TP01 Manual version 1509
Contents
1. suitable media The volumetric heat capacity C is a derived measurement C a a Non specified measurements Use in fluids pastes and gels and use for determining thermal diffusivity is possible but not specified Use in media of lower than normal 4 is possible after correcting for non linear behaviour see appendices CE requirements TPO1 complies with CE directives MEASUREMENT SPECIFICATIONS Expected accuracy 5 for thermal conductivity measurements of suitable media 20 for thermal diffusivity measurements of suitable media Resolution of Cy 10 Temperature dependence 0 15 K 41 measurement only Heating power 0 05 W typically during 180 s When switched on every 3 hours from a 12 VDC supply using a 150 Ohm serial resistor the actual power cunsption will be 0 9 Watt The average power consumption will then be 0 015 Watt Heating power m 0 8 W m nominal 1V 20 ohm 60 mm Table 3 1 List of TPO1 specifications continued on the next page TPO1 Manual version 1509 page 16 45 Hukseflux Thermal Sensors SENSOR SPECIFICATIONS Thermocouples 40 Cu CuNi E 0 15 mV K nominal value 63 response time 19 s nominal value in agar gel reference Temperature range 30 to 80 C Required readout 2 diff voltage channels Expected voltage 1 to 1 mV sensor O to 1 V heater output Voltage in2. List of symbols Thermal diffusivity Distance from the heater Heating cycle time Heating power per meter Intermediate variable Thermal conductivity Voltage output Sensitivity of the thermopile for temp gradients Sensitivity for the thermal conductivity Effective sensitivity for thermal conductivity Time Response time Temperature Differential temperature Electrical resistance Effective length of the heater Electrical resistance per meter Volumetric heat capacity Heat capacity Density Water content on mass basis Water content on volume basis Heat flux Storage term Depth below the soil surface Subscripts Property of thermopile sensor Properties of the heater Property of the reference medium Property of the hot joints Property of the cold joints Property att 0 at t 180 seconds Property of dry soil Property of water Measurement done at the 63 response time Measurement done by a heat flux sensor TPO1 Manual version 1509 W mK V Er V K A V K E V KQm t S T S T K AT K Re Q L m Rem Q m C J K m C J kg p kg m 8m kg kg Oy m m D W m S W m d m sen heat ref h C 0 180 d w 63 heat flux page 4 45 Hukseflux Thermal Sensors Introduction The TPO1 is a sensor for the long term monitoring of soil thermal conductivity thermal diffusivity and heat capacity Because of its unigue sensor principle it is extremely fast and has easy signal interpretation It is based
3. g 4 nr r Jaat q U Up E dg 11 8 4 Use of Formula 1 4 reguires the assumption that the thermal mass and the conductivity of the sensor are guite low In this situation only the parameters of the medium play a role The validity of this assumption is treated in the appendix on the use in media with low thermal conductivity The conclusion is that TPO1 can be used in the thermal conductivity range from 0 3 to 5 W mK Uo s deviation from zero is caused by a variety of factors temperature gradients in the medium and offset of the electronics are the most common ones The assumption is that these offsets do not vary during the experiment Also it is assumed that the medium properties do not change This is the reason why the heater power must be low In case of high power especially in moist media local moisture transport might take place With the TPO1 typical heater power of 0 8 W m the temperature rise will not be higher than 1 degree during a typical measurement of 3 minutes This results in negligible moisture transport TPO1 Manual version 1509 page 37 45 Hukseflux Thermal Sensors Finally there is the implicit assumption that the sensor does not move during the measurement and that the sensor dimensions are stable For large t the integral in formula 1 2 approaches a constant value pie faa q 5 amp 3 for 4at r2 gt gt 1 11 8 5 Bon q Th Thus U Uo ES in F at 11 8 6 211 J th where F at is the normal
4. diffusivity the signal fall after the calculations are thermal conductivity highly complicated measurement This operation is very simple and calculation is robust It should be noted that the resolution of this measurement is much better than the absolute accuracy Table 11 9 1 comparison of TPO1 to conventional techniques started on the previous page TPO1 Manual version 1509 page 44 45 Hukseflux Thermal Sensors 11 9 CE declaration of conformity According to EC guidelines 89 336 EEC 73 23 EEC and 93 68 EEC We Hukseflux Thermal Sensors Declare that the product TPO1 Is in conformity with the following standards Emissions Radiated EN 55022 1987 Class A Conducted EN 55022 1987 Class B Immunity ESD IEC 801 2 1984 8kV air discharge RF IEC 808 3 1984 3 V m 27 500 MHz EFT IEC 801 4 1988 1 kV mains 500V other Delft January 2006 TPO1 Manual version 1509 page 45 45
5. in humid materials particularly in soils Connection Sometimes no fixed Heater and sensor fixed in heater to distance one foil This improves sensor sensor stability Required Typically more than Typically 0 8 W m heater power 0 8 W m Thermal mass of the sensor important for Large because of the use of metal Negligible low mass plastic foils are used so that an accurate measurement of thermal diffusivity and heat capacity diffusivity can be made Sensitivity for Requires a stable The two thermopiles have temperature situation an opposite directional gradients sensitivity so that there is changes in the no sensitivity to thermal medium gradients in the medium This avoids measurement errors Table 11 9 1 comparison of TPO1 to conventional techniques continued on the next page TPO1 Manual version 1509 page 43 45 Hukseflux Thermal Sensors Thermal Curve fitting or Determination of two conductivity determination of voltage levels division by analysis the the calibration factor This d In U dt time operation is very simple and derivative of the calculation is robust natural logarithm of the sensor output for large t Thermal Complicated or not The sensor thermal mass is diffusivity possible also so low that it can be left out analysis depending on the of the equation sensor thermal Determining the 63 mass Thermal response time by looking at
