Description of the problem
How do you use a Thermistor when Dewesoft only natively supports thermocouple and RTD measurements?
Description of the solution
Depending upon availability, sometimes individuals may find themselves in possession of a thermistor sensor to measure temperature. While Dewesoft has native capabilities to measure temperature with Thermocouples (Krypton-TH, IOLITE-TH, and DSI-TH-x adapters) and RTD sensors (Krypton-RTD, IOLITE-RTD, DSI-RTD, SIRIUS-STG), Dewesoft does not have a native Thermistor compatibility. Does that mean that Thermistors cannot be used in DewesoftX? If they can, how is it connected?
Even if the Dewesoft hardware you are using doesn't measure resistance natively we can still calculate the resistance through the Wheatstone bridge and then use the correlation between resistance and temperature provided by the manufacturer to get the proper output. If you have the Resistance vs Temperature table provided by the manufacturer we can create a dedicated analog sensor in DewesoftX for the measurement, but if you don't have that we can use the Steinhart-Hart equation to calculate temperature.
For both NTC and PTC type thermistors the relationship between resistance and temperature is non-linear, so a table is needed to establish the the curve. This table is provided by the manufacturer and while they follow similar trends it will be subtly different for each sensor. Here is an example of Resistance vs. Temperature table for the Measurement Specialties G2K7D411 NTC thermistor, with the full spec sheet attached.
We are going to be using this resistance value as a part of a Half Bridge configuration in DewesoftX. Due to the nature of the relationship of measuring resistance in a Wheatstone bridge we can calculate the thermistor's resistance into a mV/V value.
R1 and R2 are already known, so that becomes 0.5. R4 is a fixed value of 2 as indicated by the spec sheet, and R3 is the value of the thermistor as temperature changes. VS will be a value of 1000 as we are working in mV/V.
Using this equation we can calculate an example where if -40°C = 43.362 kOhm the appropriate voltage will be 455.910233 mV/V.
Now that the equivalent relationship has been established we can perform it for every value in the Resistance v Temp table from above.
mV/V °C mV/V °C mV/V °C mV/V °C
| 455.910233 | -40 | -74.8778385 | 33 | -237.191301 | 54 | -343.17032 | 75 |
| 442.278276 | -35 | -83.7711617 | 34 | -243.494424 | 55 | -347.098687 | 76 |
| 425.32019 | -30 | -92.5925926 | 35 | -249.625187 | 56 | -350.701829 | 77 |
| 404.671115 | -25 | -101.32291 | 36 | -255.572346 | 57 | -354.335754 | 78 |
| 379.937568 | -20 | -109.942056 | 37 | -261.614623 | 58 | -358.000858 | 79 |
| 350.968703 | -15 | -118.42919 | 38 | -267.165324 | 59 | -361.326443 | 80 |
| 317.601459 | -10 | -126.959248 | 39 | -272.797527 | 60 | -364.677907 | 81 |
| 280.0022 | -5 | -135.122261 | 40 | -272.797527 | 61 | -368.055556 | 82 |
| 238.562092 | 0 | -143.293664 | 41 | -275.494378 | 62 | -371.080139 | 83 |
| 193.815064 | 5 | -151.253663 | 42 | -283.69906 | 63 | -374.508089 | 84 |
| 146.64311 | 10 | -159.195781 | 43 | -288.954635 | 64 | -377.577885 | 85 |
| 97.9091275 | 15 | -166.888963 | 44 | -299.041151 | 65 | -380.28169 | 86 |
| 48.6346197 | 20 | -174.536256 | 45 | -304.181745 | 66 | -383.392226 | 87 |
| 0 | 25 | -182.12824 | 46 | -308.734331 | 67 | -386.132034 | 88 |
| -9.68399592 | 26 | -189.417442 | 47 | -313.66965 | 68 | -388.888889 | 89 |
| -19.2107996 | 27 | -196.621386 | 48 | -317.99591 | 69 | -391.265597 | 90 |
| -28.6809411 | 28 | -203.729768 | 49 | -322.368421 | 70 | -414.327512 | 95 |
| -38.0683347 | 29 | -210.479574 | 50 | -326.787929 | 71 | -423.872875 | 100 |
| -47.3453749 | 30 | -217.360115 | 51 | -331.255195 | 72 | -432.18364 | 105 |
| -56.637907 | 31 | -224.112962 | 52 | -335.073069 | 73 | -439.452299 | 110 |
| -65.7708628 | 32 | -230.727073 | 53 | -339.278221 | 74 |
This data can now be brought into DewesoftX for our custom sensor. Since it is nonlinear relationship this table can then be copied into DewesoftX in the Analog Sensor Editor with Tabular Scaling.
Then this custom sensor can be applied directly to a Bridge-based channel in DewesoftX.
The sensor should be wired up to DewesoftX as follows:
If the Reference resistor and the Thermistor are reversed i.e. the Thermistor is connected to the Positive excitation and the Reference resistor is connected to the negative excitation, the measured value will be reversed and will actually go in the opposite direction of the table.
For your convenience this table has already been created and attached in this document as the G2K7D411 Thermistor.dxb. Upon downloading it you can go to the DEWESoft Analog Sensors Editor page and select Import and choose this file and it will be added to your database as NTC Thermistor.
It can then be selected in the dropdown menu of available sensors to use and it will populate. Be sure to set the range to 1000mV/V, Half Bridge, and 10V excitation after the sensor has been selected.
If you do not have a spec sheet for the thermistor we can still calculate the temperature using Math. Attached is a guide called How to - Connect a Thermistor to STG. This leverages the Steinhart-Hart equation to mathematically determine the temperature of an NTC thermistor depending on the resistance measured on the bridge and will give an output depending on certain conditions.
For your convenience these formulas have been combined into the following DewesoftX Setup file Thermistor example.dxs, attached below so you don't need to type out the formulas.
Additional Information
For more information on temperature measurements in DewesoftX, please check out the presentation from DEWESoft's 2018 Worldwide Measurement Conference on Temperature, attached below.