Thermistors as passive analog transducers

First, we need to review the nature of a thermistor to provide the motivation for measuring an electrical resistance to estimate a temperature. Then, we need to understand the method of inferring the resistance of a thermistor based on the effects of the current flowing through a half bridge circuit that includes the thermistor (15:55 min).

We will be using a thermistor that is included in a Campbell sensor used to trace the electrical conductivity and temperature of stream water. The conductivity probe includes a thermistor because the use of electrical conductivity to track the variation in total dissolved solids in environmental waters requires a correction for the influence of the water temperature on its ability to conduct electricity. Let's work through wiring the device, including the adapter that provides the resistors for the half bridge circuit (6:00 min).

Measuring a temperature from the thermistor will require using a built-in Campbell function to determine the voltage ratio across the half bridge. We can then write a CRBasic function that will estimate the electrical resistance of the thermistor from this voltage ratio and turn the resistance into a temperature based on the Steinhart-Hart calibration equation.

Link to digital object identifier for Steinhart and Hart (1968 in Deep Sea Research and Oceanographic Abstracts)

The thermistor used in the electrical conductivity probe is the same as that used in the model 107 temperature probe. The manual for a model 107 temperature probe provides a description of the theory underlying use of the transducer, including the implementation of the Steinhart-Hart equation used by Campbell's functions for calibration.

Link to the online manual for a model 107 temperature probe

PDF Preview of manual below may be dated compared to the online version from Campbell Scientific

Download the PDF file

With this documentation, we can write a program to manually make a half bridge measurement and estimate a temperature by calculating the resistance and calibrating that to temperature (20:34 min).

Note that the "AutoRangeC" option will not be available for the BrHalf measurements with a CR310. You should use the more sensitive setting, or the 34 mV setting, if working through these exercises with a CR310.

Campbell also has a built-in function that will use a similar half bridge measurement and calibration to directly calculate the temperature of a thermistor (4:49 min).

These exercises have added to the thermocouple script to provide 4 different estimates of temperature, including two estimates from the thermocouple transducer and two estimates from the thermistor transducer.

Note that the "AutoRangeC" option will not be available for the voltage measurements, the TCDiff measurements, or the BrHalf measurements with a CR310. You should use the more sensitive setting, or the 34 mV setting, if working through these exercises with a CR310.

Click this link to download the MS PowerPoint file

Link to online manual for Campbell CS547A-L (electrical conductivity and temperature transducer)

PDF Preview of manual below may be dated compared to the online version from Campbell Scientific

Download the PDF file