9. MOISTURE SENSOR OPTION
9.1 General Description
The SHAW SDT sensor is a 2 wire loop powered transmitter using a high capacitance sensing element.
Each unit is pre-calibrated before use with an accuracy of +/- 2C dewpoint measurement. The sensor signal
is converted to a voltage directly integrated into the MGA3000 series electronics. The range of
measurement is scaled as per the transmitter
9.1.1 MGA3000 Zero Calibration
The sensor zero signals is intrinsic and therefore no general zero calibration is required
9.1.2 MGA3000 Span Calibration
If the sensor is exposed to ambient air flow, the sensor will saturate and produce a 20mA
output. The span calibration of the analyser can be performed by setting the SPAN GAS to the
sensor range and performing SPAN SET in the normal way
9.1.3 Sensor Autocal
There are 2 methods of setting the autocal
1) Saturate the transmitter (Only applicable to C and F SDT transmitters)
If the sensor element is exposed to a dewpoint level above the range of the transmitter, the
sensor will saturate and the transmitter output can be set as +20C.
The easiest way to achieve this is to wrap ones hand around the sintered stainless steel
guard that protects the sensing element.
Note:- This is only recommended for transmitters used in non-toxic, non-
poisonous, clean gasses. Local Health and Safety Procedures should be followed at
all times.
2) Expose to a known moisture level. (Applicable to all versions of SDT Transmitters)
If a known gas is available or the ambient dewpoint is known, then the auto cal can be set to
the known value rather than the saturating value.
The SDT is supplied with an adjustable potentiometer to perform auto cal, which allows the
calibration span of the transmitter to be adjusted. This potentiometer is located under the
weatherproof guard and can be accessed by undoing the large silver coloured screw on the
side of the transmitter body. The auto cal is adjusted by turning the potentiometer with the
small screwdriver supplied.
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