Campbell Scientific 253-L and 257-L (Watermark 200) Soil Matric Potential Sensors - Manual (Page 18)

253-L and 257-L Soil Matric Potential Sensors
5. Overview
The 253 and 257 soil matric potential sensors provide a convenient method of
estimating water potential of wetter soils in the range of 0 to –200 kPa. The
253 is the Watermark 200 Soil Matric Potential Block modified for use with
Campbell Scientific multiplexers and the 257 is the Watermark 200 Soil Matric
Potential Block modified for use with Campbell Scientific dataloggers.
The –L option on the Model 257-L and 253-L indicates that the cable length is
user specified. This manual refers to the sensors as the 257 and 253. The
typical cable length for the 257 is 25 ft. The following two cable termination
options are offered for the 257:
• Pigtails that connect directly to a Campbell Scientific datalogger
(cable termination option –PT).
• Connector that attaches to a prewired enclosure (cable termination
option –PW).
For 253 applications, most of the cable length used is between the datalogger
and the multiplexer, which reduces overall cable costs and allows each cable
attached to the 253 to be shorter. The cable length of each 253 only needs to
cover the distance from the multiplexer to the point of measurement. Typical
cable length for the 253 is 25 to 50 ft.
The difference between the 253 and the 257 is that there is a capacitor circuit
and completion resistor installed in the 257 cable (FIGURE 5-1) to allow for
direct connection to a datalogger, while the 253 does not have any added
circuitry. For applications requiring many sensors on an analog multiplexer,
the 253 is used and one or more completion resistors are connected to the
datalogger wiring panel. A capacitor circuit is not required for the 253 on a
multiplexer because the electrical connection between the sensor and the
datalogger is interrupted when the multiplexer is deactivated. Any potential
difference between the datalogger earth ground and the electrodes in the sensor
is thus eliminated.
The 253 and 257 consist of two concentric electrodes embedded in a reference
granular matrix material. The granular matrix material is surrounded by a
synthetic membrane for protection against deterioration. An internal gypsum
tablet buffers against the salinity levels found in irrigated soils.
If cultivation practices allow, the sensor can be left in the soil all year,
eliminating the need to remove the sensor during the winter months.
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