6. in humid materials In humid materials it is recommended that the heater power remains low to avoid local transport of moisture by evaporation The 0 8 W m of TPO1 is the recommended maximum The signal U is generated by the thermopile sensor from the differential temperature around the heating wire This can be seen in figure 1 1 It gives a top view of the sensor and the surrounding medium when heating The heating wire generates a circular temperature field After some minutes the temperature difference around the sensor becomes stable Figure 11 8 1 Top view of the radial temperature distribution around the heating wire of TPO1 in two different media The thermopiles measure the difference between the temperature at Fp and rc TPO1 Manual version 1509 page 36 45 Hukseflux Thermal Sensors The temperature field around a heating wire that is switched on at t O and thereafter provides constant heat input is 00 Qa f ee 4nh Ja q T dq 11 8 1 The TPO1 sensor uses the differential temperature AT at radii rc and rp which is expressed r dat q en E 4 mA r Jaat q dq 11 8 2 A thermopile is used in the TPO1 sensor to measure AT Thermopile output U varies linearly with AT according to the relation USE AT 4 11 8 3 where Up is the sensor output at t 0 and Er is the thermopile sensitivity to thermal gradients Thus we obtain the relation Q pelet 6 3
7. systems Additionally the TPO1 measurement is quite useful to create some redundancy for the soil moisture measurement that is often done in such systems From 2 3 it can easily be seen that there is a direct relationship between the soil moisture and the volumetric heat capacity Om Cy pa Ca Cw 2 4 The latter formula gives water content on a mass basis For estimates on a volume basis one has to multiply by pp and divide by pw o Cy Ca pa pw Cw 2 5 As the properties of water are quite well known an error in will stem from errors in Cy and in pa TPO1 Manual version 1509 page 15 45 Hukseflux Thermal Sensors 3 Specifications of TPO1 TPO1 Thermal properties sensor is intended for the long term monitoring of soil thermal conductivity thermal diffusivity and heat capacity It can only be used in combination with a suitable measurement and control system GENERAL SPECIFICATIONS Suitable Media Soils in the thermal diffusivity range a of 0 05 to 1 10 m s and the thermal conductivity A range of 0 3 to 5 W mK Temperature range 30 to 80 C Required heating cycle time H aH must be of the order of magnitude of 25 10 m or larger typ 180 seconds Required depth of Preferably the soil must be all around insertion TPO1 The foil must be inserted for at least 45 mm Specified Thermal conductivity and thermal measurements diffusivity in the soils as specified under
8. to work with a calibration curve See the appendix on use of TPO1 out of normal specifications The recommended approach for using TPO1 as a sensor for thermal diffusivity and volumetric heat capacity is to measure the signal amplitude Uo Uig9 and to establish how much time it takes after switching the heater off to return to Uo 0 37 Uo Uigo this is equivalent to the 63 response time t 63 The reference thermal diffusivity has been established in water In this case arer is 0 14 10 Comparison to U to Uo Uigo Timing T 63 a a ref T ref 63 T 63 11 8 12 The calibration for thermal diffusivity is not done for each individual sensor it largely depends on the sensor dimensions The value can be found on the calibration certificate It is the same for every sensor Volumetric heat capacity is determined by Cy A a 11 8 13 It should be noted that the absolute accuracy of the measurement of the volumetric heat capacity is not very high The power of TPO1 is in detecting changes The smallest meaningfully detectable change we call resolution TPO1 Manual version 1509 page 42 45 Hukseflux Thermal Sensors 11 8 Comparison of TPO1 to conventional probes Conventional TPO1 probes Sensitivity of Typically 0 05 Typically 0 003 degree differential degree depending depending on readout temperature on readout One can work with low measurement power which is essential
9. TPO1 Thermal Properties Sensor USER MANUAL INCLUDING THERMAL DIFFUSIVITY AND VOLUMETRIC HEAT CAPACITY MEASUREMENT TPO1 Manual version 1509 Edited amp Copyright by Hukseflux Thermal Sensors http www hukseflux com e mail info hukseflux com Hukseflux Thermal Sensors Hukseflux Thermal Sensors Warning Putting more than 2 volts across the heater of TPO1 may result in permanent damage to the sensor In case of supply from a 12VDC source typically a 150 Ohm resistor must be put in series with the TPO1 heater This is the user s responsibility TPO1 Manual version 1509 page 2 45 Hukseflux Thermal Sensors Contents List of symbols 4 Introduction 5 1 Theory 8 2 Theory for meteorological applications 14 3 Specifications of TPO1 16 4 Short user guide 18 5 Putting TPO1 into operation 19 6 Installation of TPO1 21 7 Maintenance of TPO1 22 8 Reguirements for data acguisition and control 23 9 Electrical connection of TPO1 24 10 Programming for TPO1 25 11 Appendices 28 11 1 Appendix on calibration of TPO1 28 11 2 Appendix on agar gel 28 11 3 Appendix on cable extension for TPO1 29 11 4 Appendix typical thermal properties 29 11 5 Appendix trouble shooting 30 11 6 Appendix on use in media of low thermal conductivity 32 11 7 Theory of TPO1 extended 35 11 8 Comparison of TPO1 to conventional probes 43 11 9 CE declaration of conformity 45 TPO1 Manual version 1509 page 3 45 Hukseflux Thermal Sensors
10. ach 5 meters Infinite indicates a broken circuit zero indicates a short circuit Check the heater impedance Use a multimeter at the 100 ohms range Measure between two wires that are connected at opposite ends of the heater Subtract the resistance value that was measured during the previous measurement What is left is the heater resistance This should be between 10 and 20 ohms Infinite indicates a broken circuit zero indicates a short circuit Check the impedance of the sensor Use a multimeter at the 100 ohms range Measure at the sensor output Subtract the resistance value of the wiring that was measured during the first measurement What is left is the sensor resistance Warning during this part of the test please put the sensor ina thermally quiet surrounding holding the sensor foil in still air A typical sensor impedance should be between 20 and 50 ohms Infinite indicates a broken circuit zero indicates a short circuit Table 5 1 Checking the functionality of the sensor The procedure offers a simple test to get a better feeling how TPO1 works and a check if the sensor is OK continued on the next page TPO1 Manual version 1509 page 19 45 Hukseflux Thermal Sensors Check if the sensor reacts to differential temperatures Use a multimeter at the millivolt range Measure at the sensor output Generate a signal by touching the thermopile cold joints at rc with your h
