Measurement and output linearization

CRBasic instruction Therm107() measures the 107 probe thermistor and automatically converts the result to temperature. With reference to the previous 107 thermistor probe schematic, Therm107() applies 2000 mV excitation at the Vx line and measures the voltage drop across the 1 kΩ resistor at the Vs line.

The ratio of measured voltage (Vs) to excitation voltage (Vx) is related to thermistor resistance (Rs), and the 1 kΩ and 249 kΩ fixed resistors as described in the following equations:

Vs/Vx = 1000 / (Rs + 249000 Ω + 1000 Ω)

Solving for Rs:

Rs + 250000 Ω = 1000 • (Vx/Vs)

Rs = 1000 • (Vx/Vs) – 250000

The relationship of Rs to temperature is tabulated in Thermistor resistance and temperature, but is calculated by Therm107() using the Steinhart-Hart equation, described as follows:

Tc = (1 / (A + B • ln (Rs) + C • (ln (Rs))3)) – 273.15

where:

Tc = temperature in degrees Celsius (°C)

A1 = 8.271111E–4

B1 = 2.088020E–4

C1 = 8.059200E–8

1Coefficients provided by Measurement Specialties™.