For example, for an air temperature of 20 °C and a vapor pressure of 1.17 kPa, the saturation
vapor pressure is 2.34 kPa and the relative humidity is 50%. If the air temperature is increased by
5 °C and no moisture is added or removed from the air, the saturation vapor pressure increases
to 3.17 kPa and the relative humidity decreases to 36.9%. After the increase in air temperature,
the air can hold more water vapor. However, the actual amount of water vapor in the air has not
changed. Thus, the amount of water vapor in the air, relative to saturation, has decreased.
Because of the inverse relationship between relative humidity and air temperature, finding the
mean relative humidity is meaningless. A more useful quantity is the mean vapor pressure. The
mean vapor pressure can be computed online by the data logger (see
(p. 19)).
8.4.1 Measurement below 0 °C
The EE181 provides a humidity reading that is referenced to the saturated water vapor pressure
above liquid water, even at temperatures below 0 °C, where ice might form. This is the common
way to express relative humidity and is as defined by the World Meteorological Organization. If
an RH value is required referenced to ice, the EE181 readings will need to be corrected.
One consequence of using water as the reference is that the maximum humidity that will
normally be output by the sensor for temperatures below freezing is as follows:
100% RH at 0 °C
82% RH at –20 °C
95% RH at –5 °C
78% RH at –25 °C
91% RH at –10 °C
75% RH at –30 °C
87% RH at –15 °C
In practical terms this means that, for instance, at –20 °C the air is effectively fully saturated when
the sensor outputs 82% RH.
9. Troubleshooting and
maintenance
NOTE:
All factory repairs and recalibrations require a returned materials authorization (RMA) and
completion of the “Declaration of Hazardous Material and Decontamination” form. Refer to
the
page at the end of this manual for more information.
EE181 Temperature and Relative Humidity Probe
14
