Theory - Decagon Devices VP-4 Operator's Manual

3 THEORY

VP-4
3 Theory
Relative Humidity
The VP-4 utilizes a capacitance type RH sensor to measure the rela-
tive humidity of the surrounding air. Relative humidity is measured
at the same location as the temperature sensor. For this to be an ac-
curate representation of the atmospheric humidity, it is critical that
the humidity sensor be at air temperature. For most measurement
scenarios, the VP-4 should be housed in a radiation shield with ad-
equate 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-4 sensor provides a RH measurement that is referenced to
saturation vapor pressure over liquid water, even at temperatures be-
low 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 freez-
ing when referenced to ice. The figure below shows the maximum RH
the VP-4 measures at saturation, 100% RH and with temperatures
below zero. RH values below saturation can be corrected using the
correction shown in the figure for a given temperature.
Figure 4: RH Value Corrections
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