Campbell 107 User Manual

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. 107 thermistor probe schematic
8.2 Measurement and output linearization
CRBasic instruction Therm107() measures the 107 probe thermistor and automatically converts
the result to temperature. With reference to the previous FIGURE 8-1 (p. 10), 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 (p.
16), but is calculated by Therm107() using the Steinhart-Hart equation, described as follows:
T
c
= (1 / (A + B • ln (R
s
) + C • (ln (R
s
))
3
)) – 273.15
107 Temperature Probe10

Campbell 107 Specifications

General

Brand	Campbell
Model	107
Category	Measuring Instruments
Language	English