Thermistors

A thermistor's resistance changes with temperature. Most thermistors installed by WERC are either manufactured by Alpha (model 13A5001-C3) or YSI (model 44033) and temperature is inversely related to resistance. To convert: Temperature (Degrees C) = 1 / [a + b * ln (R) + c * ln (R) ^ 2] - 273.15 where, R = Thermistor resistance in Ohms a, b, and c are Steinhart-Hart equation coefficients. For Alpha model 13A5001-C3:

 a = 1.28e-3
 b = 2.36e-4
 c = 9.06e-8

For YSI model 44033:

 a = 1.47e-3
 b = 2.39e-4
 c = 1.0002e-7

To measure these values with a CR10X data logger you might use the following short program (assumes a YSI 44033 thermistor). For this example we'll assume the wiring looks like this:

 CR10X SE1 --> Thermistor +, 15.0 kOhm Resistor to CR10X E1
 CR10X AG --> Thermistor -
 CR10X E1 --> 15.0 kOhm Resistor to CR10X SE1
 One note about the thermistors, the two leads of a thermistor are interchangeable.  So either could go into the measurement channel and either could be put into analog ground.

Program:

 1:  Excite-Delay (SE) (P4)
  1: 1        Reps
  2: 1        2.5 mV Slow Range
  3: 1        SE Channel
  4: 1        Excite all reps w/Exchan 1
  5: 4        Delay (0.01 sec units)
  6: 2500     mV Excitation
  7: 1        Loc [ thermRES  ]
  8: 0.0004   Multiplier
  9: 0        Offset
 2:  BR Transform Rf[X/(1-X)] (P59)
  1: 1        Reps
  2: 1        Loc [ thermRES  ]
  3: 15.0     Multiplier (Rf)
 3:  Z=X*F (P37)
  1: 1        X Loc [ thermRES  ]
  2: 1000     F
  3: 1        Z Loc [ thermRES  ]
 4:  Steinhart-Hart Equation (P200)
  1: 1        Reps
  2: 1        Source Loc (R)(Ohms) [ thermRES  ]
  3: 2        Destination Loc (Deg C) [ thermTEMP ]
  4: 1.47     A
  5: -3       x 10^n
  6: 2.39     B
  7: -4       x 10^n
  8: 1.0002   C
  9: -7       x 10^n

A note about the program. In the first program step the thermistor is excited with 2500 mV of DC electricity. after a delay of 0.04 seconds the voltage drop across the thermistor is recorded. In this circuit the thermistor is placed in series with the resistor, Rf, going between E1 and SE1. Because we know the total excitation voltage (2500 mV) and the resistance, Rf (15.0 kOhms) we can use the P59 program instruction to calculate the resistance of the thermistor based on the voltage measured by SE1. Next we convert that resistance from k-Ohms to Ohms for use in the Steinhart-Hart equation.

If you are working with an older data logger the program instruction P200 may not be in its program library (the program won't compile when you send this program to the data logger). You can gain access to this program instruction by updating the operating system (OS) in the data logger by visiting the Campbell Scientific software download site. Use DevConfig, part of the Loggernet package to send the OS to the data logger.