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How the Humidity/Temperature Sensors Work
The following sections provide a detailed overview of how the humidity sensors in the AMS operate, and outlines
some of the cautions you should be aware of, when recording and interpreting the RH readings generated by the
AMS. In particular, please note that varying temperatures, high air-flow environments, and hysteresis can have
significant effects on the RH readings.
General Principles
The AMS uses solid state MEMS humidity/temperature sensors. Each sensor contains a capacitive-type humidity
sensor, a band-gap temperature sensor, and specialized analog and digital circuitry, all inside the IC package. The
capacitor inside the sensor contains a polymer between the electrodes, which absorbs and desorbs water
molecules as the relative humidity changes. The change in capacitance is translated into a relative humidity output
by the internal circuitry. Each sensor is internally calibrated at the time of manufacture to be within 2% relative
humidity over most of the 0
–
100% RH range. However, in order to increase accuracy, the AMS undergoes an
additional calibration that corrects the factory calibration and extends it to lower RH values. Thus the internal
sensor calibration is corrected via look-up tables in the AMS firmware. Then these values are displayed in
MultiView.
Relative Humidity versus Absolute Humidity
Relative humidity (RH) measures the amount of moisture in the air relative to the maximum amount of moisture
that can be held in the air at that temperature. Zero % (0%) RH means there is a negligible amount of moisture in
the air. One hundred % (100%) RH means that the air cannot contain any more moisture before the moisture
begins to condense. Relative humidity is extremely temperature sensitive. Hence the reason for including a very
precise (± 0.3 °C accuracy) air-temperature sensor.
Absolute humidity measures the total amount of moisture in the air and is independent of the temperature.
Absolute humidity can be specified in a number of ways: dew point, ppm on a volume basis, and ppm on a weight
basis (kg/kg of dry air). Appendix A (see
on page
) contains a table
that gives corresponding values for these units at room temperature. Appendix A also contains a psychrometric
chart that allows you to convert humidity to dew point, vapor pressure, and the parts-per-million concentration of
water on a volume or weight basis.
Effect of Temperature
As noted above, relative humidity can be extremely sensitive to the local air temperature. Assuming the air in a
chamber all contains the same amount of absolute humidity, the relative humidity will vary greatly, if there is local
heating of the air.
For instance, at 85% RH and 20°C, the dew point will be 17.4° C. If the air near the sensor is heated by only 1°C, the
RH will change to 80% RH
—
a 5% RH change for just a 1°C change in temperature. At low RH the situation is
somewhat better; a 1°C change in temperature at 10% RH only causes a 0.6 % RH change.
In order to record relative humidity accurately, the air temperature must be extremely uniform. The
humidity/temperature sensors in the AMS should be shielded from any external heat source
—
such as a strong
convective-flow of heated or cooled air, radiant heat, and any heat conduction into the AMS housing.
The AMS humidity/temperature sensors are designed so that there is very little self-heating from the internal
circuitry. In addition, the central AMS humidity/temperature sensor (
labelled “
HT1
”
) that resides in the central
housing, has constricted heat paths to the sensor in order to minimize heat conduction to the HT1 sensor from the
other AMS internal electronics. Please note that the air-temperature sensor on the HT sensors will display a
different temperature from the internal electronics temperature sensor.