11. acity d the depth of installation of the soil heat flux sensors ti t the length of the measurement interval in seconds TPO1 Manual version 1509 page 14 45 Hukseflux Thermal Sensors At an installation depth of 6 cm the storage term typically represents up to 50 of the total flux amp When the temperature is measured closely below the surface the response time of the storage term measurement to a changing amp is in the order of magnitude of 20 minutes while the heat flux sensor amp heatftux buried at twice the depth is a factor 4 slower square of the depth This implies that a correct measurement of the storage term is essential to a correct measurement of with a high time resolution At present the volumetric heat capacity Cy is estimated from the heat capacity of dry soil Ca the bulk density of the dry soil pa the water content on mass basis Om and Cy the heat capacity of water Cy pa Ca Om Cw 2 3 The heat capacity of water is known but the other parameters of the eguation are much more difficult to determine and are dependent on location and time For determining bulk density and heat capacity one has to take local samples and to perform careful analysis The soil moisture content measurement is difficult and suffers from various errors TPO1 gets around these problems by performing a direct measurement This is a big advantage as such and sufficient reason for application in Bowen Ratio
12. and or by bringing the thermopile outer joints into contact with a hot object like a mug filled with hot coffee or tea If a 1 Volt battery or power source is available it is also possible to connect the power source to the heater and see if a signal is generated The thermopile should react by generating a millivolt output signal Table 5 1 Checking the functionality of the sensor The procedure offers a simple test to get a better feeling how TPO1 works and a check if the sensor is OK started on the previous page The TPO1 should be connected to the measurement and control system as described in the chapter 9 on the electrical connection The programming of data loggers is the responsibility of the user Please contact the supplier to see if directions for use with your system are available Programs are available for Campbell Scientific CR10X and CR1000 TPO1 Manual version 1509 page 20 45 Hukseflux Thermal Sensors 6 Installation of TPO1 TPO1 is generally installed at the location where one wants to measure The more the foil of TPO1 is in contact with the soil the better In order to meet its specifications the sensor must be inserted into the medium for at least 45 mm The sensor should not move during the measurement Usually it is sufficient to prepare the path for inserting the sensor foil into the soil simply by temporarily inserting a knife When the knife is taken out the p
13. ath has much less resistance to insertion of the sensor than the same medium in its original state Repeated insertion of TPO1 into various soils is possible but in general not recommended Needle type sensors like TPO2 and TPO8 are better suited to that purpose If necessary it should be done with care The sensor is fairly robust However it is not sufficiently rigid to be inserted into e g sand without preparing the sample In meteorological applications permanent installation is preferred The sensor orientation should be such that the flow of water through the soil is not obstructed This implies that the heating wire will usually be lying horizontally and the foil will be perpendicular to the soil surface It is recommended to fix the location of the sensor by attaching a metal pin to the cable Attachment of the pin to the cable can be done using a tie wrap Table 6 1 General recommendations for installation of TPO1 TPO1 Manual version 1509 page 21 45 Hukseflux Thermal Sensors 7 Maintenance of TPO1 Once installed TPO1 is essentially maintenance free Usually errors in functionality will appear as unreasonably large or small measured values As a general rule this means that a critical review of the measured data is the best form of maintenance It is advisable to check the quality of the cables at regular intervals It is advisable to check the calibration once every 2 years This is ty
14. cations for cable extension of TPO1 11 4 Appendix typical thermal properties Thermal conductivity Thermal diffusivity 10 W mk m s Water Agar 0 60 0 14 gel Perfectly dry 0 17 sand Dry moist 0 30 0 26 sand Glycerol 0 27 0 09 Air 0 026 21 4 Wet sand 2 2 0 57 Table 11 4 1 Table giving the typical values of the thermal conductivity and the thermal diffusivity of some materials at 20 C TPO1 Manual version 1509 page 29 45 Hukseflux Thermal Sensors 11 5 Appendix trouble shooting This paragraph contains information that can be used to make a diagnosis whenever the sensor does not function It is recommended to start any kind of trouble shooting with a simple check of the sensor and heater impedance and a check to see if the thermopile gives a signal First check the sensor according to table 5 1 This table offers a simple test to see if the connections are OK and if the sensor is still functioning If this check does not produce any outcome proceed to the next table No signal from Check the sensor impedance as in table 4 1 the sensor This can also be done while the sensor is still in place Check the data acquisition system by applying an artificially generated voltage to the input Preferably a millivolt generator is used for this purpose Check the heater connection Check the heater impedance Check the functionality of the heater by putting it on When it i
15. curve See the appendix on use of TPO1 out of normal specifications TPO1 Manual version 1509 page 12 45 Hukseflux Thermal Sensors The recommended approach for using TPO1 as a sensor for thermal diffusivity and volumetric heat capacity is to measure the signal amplitude Uo Uigo and to establish how much time it takes after switching the heater off to return to Ust 0 37 Uo Uigo this is equivalent to the 63 response time 1 63 The reference thermal diffusivity has been established in water In this case arer is 0 14 10 Comparison to U to Uo Uigo Timing T 63 a a ref T ref 63 T 63 1 5 The calibration for thermal diffusivity is not done for each individual sensor it largely depends on the sensor dimensions The value can be found on the calibration certificate It is the same for every sensor Volumetric heat capacity is determined by Cy ArA a 1 6 It should be noted that the absolute accuracy of the measurement of the volumetric heat capacity is not very high The power of TPO1 is in detecting changes The smallest meaningfully detectable change we call resolution TPO1 Manual version 1509 page 13 45 Hukseflux Thermal Sensors 2 Theory for meteorological applications There are several motives for using TPO1 In meteorology the main measurement objectives typically are 1 using TPO1 it is no longer necessary to separately take soil samples at every measurement locati
