Measurement and output linearization

CRBasic instruction Therm109() measures the 109SS probe thermistor and automatically converts the result to temperature. With reference to the previous 109SS thermistor probe schematic, Therm109() applies 2500 mV excitation at the Vx line and measures the voltage drop across the 24.9 kΩ resistor at the Vs line.

The ratio of measured voltage (Vs) to excitation voltage (Vx) is related to thermistor resistance (Rs) and the 24.9 kΩ bridge resistor as described in the following equations:

Vs/Vx = 24900 Ω / (Rs + 24900 Ω)

Solving for Rs:

Rs + 24900 Ω = 24900 Ω • (Vx/Vs)

Rs = 24900 Ω • ((Vx/Vs) – 1)

The relationship of Rs to temperature is tabulated in Thermistor resistance and temperature, but is calculated by Therm109() 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 = 1.129241E–3

B1 = 2.341077E–4

C1 = 8.775468E–8

1Coefficients provided by Measurement Specialties™.