Campbell Scientific CS215 Temperature and Relative Humidity Probe - Manual (Page 36)

Appendix C. Environmental Performance
C.3 Operating Range of RH Element
The RH sensor is specified to work over the entire humidity range of 0 to
100% RH for the temperature range –20 to +60 °C. It will give readings over
an extended range as shown in FIGURE C-1 (although the electronics of the
CS215 probe are not specified to operate beyond +70 °C).
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. Upon
returning to normal conditions, the calibration will settle back to the “standard”
calibration over the course of several days. 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 to 30 °C at > 74 %RH for 48h
(re-hydration).
FIGURE C-1. Normal operating conditions of RH element
C.4 Measurement Below 0 °C
The CS215 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 to be 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.
C-2