Product guide

Humidity and Dew-Point Instruments
© Michell Instruments 2009-2011
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Any calibration corrections identifi ed on a certifi cate
of calibration should be applied to measured values
obtained using that instrument. If the calibration
corrections cannot be applied, the quoted uncertainty in
the results should include an allowance for this.
What is checking?
A measurement check is not the same thing as a
calibration. A check is a test to confi rm whether or not
some condition is fulfi lled (for example to confi rm that
the performance of an instrument has not drifted).
The outcome of a check is not normally a certifi cate of
calibration. However, checks of a measuring instrument
against another (stable) instrument are often useful at
intervals in between calibrations.
6 Recommended practices in
humidity measurements
6.1 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, vapour 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.
• Cleanliness of the environment will affect different
hygrometers in different ways. Dust and airborne
droplets should be avoided or fi ltered 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 confi rmed whether the
calibration is valid for the gas in use.
• Avoid using instruments in direct sunlight or near any
other source of heat, unless they are suitably shielded
to prevent measurement errors.
6.2 Sampling in general
• Relative humidity measurements should be carried
out at a representative temperature. Failure to
allow temperature equilibration will lead to a false
indication of the relative humidity.
• Variations in vapour pressure from place to place
can occur where an environment is subject to any
addition or removal of water. If so, care must be
taken over where to make a measurement in order to
obtain a representative result.
• Sources and sinks of water vapour should be avoided
in any sampling system. Invasion of stray water
can be minimised by attention to leaks, hygroscopic
materials, droplets and condensation. The lower the
humidity, the more critical these precautions are.
• Hygroscopic materials should be avoided. Many
materials contain moisture as part of their structure,
particularly organic materials (whether natural or
synthetic), salts (or anything which contains them),
and anything which has small pores. Temperature
changes can increase the tendency of these materials
to affect the humidity of the surrounding air.
A Guide to the Measurement of Humidity