Engineering Bulletin No 29 Calibration of a Kelvin Varley Voltage Divider Engineering Bulletin No 24 A Resistance Bridge Made From a Voltage Divider Engineering Bulletin No 38 these references are available on IET s website Consult IET Labs for assistance and additional information Operation RV 722 13 RV 722 14 Chapter 5 DECADE ADJUSTMENT This chapter describes an additional option of the IET RV 722 to adjust the resistors in order to bring them into initial specifications This option was not available in the esi RV 722 Therefore this process is OPTIONAL 5 1 Introduction to decade adjustment Itis important to make a distinction between calibra tion and adjustment Calibration is the official sometimes accredited measurement of a value or a parameter as commonly used calibration is incor rectly applied to the setting of a parameter e g with a potentiometer Adjustment is the proper term for setting This chapter will deal with the adjustments required to bring the RV 722 into specifications As
31. ry to remove the stick on feet on the bottom ofthe unit it they interfere mechanically with placement For bench models no installation as such is required because this instrument series is not powered Since the RV 722 is a high accuracy instrument it is recom mended that a bench space be provided that would not expose it to abuse and keep it protected from temperature extremes and contaminants 3 3 Repackaging for Shipment Ifthe instrument is to be returned to IET Labs contact the Service Department at the number or address shown on the front cover of this manual to obtain a Returned Material Authorization RMA number and any special shipping instructions or assistance Proceed as follows 1 Attachatag to the instrument identifying the owner and indicate the service or repair to be accomplished Include the model number the full serial number of the instrument the RMA number and shipping address 2 Wrap the instrument in heavy paper or plastic 3 Protect the front panel and any other protru sions with cardboard or foam padding 4 Place instrument in original container or equally substantial heavy carton 5 Use at least 3 inches of packing material around all sides of instrument 6 Seal box with strong tape or strapping 7 Mark shipping container DELICATE INSTRUMENT FRAGILE etc 3 4 Storage If this instrument is to be stored for a lengthy period of time it should be sealed in plastic and s
32. t mode Resistance bridges such as the MI 6000 Series the Guildline 6622A Series The IET formerly esi Model 242 along with resis tance standards bridge measurement system such 85 de scribed in the performance verification of this manual used in a resistance measurement mode along with an IET Labs formerly esi Transfer Standard Series 1010 The significant advantage ofthe procedure here is that it is self consistent in the sense that the first resistor is measured and the second and all subsequent resistors are set to match the first This produces the required linearity performance without having to attempt to set all the resistors to an absolute perfect value Figure 5 1 Internal Rear Panel of RV 722 Showing Calibrate Operate Switch Decade Adjustment 15 RV 722 16 5 3 Resistor Adjustment Procedure Before starting the adjustment procedure allow the RV 722 unit to stabilize at 23 C for at least 24 hours To set up the unit for adjustments do the following 1 2 Decade 107 Decade 10 Decade 10 Review section 5 2 2 Set up the necessary measurement equipment Unscrew the screws on the top rear and bot tom ofthe unit and remove the housing Setthe CALIBRATE OPERATE switch to CALIBRATE Setting Requirements Measure across Accuracy 4 ppm Resolution 0 1 ppm Repeatability 2 ppm Matching 1 ppm 5 3 1 Adjusting the first three decades There are 11 resistors in
33. tored in a dry location It should not be subjected to temperature extremes Extended exposure to such temperatures can result in an irreversible change in resistance and require recalibration Installation RV 722 Chapter 4 OPERATION 4 1 Initial Inspection and Setup This instrument was carefully inspected before ship ment It should be in proper electrical and mechanical order upon receipt An OPERATING GUIDE like the typical one shown in Figure 2 1 is attached to the case ofthe instrument to provide ready reference to specifications 4 2 General Considerations for Use of a Voltage Divider A Kelvin Varley voltage divider is designed for use in a null balance circuits or a voltage source for high impedance circuits See appendix It cannot be used as simple variable resistor because of the additional resistance in series with the tap see Figure 1 2 The resistance ratios between the taps of a Kelvin Varley divider are not linearly related to the voltage ratios Although many new high performance DMM s Digital Multimeters claim to have a divider capabil ity that capability is limited to one or two decades and a Kelvin Varley voltage divider such as the RV 722 remains the choice for all precision divider applications requiring a fraction of a ppm Operation 4 3 Power Limitations N CAUTION N Excessive current can damage the instrument To avoid damaging the RV 722 it is necessary to take certain pre
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35. y devia tion When a load 15 applied the output becomes Vour SV DEV R R where R is the output resistance which may be de termined by shorting the input leads and measuring across the output leads or can be approximated by using Figure 1 4 R ranges from zero to about 66 R is the load resistance applied at the output R our pee Figure 4 2 Th venin Equivalent Circuit of a Kelvin Varley Voltage Divider to Show Loading Effects It may be seen that it is the relative size of the term to the linearity deviation that determines the Importance ofthe loading error In particular for the RV 722 R must be greater than 1 TO i e 10 for the loading effect to be lt 03 ppm for the maximum It will be smaller for lower output resistances at other settings A bridge circuit under balanced conditions will effec tively draw no current from the RV 722 and approach a true open circuit Whena load is placed across the output ofthe RV 722 the output voltage will change This change in output voltage may or may not have to be considered in the measurement being made depending on the degree of accuracy desired With high impedance loads the effect of the load may be less than the effect of the linearity deviation In this case the change in output voltage due to output loading can be neglected The RV 722 can be represented by an equivalent generator and output resistor in series as shown in Figure 4 2 Wit
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