16. ductivity sensor is valid after typically 3 minutes 180 seconds of heating when F eguals 1 A E Q Vio Uo 1 2 or A E Uneat Uigo Uo Re L 1 3 Re L and E are parameters that are given as part of the delivery Lis 0 06 m E Re and L can be combined to one factor so that one obtains A Eer Uneat Uigo Uo 1 4 with Eeff E Re L 1 5 All one has to know is the sensor output voltage before and after heating for about 3 minutes and the heater power The heater power is typically calculated from the known heater resistance also delivered with the sensor and the voltage across the heater NOTE in case current measurements are available the above calculations may be re written using Q I Re L The sensor is calibrated for measurements of See the appendix on calibration Although formal calibration traceability is to international standards of NPL in practice the reference medium is agar gel see the appendix on agar gel The calibration is valid for the range of 0 3 to 5 W mK In this range of thermal conductivity s the properties of the medium dominate over the sensor properties When the thermal conductivity of the medium is lower the thermal conductance of the sensor foil itself particularly the copper starts dominating This results in a change of sensitivity or non linear behaviour It is possible to work in a wider range but one will have to work with a calibration
17. e 2 Both heating wire and sensor are incorporated in a very thin plastic foil The low thermal mass makes it suitable for estimating thermal diffusivity a Dividing A by the thermal diffusivity a gives the volumetric heat capacity C which varies with water content The thermopile signal minus the initial offset U Uo when heating with power Q depends on A and a of the medium U Uo CE Q A Flat E is a calibration constant t is time F is a function that equals 1 for large at By looking at the steady state signal amplitude A can be determined C and a can be found by looking at the 63 response time for F The detection of changes in C and water content is the strong point of TPO1 the resolution is much better than the accuracy The product manual can be obtained via e mail Programs for use with the Campbell Scientific CR10X and CR1000 are available Hukseflux has a broad product range of sensors for thermal conductivity measurement please consult the product catalogue TPO1 is a design is completely new Development of this measurement method was done at Hukseflux TPO1 Manual version 1509 page 6 45 Hukseflux Thermal Sensors V 1 t va Figure 0 1 above TPO1 sensor thermopiles 1 heating wire 2 cable 3 Dimensions in mm Below graphs in different soil types signal amplitude varies with 1 A signal response time varies with a All dimensions are in mm The general
18. e suitable for fluids with a low coefficient of thermal expansion and a high viscosity TPO1 Manual version 1509 page 34 45 Hukseflux Thermal Sensors 11 7 Theory of TPO1 extended As indicated in the introduction the TPO1 design is a modification of the well known non steady state probe This technigue utilizes the temperature measurement around a heating wire to analyze soil properties All non steady state probes are based on the same phenomenon that one can determine the thermal properties of a medium from the temperature response to heating After an initial transition period the temperature rise close to the heater depends only on the thermal conductivity of the surrounding medium and no longer on heat capacity Generally this method avoids the necessity to reach a true thermal eguilibrium with constant temperatures Non steady state technigues are fast and also there is no need for careful sample preparation Sensors based on this principle are therefore suitable for guick experiments and also for field use Provided that the thermal mass of the sensor itself is small during the initial transition period the time response of this type of probe is proportional to the thermal diffusivity of the surrounding medium Some conventional sensor designs have a temperature measurement at a large distance from the heater typically some centimetres away sometimes using a physically separated heater and probe Other des
19. ero In practice it will have a value different from zero typically between 20 and 20 microvolts Store Uo t 0 seconds Switch heater on Att 0 the zero reading is taken After this the heater is switched on Table 10 1 Typical ingredients of a program for measurement and control of TPO1 continued on the next page TPO1 Manual version 1509 page 26 45 Hukseflux Thermal Sensors Measure the heater It is also possible to use a voltage Uheat current measurement Calculate the heater power per meter Q Measure the steady state thermopile output U t 180 seconds typically Store Uigo Switch off the heater Start the timer Calculate the tension to compare to Uigo0 0 63 Uo U180 If U is smaller than the above mentioned level take the timer reading T 63 Store Uo at t 360 seconds typically Determine the This is to compensate for average Uo drifting zero offset Calculate the thermal conductivity of the medium i a and Cy Validate the For example if one knows measurement between which limits the thermal properties can vary for the medium under evaluation Repeat either on user demand or on a regular time schedule Table 10 1 Typical ingredients of a program for measurement and control of TPO1 started on the previous page TPO1 Manual version 1509 page 27 45 Hukseflux Thermal Sensors 11 Appendic
20. es 11 1 Appendix on calibration of TPO1 Although formal calibration traceability is to international standards of NPL in practice the reference medium is agar gel Calibration of TPO1 can be done in any laboratory that has the necessary electronic eguipment The reguirements for power supply and readout can be found in the chapter on reguirements The procedure for calibration is as follows There should be perfect contact between the foil of the sensor and the gel One can perform a calibration by doing a normal measurement in agar gel Knowing the thermal properties of the gel E can be calculated and 1 63 can be timed Calibration of the heater is generally not considered to be necessary The resistance generally is very stable If necessary it can be done using a simple current meter and a voltage source The calibration constant for the heater is expressed in Ohms per meter The heater length effective is a constant and can be found in the specifications 11 2 Appendix on agar gel The procedure for calibration relies on the use of agar gel This is a water based gel of which the ingredients can be bought in every pharmacy In most countries agar is also available in food stores that sell environmentally friendly foods The agar gel is often used for growing bacteria The agar itself does not significantly influence the thermal properties of water but reduces the effects of convection The properties of agar gel cl
