7.4 Soil temperature installation
The 109SS tends to measure the average temperature over its length, so burying the probe such
that the measurement tip is horizontal to the soil surface at the desired depth is usually preferred.
The maximum burial depth for soil that could become saturated with water is dictated by the
maximum water pressure allowed for the sensor, which is 21 psi.
One or two coils of cable should also be buried in a shallow installation. Burial of some cable
mitigates the effect of solar heating of the above ground cable on the temperature
measurement.
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. 109SS thermistor probe schematic
8.2 Measurement and output linearization
CRBasic instruction
Therm109()
measures the 109SS 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.
109SS Temperature Probe
8
