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VP-3 User's Manual
2. Measurements
2. Measurements
Relative Humidity
The VP-3 utilizes a capacitance type RH sensor to measure the
relative humidity of the surrounding air. Relative humidity is
measured at the same location as the temperature sensor. For
this to be an accurate representation of the atmospheric humidity,
it is critical that the humidity sensor be at air temperature. For
most measurement scenarios, the VP-3 should be housed in a
radiation shield with adequate air flow to allow the sensor to
come into equilibrium with air temperature. This is not as critical
for non-greenhouse, indoor monitoring applications where
radiation loading is small.
The VP-3 sensor provides a RH measurement that is referenced
to saturation vapor pressure over liquid water, even at
temperatures below freezing where ice is likely to be present
instead of super-cooled water. Although this is the standard way
to define RH (WMO, 2008), it has the disadvantage of providing
incorrect RH values below freezing when referenced to ice. The
figure below shows the maximum RH that will be measured by
the VP-3 sensor (at saturation, 100% RH) at temperatures below
zero. RH values below saturation can be corrected using the
correction shown in the figure for a given temperature.
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