WM281/291 User’s Manual
Michell Instruments
29
appendix F
Appendix F
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.
Definition 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, airflow, 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 confidence, but in the event of one instrument
drifting among a set of three, it can be seen which reading is most suspect.
•
Cleanliness of the environment will affect different hygrometers in different ways. Dust
and airborne droplets should be avoided or filtered out if possible. Contaminants can come
from the most surprising sources, ordinary urban pollution, for example.
•
The readings given by some types of hygrometer are sensitive to gas type. For any
Instrument which reads in terms of mass per unit volume, e.g. in grams per cubic metre,
it must be confirmed whether the calibration is valid for the gas in use.