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. 109 thermistor probe schematic
8.2 Measurement and output linearization
CRBasic instruction
Therm109()
measures the 109 probe thermistor and automatically converts
the result to temperature. With reference to the previous
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
16), but is calculated by
Therm109()
using the Steinhart-Hart equation, described as follows:
T
c
= (1 / (A + B • ln (R
s
) + C • (ln (R
s
))
3
)) – 273.15
109 Temperature Probe
10