21. he medium gets too low this is no longer valid A significant portion of the heat will be conducted by the thermopile itself from the hot to the cold joints This implies that the signal level gets too low and the thermal conductivity is overestimated This kind of behaviour can easily be demonstrated by performing a measurement in air At Hukseflux laboratories one also has been testing well characterized glass pearls with a thermal conductivity of 0 19 W mK Our experiments have shown that for general users it cannot be recommended to use TPO1 outside its specified measurement range because the method becomes quite sensitive to errors TPO1 Manual version 1509 page 32 45 Hukseflux Thermal Sensors dry moist agar saturated air sand sand gel sand theoretical thermal conductivity W mK 0 02 0 19 0 30 0 61 2 22 measured thermal conductivity W mK 0 16 0 29 0 32 0 61 2 11 deviation from ideal 689 53 8 0 5 Table 11 6 1 The variation of the measurement result measured thermal conductivity when measuring in media of different thermal conductivity s The TPO1 is specified in the range from 0 3 to 5 W mK Here E is fairly constant When measuring at lower thermal conductivity the behaviour becomes non linear When using TPO1 in this region one will have to calibrate specifically for this and one will have to work with a modified algorithm for data interpre
22. igns measure the temperature rise of the heater itself TPO1 uses a new technigue which depends heavily on a very sensitive temperature gradient sensor A differential temperature sensor 2 thermopiles measures the radial differential temperature around the central heating wire with record breaking sensitivity This technigue is easier to employ than conventional technigues because the interpretation of the signals is very easy The principle of measurement is clarified in figures 0 1 1 1 and 1 2 A thermopile essentially is a number of thermocouples in series A thermocouple delivers an output signal that is proportional to the differential temperature between the hot joints and the cold joints Multiple thermocouples in series a thermopile will produce a proportionally larger signal In case of TPO1 the hot joints are located near the heating wire at 1 mm distance rp and the cold joints are located far away from the heater at about 5mm rc There are two rows of each 20 thermocouples copper constantan which results in a signal of about 1 6 mV TPO1 Manual version 1509 page 35 45 Hukseflux Thermal Sensors when the medium at 1 mm from the heater differs 1 degree Celsius from the medium at 5 mm from the heater This sensitivity is not equalled by any other sensor that is known to us It opens the possibility to reduce the sensor dimensions considerably and to use low heater power which is essential for accurate measurements
23. ing F one can find a One of the easiest ways of doing this is looking at the 63 response time of the measurement The common procedure is to first determine the thermal conductivity and after the heating cycle to determine how much time it takes to fall by 63 of the amplitude towards the original signal level When applying TPO1 as a thermal diffusivity sensor an essential feature is that both the heater and the thermopile are incorporated in a very thin plastic sheet This implies that the sensor itself does not play a significant role from a thermal point of view The low thermal mass makes it suitable for estimating thermal diffusivity a without correcting for the sensor mass contrary to some steel needle probes that have a relatively high mass TPO1 Manual version 1509 page 39 45 Hukseflux Thermal Sensors dry sand 0 3 agar gel 0 6 wet sand 2 4 mV at 0 11W power 0 200 400 600 800 1000 time s Figure 11 8 2 Examples of pulse responses in different media The graph shows the original pulse responses in various media The thermal conductivity of the various media is indicated in W mK TPO1 Manual version 1509 page 40 45 Hukseflux Thermal Sensors 0 6 T E 0 5 O EENT dry sand 320s agar ge S wet sand 2 v 5 0 2 5 2 2 ox o 0 T T 1 500 1000 time scaled with thermal diffusivity Figure 11 8 3 The previous graph is now twice scaled for am
24. ions based on this Table 8 1 Requirements for data acquisition and control TPO1 Manual version 1509 page 23 45 Hukseflux Thermal Sensors 9 Electrical connection of TPO1 In order to operate TPO1 should be connected to a measurement and control system as described in the previous chapter A typical connection is shown in figure 9 1 For the purpose of making a correct measurement of the heater power Q there is a 4 wire connection to the heater Two wires carry the current the others are used for the measurement Through these wires there is a negligible current so that there is no voltage drop across the wires and the true voltage across the heater wire is measured The voltage should be of the order of magnitude of 1 to 2 Volt Warning putting more than 2 volt across the heater may result in permanent damage to the sensor In case of supply from a 12VDC source typically a 150 to 200 Ohm resistor must be put in series with the TPO1 heater This is the user s responsibility e 5 e 4 e 3 gt 2 0 L e 1 Figure 9 1 Typical connection of TPO1 The relay serves to switch the heater on and off The thermopile output is connected to a differential voltage measurement The voltage across the heater is also measured by a differential voltage channel It might be that a measurement system already has a current channel In this case it is not necessary
25. ized integral that approaches a value of 1 for large t r aat er Fs F at dq ank 11 8 7 r J4at q Th It should be noted that while AT approaches a constant value for large t the absolute temperature is still rising with In 4at r Formula 1 6 suggests that we can define a new constant E which depends only on sensor geometry and thermopile sensitivity E Er 1 27 In rd th 11 8 8 Sensitivity and geometry vary from sensor to sensor Thus E is an individual sensor property that must be determined by sensor calibration The result is a sensor which reacts to a heating pulse in the following way U Uo E Q 2 Flat 11 8 9 TPO1 Manual version 1509 page 38 45 Hukseflux Thermal Sensors E isa calibration constant expressing the sensor sensitivity to thermal conductivity of the medium tis time F is a function that eguals 1 for large t By looking at the steady state signal amplitude can be determined The function F describes the speed at which the process takes place This process scales with the thermal diffusivity of the medium As the thermopile only extends to 6 10 m from the heater and the worst case value for thermal diffusivity is 0 1 10 m s the stabilisation of the differential temperature typically takes 180 seconds Formula 11 8 9 shows that the pulse response of the sensor signal scales with Q 4 for the amplitude and with a for the time response By curve fitt
