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Using Thermistors in VI Logger with a Traditional DAQ Device

Primary Software: Other NI Software>>VI Logger
Primary Software Version: 2.0.1
Primary Software Fixed Version: N/A
Secondary Software: N/A
Hardware: Multifunction DAQ (MIO)

Problem:
How do I use a thermistor, with its Steinhart-Hart coefficients, in VI Logger for a Traditional DAQ device?

Solution:
Since Traditional NI-DAQ Virtual Channels don't have a built in thermistor sensor type for analog input, there is no direct way to acquire temperature readings with them:

Thermistors have a resistance-temperature relationship which is approximated using the Steinhart-Hart equation:

Manufacturers will provide the different a, b and c coefficients for the Steinhart-Hart equation to best approximate a specific thermistor model.

To obtain temperature data via a thermistor using VI Logger and a Traditional DAQ device requires the creation of a channel to measure the resistance of the thermistor. In the image below the channel is called Resistance:

A Calculated Channel must be created in order to apply the Steinhart-Hart equation to the resistance data to convert it to valid temperature data. This is done by selecting a User Defined formula within the Formula Settings. The Steinhart-Equation should then be entered in the User defined formula field. In the figure below the equation has been entered with 1, 2 and 3 as the a, b and c coefficients respectively. The formula also refers to the Resistance virtual channel created in the previous step. The remaining settings will be dependent on the thermistor properties and should be filled out according to specifications provided with the sensor:

The Calculated Channel should look similar to the figure below once it has been created:

Since the channel is log enabled, now if the VI Logger Task is run the data logged will be the converted temperature data from the thermistor.

Related Links:
NI 435x User Manual
NI Developer Zone: Thermistor Fundamentals
NI Developer Zone: Measuring Temperature with an RTD or Thermistor

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Report Date: 02/17/2006
Last Updated: 07/20/2007
Document ID: 3UGAOI9N

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