Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
ELECTRONIC CONTROL FUNDAMENTALS
126
Fig. 10. Capacitance Type Relative Humidity Sensor.
A relative humidity sensor that generates changes in both
resistance and capacitance to measure moisture level is
constructed by anodizing an aluminum strip and then applying
a thin layer of gold or aluminum (Fig. 11). The anodized
aluminum has a layer of porous oxide on its surface. Moisture
can penetrate through the gold layer and fill the pores of the
oxide coating causing changes in both resistance and
capacitance which can be measured by an electronic circuit.
Fig. 12. Capacitance Type RH Sensor with Integral
Temperature Compensation Sensor.
CONDENSATION AND WETTING
Condensation occurs whenever the sensors surface
temperature drops below the dew point of the surrounding air,
even if only momentarily. When operating at levels of 95% rh
and above small temperature changes can cause condensation.
Under these conditions where the ambient temperature and the
dew point are very close, condensation forms quickly, but the
moisture takes a long time to evaporate. Until the moisture is
gone the sensor outputs a 100% rh signal.
When operating in high rh (90% and above), consider these
strategies:
1. Maintain good air mixing to minimize local temperature
fluctuations.
2. Use a sintered stainless steel filter to protect the sensor
from splashing. A hydrophobic coating can also suppress
condensation and wetting in rapidly saturating/
desaturating or splash prone environment.
3. Heat the rh sensor above the ambient dew point
temperature.
NOTE: Heating the sensor changes the calibration and
makes it sensitive to thermal disturbances such
as airflow.
Quartz Crystal Relative Humidity Sensor
Sensors that use changes in frequency to measure relative
humidity (Fig. 13) can use a quartz crystal coated with a
hygroscopic material such as polymer plastic. When the quartz
crystal is energized by an oscillating circuit, it generates a
constant frequency. As the polymer material absorbs moisture
and changes the mass of the quartz crystal, the frequency of
oscillation varies and can be measured by an electronic circuit.
Most relative humidity sensors require electronics at the
sensor to modify and amplify the weak signal and are referred
to as transmitters. The electronic circuit compensates for the
effects of temperature as well as amplifies and linearizes the
measured level of relative humidity. The transmitters typically
provides a voltage or current output that can be used as an input
to the electronic controller.
Fig. 11. Impedance Type Relative Humidity Sensor.
TEMPERATURE COMPENSATION
Both temperature and percent rh effect the output of all
absorption based humidity sensors. Applications calling for
either high accuracy or wide temperature operating range
require temperature compensation. The temperature should be
made as close as possible to the rh sensors active area. This is
especially true when using rh and temperature to measure
dewpoint. Figure 12 shows an rh sensor with the temperature
sensor mounted directly on the substrate.
ELECTRODE
ANODIZED
ALUMINUM STRIP
POROUS
OXIDE LAYER
ELECTRODE
THIN
GOLD LAYER
C3101
B. MOISTURE SENSITIVE MATERIAL BETWEEN ELECTRODE PLATES
AND THIRD CONDUCTIVE PLATE.
A. MOISTURE SENSITIVE MATERIAL BETWEEN ELECTRODE PLATES.
WIRES TO CONTROLLER
OR SENSING CIRCUIT
MOISTURE
SENSITIVE
POLYMER
GOLD FOIL OR OTHER
TYPE OF ELECTRODE PLATES
WIRES TO CONTROLLER
OR SENSING CIRCUIT
C3100
GOLD FOIL OR
OTHER TYPE OF
ELECTRODE PLATES
MOISTURE
SENSITIVE
POLYMER
ULTRA THIN LAYER OF
CONDUCTIVE MATERIAL
PROTECTIVE
POLYMER
POROUS
PLATINUM
DIELECTRIC
POLYMER
CERAMIC
SUBSTRATE
ELECTRODE
FINGERS
1000 OHM
PLATINUM RTD
M10685