26. ly and to use low heater power which is essential for accurate measurements in humid soils In humid soils so not saturated and also not perfectly dry it is recommended that the heater power remains low to avoid local transport of moisture by evaporation The 0 8 W m of TPO1 is the recommended maximum TPO1 Manual version 1509 page 9 45 Hukseflux Thermal Sensors A voltage signal U is generated by the thermopile sensor driven by the differential temperature around the heating wire This can be seen in figure 1 1 It gives a top view of the sensor and the surrounding soil when heating The heating wire generates a circular temperature field After some minutes the temperature difference around the sensor becomes stable EX oy ff Pp Pa 1 2 3 4 5 Figure 1 1 Top view of the radial temperature distribution with isotherms 3 around the heating 2 wire of TPO1 1 in two different media right high thermal conductivity left low thermal conductivity The thermopiles measure the difference between the temperature at r 4 and r 5 TPO1 Manual version 1509 page 10 45 Hukseflux Thermal Sensors The result is a sensor which reacts to a heating pulse in the following way U Uo E Q 2 Flat 1 1 E isa calibration constant expressing the sensor sensitivity to thermal conductivity of the medium t is time F is a function that equals 1 for large t By looking at
27. measurement principle is to look at the sensor response when the heater is switched on At a certain heating power large signals indicate a soil with low thermal conductivity A The sensor response time is proportional to the soil thermal diffusivity a The division of 4 by a gives the volumetric heat capacity Cy TPO1 Manual version 1509 page 7 45 Hukseflux Thermal Sensors 1 Theory The following chapter gives a summary of the TPO1 theory A more extensive explanation can be found in an appendix of this manual As indicated in the introduction the TPO1 design is a modification of the well known non steady state probe This technigue utilizes the temperature measurement around a heating wire to analyze soil properties All non steady state probes are based on the same phenomenon that one can determine the thermal properties of a medium from the temperature response to heating After an initial transition period the temperature rise close to the heater depends only on the thermal conductivity of the surrounding medium and no longer on heat capacity This method avoids the necessity to reach a true thermal eguilibrium with constant temperatures Non steady state technigues are fast and also there is no need for careful sample preparation Sensors based on this principle are therefore suitable for guick experiments and also for field use Provided that the thermal mass of the sensor itself is small during the initial transiti
28. on a differential temperature sensor with record breaking sensitivity and extremely low thermal mass TPO1 is designed for long term permanent installation in soils It covers the thermal conductivity A range of 0 3 to 5 W mK which is sufficient for most inorganic soil types Main applications are found in soil physics agricultural meteorology soil energy balance monitoring Additional applications can be found for modelling of local conditions for oil pipelines and high voltage electrical cables TPO1 serves to estimate the so called storage term A typical TPO1 is incorporated in a meteorological system in which also wind humidity heat flux and radiation are measured Application of TPO1 takes away the necessity to separately analyse soil samples for their thermal properties It is no longer necessary to use the soil moisture measurement to determine the storage term On the other hand the TPO1 measurement can be used as a redundant measurement for the soil moisture the heat capacity is a linear function of the heat capacity During measurement the control and data storage are typically taken care of by a data logging and control system TPO1 Manual version 1509 page 5 45 Hukseflux Thermal Sensors The core of TPO1 is a differential temperature sensor 2 thermopiles 1 measuring the radial differential temperature with record breaking sensitivity The sensor performs a temperature measurement around a heating wir
29. on and analyze them for their thermal properties 2 using TPO1 it is no longer necessary to use the soil moisture measurement to analyze the storage term 3 the TPO1 measurement can be used as a redundant measurement for the soil moisture measurement In meteorological measurements the heat flux at the surface is usually measured using a heat flux plate This plate gives an output that is directly proportional to the heat flux through it For various practical and theoretical reasons the heat flux plate cannot be installed directly at the surface The main reason is that it would distort the flow of moisture and be no longer representative of the surrounding soil both from a moisture and from a thermal spectral point of view Also in case of installation close to the surface the sensor would be more vulnerable and the stability of the installation becomes an uncertain factor For these reasons the flux at the soil surface is estimated from the flux measured by the heat flux sensor heattlux plus the energy that is stored in the layer above it S P D heatfux S 2 1 The parameter S is called the storage term The storage term is calculated using an averaged soil temperature measurement combined with an estimate of the volumetric heat capacity of the volume above the sensor S T1 T2 Cv d ti t2 2 2 Where S is the storage term T1 T2 is the temperature change in the measurement interval Cy the volumetric heat cap
30. on period the time response of this type of probe is proportional to the thermal diffusivity of the surrounding medium TPO1 uses a new technigue which depends heavily on a very sensitive temperature gradient sensor A differential temperature sensor 2 thermopiles measures the radial differential temperature around the central heating wire with record breaking sensitivity This technigue is easier to employ than conventional technigues because the interpretation of the signals is very easy The principle of measurement is clarified in figures 0 1 1 1 and 1 2 TPO1 Manual version 1509 page 8 45 Hukseflux Thermal Sensors A thermopile essentially is a number of thermocouples in series A thermocouple delivers an output signal that is proportional to the differential temperature between the hot joints and the cold joints Multiple thermocouples in series a thermopile will produce a proportionally larger signal In case of TPO1 the hot joints are located near the heating wire at 1 mm distance ry and the cold joints are located far away from the heater at about 5 mm rc There are two rows of each 20 thermocouples copper constantan which results in a signal of about 1 6 mV when the medium at 1 mm from the heater differs 1 degree Celsius from the medium at 5 mm from the heater This sensitivity is not equalled by any other sensor that is known to us It opens the possibility to reduce the sensor dimensions considerab
