A thermistor can be used to measure 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
As the DAQ module Analog Input measures only voltage, we will need to provide a current source to convert the resistance to voltage. The EMANT380 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. In our exercise, we will drive 0.1mA into the thermistor. As the thermistor has a nominal value of 10 kohm at 25 oC, at this temperature the voltage across the thermistor will be (0.1mA * 10 kohm) = 1V.
The parameter 0.1 (variable is double data type) sets the current output to 0.1 mA. Value must be between 0 to 1 mA
math.log is one of the methods from the math Class. The math Class provides constants and methods for trigonometric, logarithmic, and other common mathematical functions.
The temperature is converted from oK to oC