Measuring Temperature with Thermistors, Data Acquisition (DAQ) & LabVIEW
Objective
- Learn Formula Node
- Learn Analog Output (Current)
- Learn how the Thermistor measures temperature
Problem
Severe acute respiratory syndrome (SARS) is a respiratory or airways infection that broke out in Asia in early 2003. It is highly infectious and death from the disease is not uncommon. Fever is one of the first signs of SARS. On June 1, 2003, in Taiwan, a National Temperature Monitoring Campaign was launched. In the campaign, fever was defined as forehead or axillary temperature >37C. You have been tasked to create a fever detector.
Solution
Warning: This is intended as proof of concept exercise and not intended for real life use.
A thermistor will be used to measure the forehead temperature. Thermistors are widely used in industrial applications because of their sensitivity, small size, ruggedness and low cost. Thermistors have an electrical resistance that varies non-linearly with temperature. The R-T characteristics of most thermistors can be described by the Steinhart-Hart equation:
1/T = A + B*(Ln R) + C*(Ln R)3
T is the absolute temperature (in Kelvin) and A, B, and C are constants which can be determined by measuring three sets of resistance and temperature values during calibration.
Most thermistors have a negative temperature coefficient (NTC), their resistance decreases with increasing temperature. Thermistors are specified according to its nominal resistance at 25 oC and commonly available thermistors range from 250 ohms to 100 kohms
The thermistor that we are using has the following characteristics
- Nominal resistance @ 25 oC: 10 kohms
- negative temperature coefficient (NTC)
- Steinhart-Hart equation parameters:
- A= 0.001129148
- B= 0.000234125
- C= 8.76741E-8
As the DAQ module Analog Input measures only voltage, we will need to provide a current source to convert the resistance to voltage. The EMANT300 has an 8 bit current DAC (digital to analog converter). As the DAC has 8 bits resolution, we can drive the resistance from 0 to 1mA in 255 steps with increments of about 39uA.
- Connect the thermistor to the Light Application Adaptor screw terminals labeled IDAC and AGND
- Connect a wire from IDAC to AIN3
- Connect a wire from AGND to AIN2