INSTUM_01378 Pag. 6/12
3 Application note
3.1 Measuring humidity in high temperature environments
In very wet tropical areas, humidity values may acquire a very low dynamic range, close to
saturation conditions and, at the same time, to have a low temperature dynamic range and very low
wind speed values. In certain periods of the year, these conditions may occur even in milder areas
for short periods of time.
In such conditions, the hygrocapacitive element can get wet (from dew) and have no time to get dry
and restore its measuring capacity. In this case, even if the humidity value decreases, the humidity
measurement stabilizes to the saturation value or is – at any rate – underestimated.
Two conditions may occur. The first one refers to every category of “exchange type” humidity
sensor; the second one is typical of capacity-measurement humidity sensors:
a) The layer of dew over the measuring element can reach 0,25 mm. Penman’s theory states that
8 hours are needed for the layer of dew to evaporate (30°C, 80 RH% with no air speed), or 4
hours (30°C, 60 RH% with no air speed). These time periods can be shorter in case of forced
ventilation (not less than 1.5 m/s).
b) The manufacturers of hygrocapacitive elements inform that a persistent humidity level on the
element could produce a secondary absorption phenomenon, as well as cause a temporary shift of
the humidity measurement equal to about +6%. This error disappears when the element is exposed
for some hours to lower humidity values.
These considerations confirm that the hygrocapacitive element can be used when humidity values
reach a sufficient dynamic range to enable the sensor adjustment based on the “stress” condition.
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