Placement of the cable inside a rugged conduit may be necessary for long cable runs, especially
in locations subject to digging, mowing, traffic, use of power tools, or lightning strikes.
8. Operation
8.1 Sensor schematic
FIGURE 8-1. 108 thermistor probe schematic
8.2 Measurement and output linearization
CRBasic instruction
Therm108()
measures the 108 probe thermistor and automatically converts
the result to temperature. With reference to the previous
Therm108()
applies 1000 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 40 kΩ fixed resistors as described in the following equations:
Vs/Vx = 1000 / (Rs + 40000 Ω + 1000 Ω)
Solving for Rs:
Rs + 41000 Ω = 1000 • (Vx/Vs)
Rs = 1000 • (Vx/Vs) – 41000 Ω
The relationship of Rs to temperature is tabulated in
Thermistor resistance and temperature
16), but is calculated by
Therm108()
using the Steinhart-Hart equation, described as follows:
T
c
= (1 / (A + B • ln (R
s
) + C • (ln (R
s
))
3
)) – 273.15
108 Temperature Probe
10
