USING THERMISTORS. Using thermistors with a YDOC ML-x17 Data Logger. Application Note Using Thermistors

Transcription

1 Application Note Using Thermistors Using thermistors with a YDOC ML-x17 Data Logger Title : Application Note Using Thermistors Date : Feb with an YDOC ML-x17 data logger Version : 1.0 Test Engineer : Remco Kleine Author : Remco Kleine Reference : 19AN001 Manufacturers of low power instruments Ref: 19AN001 page 1

2 Contents 1 Summary Introduction Connecting a thermistor to the option board Circuit description Calculating the resistance from the measured voltage Example PT Calculated Channels Converting Resistance to Temperature Considerations NTC thermistors NTC of 32k6 example Appendix: PT1000 Resistance Table Manufacturers of low power instruments Ref: 19AN001 page 2

3 1 Summary This document describes the application of connecting thermistors to the YDOC ML-x17 data logger, making use of an analog input option board. The user can adjust the noted values to his own needs. 2 Introduction Thermistors are very commonly used sensors to measure temperature. They are in fact, just resistors with a high temperature coefficient. They vary with temperature. There are NTC and PTC thermistors ( Negative temp. coefficient and positive coefficient). A well known PTC is a PT100 or a PT1000. There have a linear response. NTC s commonly have a NOT linear response. This application note takes the user through the process of connecting and configuring the thermistor to the data logger. The part of connecting the thermistor covers the electrical properties and the configuring handles the mathematical functions to convert from the electrical unit to the real life temperature Manufacturers of low power instruments Ref: 19AN001 page 3

4 3 Connecting a thermistor to the option board 3.1 Circuit description For connecting a thermistor to the ML-x17 data logger, we suggest the use of an analog (differential) option board. ( Such as the ML-OI-AD-2000MV) The reasons for this will be explained in this document. Below the circuit is drawn. This general circuit supports all 2-wire thermistors. ( all values, all types). Figure 1: Circuit diagram Manufacturers of low power instruments Ref: 19AN001 page 4

5 The thermistor is connected to a differential input, using two series resistors. The series resistors shift the voltage level of the thermistor to the middle between GND and the 5V excitation. So, the voltage for the data logger to read has an offset of 2.5V. This is ideal for the internal opamps and it also eliminates noise on the inputs for a big part. The voltage to read now is a differential voltage. 3.2 Calculating the resistance from the measured voltage The resistance of a thermistor is directly related to the temperature and is covered in the next chapter. How to convert the measured voltage into resistance is shown below. Rth = Umeas(R1 + R2) Vref Umeas If we use two same resistors, as suggested, and use the V for excitation the equation is: 2R1 Rth = Umeas 5 Umeas The user can change the value of the reference resistors, to suit the use thermistor. 3.3 Example PT1000 In the following example a PT1000 is used ( a resistance of degrees Celsius) and we use 10k reference resistors (R1=R2=10k) to keep the measurement current low to avoid heat-up of the thermistor. In this case the circuit looks like: Make sure to use high precision resistors (0.1% or better). Manufacturers of low power instruments Ref: 19AN001 page 5

6 The formula for calculating the resistance of the thermistor is: Rth = Umeas 20k 5 Umeas Total resistance = 10k + 10k + R PT1000 = (about 21k) The total current is about.: 5V 21k = 238 ua, which is rather low not heating-up the thermistor too much. The expected measuring voltage is I * R = 238uA * 1000 ohm = 238mV A measurement range of 500 mv is a good match as 238mv is about half-way the range. Because we want to measure voltage we set the parameter value at 500mV to 500. Then we read the voltage over the resistor 1 to 1. We can test this easily in the field test -> analog input test. Manufacturers of low power instruments Ref: 19AN001 page 6

7 In this case, the thermistor is connected to Port 2 of the option board. We measure 244.5mV. During this field test, the thermistor is continuously powered, so you can verify it with a digital multi meter. If your voltage measurement is confirmed, you can enter the formula into a calculated channel (In normal mode, where the thermistor is only powered for a fraction of time, it is not convenient to verify the value with a multi meter). 3.4 Calculated Channels For converting the measured voltage into a resistance, we use the calculated channels of the data logger. You can enter a mathematical formula to alter you value. So, in this case we want to enter the formula from page 6 into the formula editor: Manufacturers of low power instruments Ref: 19AN001 page 7

8 We see that the resistance of the PT1000 is 1027 ohm, which we can manually look it up in a table (see: Appendix) or calculate it with a formula. 3.5 Converting Resistance to Temperature For the PT1000 in our example the following formula is valid: T = Rth Rth Rth0 is the resistance at 0 degrees Celsius ( 1000 ohm) If we enter this formula into the formula editor, it looks like this: The calculated temperature is shown ( 7.22 Degrees in our example) and can be verified by the user. Now you are finished. The thermistor is correctly connected to the data logger, via an option board. Also you can verify the obtained temperature value with a PT1000 table. ( see appendix) Manufacturers of low power instruments Ref: 19AN001 page 8

9 4 Considerations In our example of the PT1000, we used 10k as resistors. This value is chosen with these parameters taken into account. - Not too much current for low power aspects - Also too high current heats the thermistor. Consult your sensor manual for info - Not too high resistance in respect to the range of the ADC - Use precision resistor with a low temp coefficient. - Try to adjust the voltage across the thermistor to match the voltage range of the ADC 5 NTC thermistors NTC resistors are a little more difficult, because they often are NOT linear. But you cam enter a polynomial formula to convert from resistance to temperature. But the measurement of the resistance will be the same. Of course values of components may vary, to adjust the thermistor to the input. 5.1 NTC of 32k6 example When we have an NTC thermistor of for example 32.6k and we would use the same reference resistors of 10k (R1=R2=10k), the total resistance would be: 10k+10k+32.6k=52.6k The nominal current would be: 5V / 52.6k = 95uA The nominal voltage at the input will be: I * Rth = 95uA * 32.6K = 3097mV. Attention: 3097mV is outside the max measurement range, which 2000mV. A good choice would be to increase the value of the series resistors to 100k instead. The nominal current would be: 5V/ 232.6k = 21.5uA. The nominal voltage at the input (across the thermistor) will be: I * Rth = 21.5uA * 32.6k = 700 mv. A measurement range of 1000mV is a good match as 700mV falls well within the range. Manufacturers of low power instruments Ref: 19AN001 page 9

10 6 Appendix: PT1000 Resistance Table Manufacturers of low power instruments Ref: 19AN001 page 10