31. osely resemble those of water Thermal conductivity 0 6 W mK at 20 degrees C Thermal diffusivity 0 14 10 m s Generally preparation of agar gel can be done by cooking about 4 grams of agar in 1 litre of water for about 20 minutes stirring regularly The solution can be put in a pot and be allowed to cool down and solidify This typically takes some hours Once at room temperature the TPO1 can be inserted into the agar gel TPO1 Manual version 1509 page 28 45 Hukseflux Thermal Sensors 11 3 Appendix on cable extension for TPO1 It is a general recommendation to keep the distance between data logger and sensor as short as possible Cables generally act as a source of distortion by picking up capacitive noise TPO1 cable can however be extended without any problem to 100 meters If done properly the sensor signal although small will not degrade because the sensor impedance is very low Also the 4 wire connection of the heater is immune to cable extension Cable and connection specifications are summarized below Cable 6 wire shielded copper core Core 0 1 Q m or lower resistance Outer preferred 5 mm diameter Outer sheet preferred polyurethane for good stability in outdoor applications Connection Either solder the new cable core and shield to the original sensor cable and make a waterproof connection or use gold plated waterproof connectors Table 11 3 1 Specifi
32. pically done using locally prepared agar gel For preparation please consult the appendix on agar gel TPO1 Manual version 1509 page 22 45 Hukseflux Thermal Sensors 8 Reguirements for data acguisition and control Capability to measure voltage signals 5 microvolt resolution or better for the thermopile signal typically using a 1 mV range Around 1 Volt range for the heating power For calculation of the heater power a current measurement can be used as an alternative to the voltage measurement Capability of switching 1 volt at 0 5 A this is for one sensor only worst case Capability of timing For thermal diffusivity only with a 1 second accuracy For determining the 63 response time at the moment heating stops a timer has to be started Requirements for power supply of the heater Capability to supply 1 Volt at 0 5 A In meteorological applications this is typically done for 3 minutes every 3 hours at 0 05 Watt The average required power across the day in this case is 0 0008 Watt If power is taken from a 12 VDC source a resistor of 150 ohms must be put in series In this case the power consumption is higher 0 9 Watt average 0 015 Watt Capability for the data logger or the software To store data to subtract and to perform the calculation of power Q and thermal conductivity A For thermal diffusivity Storing the data of the timer possibly do calculat
33. plitude and for time The curves have similar shapes indicating that the theory correctly predicts sensor behaviour Taking agar gel as a reference with 0 6 W mK and a 1 4 10 m s one can estimate thermal diffusivity values for other materials The central formula for using TPO1 as a thermal conductivity sensor is valid after typically 3 minutes 180 seconds of heating when F equals 1 x E Q Uigo Uo 11 8 10 or 4 E Uneat Uigo Uo Re L 11 8 11 The calibration factor E is delivered with the sensor All one has to know is the sensor output voltage before and after heating for about 3 minutes and the heater power The heater power is typically calculated from the known heater resistance also delivered with the sensor and the voltage across the heater TPO1 Manual version 1509 page 41 45 Hukseflux Thermal Sensors The sensor is calibrated for measurements of See the appendix on calibration The reference medium is agar gel see the appendix on agar gel The calibration is valid for the range of 0 3 to 5 W mK In this range of thermal conductivity s the properties of the medium dominate over the sensor properties When the thermal conductivity of the medium is lower the thermal conductance of the sensor foil itself particularly the copper starts dominating This results in a change of sensitivity or non linear behaviour It is possible to work in a wider range but one will have
34. polarity the power supply of the sensor has a leak to the millivolt measurement This is most probably caused by a broken cable Please check the cabling the connector at the sensor and the connection at the data acguisition Check if the data acguisition system has sufficient sensitivity This should be in the microvolt range Signal shows Check is there are no large currents in your unexpected system which can cause a ground loop If these variations are there switch them off and see if any of these is causing the disturbance Check the surroundings for large sources of electromagnetic radiation Radar installations microwave emitters etc Inspect the sensor itself The surface should be smooth and have no scratches Table 11 5 1 Extensive checklist for trouble shooting started on the previous page TPO1 Manual version 1509 page 31 45 Hukseflux Thermal Sensors 11 6 Appendix on use in media of low thermal conductivity In the thermal conductivity range from 0 3 to 5 W mK the measurement accuracy of TPO1 is well specified When measuring in lower thermal conductivity media the conductivity of the sensor itself starts playing a significant role The model that is described in paragraph on theory assumes that the medium properties dominate The heat that is generated by the heater is supposed to be distributed into the medium as if the sensor were not there If the thermal conductivity of t
35. put 1 2 VDC nominal switched Thermopile 20 50 ohm resistance Heater resistance 10 20 ohm 60mm effective length L Sensor dimensions Foil 60 by 20 by 0 15 mm Connector block 43 by 24 by 10 mm Cable length 5 metres Weight including 5 m 0 3 kg cable CALIBRATION Calibration to the guarded hot plate of National traceability Physical Laboratory NPL of the UK Recalibration interval Every 2 years typically using locally made agar gel as a reference Table 3 1 List of TPO1 specifications started on the previous page TPO1 Manual version 1509 page 17 45 Hukseflux Thermal Sensors 4 Short user guide Preferably one should read the introduction and the section on theory Really important items are put in boxes The sensor should be installed following the directions of the chapter 5 Essentially this reguires a data logger and control system capable of switching readout of voltages comparing values timing with 1 second accuracy and capability to perform calculations based on the measurement The first step that is described in chapter 5 is an indoor test for the thermal conductivity measurement The purpose of this test is to see if the system works It can be done in a very simple way using agar gel alternatively glycerol and air If the sensor works for the thermal conductivity measurement this implies that it is fully functional The bottom line i
