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in_m22.doc
Temperature operating range and limits
The M22 series can operate within -40 to 60
°
C (-40 to 140
°
F) at the electronics.
The temperature operating range of the HygroClip S probe is -40…85
°
C (-40…185
°
F).
Operating the transmitter and/or its probe outside of the temperature limits can result in
permanent damage.
Humidity limits
As far as possible, avoid sudden condensation at the sensors. When measuring at high humidity,
condensation may occur on the humidity sensor due to a sudden difference in temperature with the
environment. This does not damage the sensor. However, this will produce an overflow reading (an
output signal of more than 100 %RH) for as long as condensation is present on the humidity sensor.
Temperature compensation of the humidity sensor
Practically every make of relative humidity sensor requires a compensation for the effect of
temperature on the humidity output signal in order to measure accurately over a wide range of
temperature conditions. In the specific case of an instrument using a capacitive sensor, compensation
is required because the dielectric characteristics of both the water molecule and the hygroscopic
polymer used in the sensor vary with temperature.
The HygroClip S probe uses the temperature data from the Pt100 RTD to provide automatic
compensation for the effect of temperature on the humidity sensor. The temperature compensation
uses normal room temperature as a reference. Because of this, calibration of the HygroClip S is done
at normal room temperature rather than at the temperature of operation at the sensor.
Output range
The range of the relative humidity output is 0 to 100%RH. The temperature output depends on the
range specified when ordering - see identification labels (a) on shipping box and (b) inside electronics
module under the printed circuit board.
Output signals
The M22 series provides two 4-20 mA output signals. These signals are linear and are consistent with
the requirements of most data/signal processing instrumentation (panel meter, controller, computer
card, etc.).
The M22 behaves as a variable source of current and adjusts the current flowing through the terminals
as a function of relative humidity and temperature. The output signal may be read with any current
sensing device having a maximum impedance of 250 ohms. When several devices are connected in
series with the transmitter, the resulting impedance should not exceed 250 ohms, wiring included.
If so desired, the loop of one of the signal can be left open without affecting operation of the other
output (humidity or temperature only).
