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SF52 User’s Manual
Michell Instruments 15
APPENDIX C
Appendix C Recommended Practices in Humidity Measurements
The following text is reproduced with kind permission from the National Physical Laboratory. It
is originally published in the booklet, A Guide to the Measurement of Humidity.
Defi nition of Relative Humidity
Relative Humidity – The ratio of the actual vapor pressure to the saturation vapor pressure
over a plane liquid water surface at the same temperature, expressed as a percentage. This is
commonly understood when the term ‘X percent relative humidity’ is used.
For actual vapor pressure, e, and saturation vapor pressure, e
s
e
relative humidity (in %) = ––– x 100
e
s
USAGE: The phrase ‘relative humidity’ is commonly abbreviated RH although this is not a
recognized abbreviation. Values of relative humidity are commonly expressed in units of percent
relative humidity (% RH).
Recommended practices in humidity measurements
General practical recommendations
• Where relative humidity is of interest, a direct measurement of relative humidity
is usually best. Where an absolute measure of humidity is needed, choose dew
point, vapor pressure or similar measurements.
• Establish the measurement requirements at the purchasing stage in order to
have the right instrument for the job.
• Allow hygrometers to equilibrate in any new environment. This is particularly
necessary after changes in temperature due to transportation or storage.
Depending on the instrument and on how great the change in conditions, this
may require from only a few minutes to many hours.
• Follow Michell Instruments’ care instructions for the instrument. Some instruments
need routine cleaning or other maintenance. Before using any solvent cleaner,
check with Michell Instruments that this will not harm the sensor or other
materials of construction.
• Wherever possible, ensure that hygrometers are calibrated under the conditions
of use, i.e. at similar values of humidity and temperature, and (if relevant) in
similar conditions of pressure, airfl ow, etc.
• Keep a record of calibrations and any adjustments to the hygrometer. This
will show the long-term stability of the instrument and allow the associated
uncertainty to be assessed.
• Check instruments, if possible, at intervals between calibrations, by comparison
with another (stable) instrument, to monitor for long-term drift. Routine checks
are also useful before and after subjecting an instrument to transportation or
other stress, which might lead to a shift in its performance. Where the check
is against two (or more) instruments this is even better: not only does this add
confi dence, but in the event of one instrument drifting among a set of three, it
can be seen which reading is most suspect.