Humidity measurement set-up - sampling
Correct sampling is about making sure the measurement is representative of the condition you want to measure.
Sampling format of a hygrometer can mean its configuration:
- a probe may sample free air
- a flow of gas may be sampled through a tube
Avoid:
- water being spuriously added to the measured gas i.e. don't measure near pooled water
- unintended removal of water from sample gas i.e. don't have accidental condensation in tubing upstream of the hygrometer where there is risk of condensation - Localised heating (such as electrical "trace heating" of sample tubing) protects against this, by keeping the gas above the condensation temperature (above the dew point or frost point)
- unrepresentative measurement i.e. measuring at a cold spot in a room. This could give an over-estimate of the typical relative humidity, even though water vapour might be evenly spread across the space
Repeated readings
If conditions are varying (such as often happens under cycling control of an air conditioning system), take repeated readings over a period of time to get an idea of typical range. From repeated readings, you can calculate an average or mean. This can also reduce the uncertainty due to short-term instability of the instrument.
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