SD Series Dry Cabinets Setup and Operation Manual | A55-7379-00 Rev: 2.0.0 Page 94
The current barometric pressure also has a small effect on the results so, to refine the result, measure the
pressure and enter it into the %RH equation 14 where P is the pressure in kiloPascals (kPa), Td is the dry bulb
temperature in ºC and Tw is the wet bulb temperature in ºC and e is the “natural” exponent value:
The equation can be broken down into simple steps:
1. The barometric pressure (P) is assumed to be: 101.3 kPa
2. A conversion factor is calculated: A = 0.00066(1.0 + 0.00115 Tw)
3. The saturation vapor pressure is calculated at temperature Tw:
eswb = e[(16.78 Tw – 116.9)/(Tw + 237.3)]
4. The water vapor pressure is calculated: ed = eswb – A P (Td - Tw)
5. The saturation vapor pressure is calculated at temperature Td:
esdb = e[(16.78 Td – 116.9)/(Td + 237.3)]
6. The % RH is then calculated: RH = 100(ed/esdb)
Aluminum Oxide Capacitive Sensor:
This sensor, like others, has electrical properties that change as function of humidity. It is sensitive to the wa-
ter vapor pressure so more calculations are required to convert the measurement into relative humidity. The
actual conversion equation is provided by the manufacture of this type of sensor.
Because it is sensitive to water vapor partial pressure, it is a good choice for moisture detection in natural gas
lines and other industrial atmospheres. This sensor type must be kept dry and the gases being sampled must
be clean and free of solids, which can harm the sensor.
To complete the calculation, atmospheric pressure must be accurately measured. This sensor type can also
take a long time – up to 24 hours – to stabilize.
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