User manual

CS215 Temperature and Relative Humidity Probe
A-2
When used outside the range of normal conditions or when subject to prolonged
periods of condensation or freezing the sensor calibration may be temporarily
altered, normally resulting in a change of <+3% RH. Once the sensor returns to
normal conditions the calibration will settle back, over the course of several days,
to the “standard” calibration. In laboratory conditions it is possible to speed up this
process by a reconditioning process, as follows: 80-90 °C at < 5 %RH for 24h
(baking) followed by 20-30 °C at > 74 %RH for 48h (re-hydration).
Figure A-1 Normal Operating Conditions of RH Element
A.4 Measurement below 0˚C
The CS215 provides a humidity reading that is referenced to the saturated water
vapour 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 CS215 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
95%RH at -5˚ C
91%RH at -10˚C
87%RH at -15˚C
82%RH at -20˚C
78%RH at -25˚C
75%RH at -30˚C
In practical terms this means that, for instance, at -20˚C the air is effectively fully
saturated when the sensor outputs 82%RH.