Campbell Scientific 108-L Temperature Probe - Manual (Page 16)

Model 108 Temperature Probe
7.4 Water Temperature Installation
The 108 can be submerged to 15 m (50 ft) or 21 psi. The 108 is not weighted,
so a weighting system should be added, or the probe secured to a fixed,
submerged object such as a piling.
7.5 Soil Temperature
The 108 tends to measure the average temperature over its length, so it should
generally be buried such that the measurement tip is horizontal to the soil
surface at the desired depth.
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.
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.
8. Operation
8.1 Sensor Schematic
FIGURE 8-1. 108 thermistor probe schematic
8.2 Measurement and Output Linearization
Campbell Scientific dataloggers measure the 108 probe thermistor and convert
the result to temperature. With reference to the previous FIGURE 8-1, 108
thermistor probe schematic, a precise excitation voltage is applied at the Vx
line and the voltage drop across the 1 kΩ resistor is measured 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 equation:
10