36. s on check the voltage across the heater Check the sensor connection Signal too high Check the data acquisition system by applying or too low an artificially generated voltage to the input Preferably a millivolt generator is used for this purpose Put on the heater Measure the voltage across the heater This should be between 1 and 2 volts Check the sensor output in air with the heater on This should be in the order of magnitude of 2 millivolts Table 11 5 1 Extensive checklist for trouble shooting continued on the next page TPO1 Manual version 1509 page 30 45 Hukseflux Thermal Sensors Check the zero level of the data acguisition system by putting a 50 ohm resistor in place of the sensor The data acguisition system should read less than 20 microvolts Now put the heater on The signal should not react to this by more than 10 microvolts If there is a larger reaction there is a ground loop from the heater to the sensor Check the electrical connection Put the sensor in wet sand Put the heater off Take down the signal level Put the heater on Take down the signal If there is no reaction noted the sensitivity of your readout is too low or the power is too low Reverse the polarity of the power supply or change the connection of the heater current wires by exchanging their positions Take down the signal If the signal level changes by more than 10 with the change of
37. s that all one has to do during a thermal conductivity measurement is to put the sensor in the medium determine two voltages at two different times about 3 minutes apart and to calculate using the measured values and the calibration factor and heater resistance value that are delivered with each sensor For a thermal diffusivity or heat capacity measurement one has toadd a timer to compare the actual response time to a reference value The second step is to make a final system set up The set up is strongly application dependent but it usually involves complete programming and automation of the system possibly also for thermal diffusivity Directions for this can be found in chapters 6 to 10 TPO1 Manual version 1509 page 18 45 Hukseflux Thermal Sensors 5 Putting TPO1 into operation First test the sensor functionality by checking the impedance of the sensor and heater and by checking if the sensor works according to the following table Check the 4 wire connection of the heater Use a multimeter at the 100 ohms range Measure between two wires that are connected at the same end of the heater The measurement will give the value of twice the cable resistance Repeat at the other end of the heater Take down the measured value This is the cable resistance The typical impedance of the wiring is 0 1 ohm m A typical impedance should be 1 ohm for the total resistance of two wires back and forth of e
38. tation The measurements at thermal conductivity s of 0 02 and 0 19 W mK were done in air and glass pearls respectively While perfectly dry sand may have a thermal conductivity of 0 17 the moisture content only needs to be around 2 by weight to increase the thermal conductivity to 0 3W mK TPO1 Manual version 1509 page 33 45 Hukseflux Thermal Sensors Appendix on the use in fluids TPO1 has been used in several fluids with mixed success The theory behind TPO1 assumes that the transport of heat is only performed by conduction and not as can happen in fluids by convection When comparing measurements in agar and water the following picture emerges In theory the two should have egual thermal conductivities because agar is made up for more than 99 of water the only difference being that agar is a gel The measurement result when measuring with TPO1 is that the amplitude of the signal U is higher when measuring in agar indicating that when measuring in water there also is convective loss This becomes even clearer when putting the heater power up It can than be seen that the signal is no longer steadily rising but that there is a fluctuation It seems clear that the temperature rise of the heater is such that there is thermal convection taking place When used in glycerol the convective loss seems to be lower Use of TPO1 in fluids can only be recommended for experimental purposes The method might very well b
39. the steady state signal amplitude can be determined The function F describes the speed at which the process takes place This process scales with the thermal diffusivity of the medium As the thermopile only extends to 6 10 m from the heater and the worst case value for thermal diffusivity is 0 1 10 m s the stabilisation of the differential temperature typically takes 180 seconds Formula 1 1 shows that the pulse response of the sensor signal scales with Q for the amplitude and with a for the time response By curve fitting F one can find a One of the easiest ways of doing this is looking at the 63 response time of the measurement The common procedure is to first determine the thermal conductivity and after the heating cycle to determine how much time it takes to fall by 63 of the amplitude towards the original signal level When applying TPO1 as a thermal diffusivity sensor an essential feature is that both the heater and the thermopile are incorporated in a very thin plastic sheet with extremely low thermal mass This implies that the sensor itself does not play a significant role from a thermal point of view The low thermal mass makes it suitable for estimating thermal diffusivity a without correcting for the sensor mass contrary to some steel needle probes that have a relatively high mass TPO1 Manual version 1509 page 11 45 Hukseflux Thermal Sensors The central formula for using TPO1 as a thermal con
40. to use a 4 wire connection It is however necessary to change the calculation of the heater power TPO1 Manual version 1509 page 24 45 Hukseflux Thermal Sensors 10 Programming for TPO1 The thermal conductivity should be calculated from 15 E Q Uo Uis0 10 1 In case of a connection of TPO1 as described in the paragraph on electrical connection Q Uheat Re L 10 2 Re Land E are parameters that are given as part of the delivery Lis 0 06 m E Re and L can be combined to one factor so that one obtains Eerr Uneat Uo U180 10 3 with Eerr E Re L 10 4 The 63 response time for the determination of the thermal diffusivity t rer 63 has been established in water The nominal value can be found in the list of specifications it is in the order of magnitude of 0 3 minutes Comparison to U to Uo Uigo gives t 63 a a ref T ref 63 T 63 10 5 TPO1 Manual version 1509 page 25 45 Hukseflux Thermal Sensors Sensor The heater resistance per specific meter and the thermopile part sensitivity are parameters that entering differ for each sensor and have Re L a ref to be entered into the software T ref 63 algorithm This is typically done and E in the data logger but could also be done in a later stage during processing Repetitive Measure Uo If the temperature gradients loop through the medium are zero and the electronics are perfect this signal will be equal to z
Download Pdf Manuals
Related Search