Product guide

91
Humidity and Dew-Point Instruments
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3.3 Instrument format and usage
Sampling – Sampling is an important issue in all
humidity measurement. The sampling approach may
be that of ‘immersion’, where the instrument sits wholly
or partly in the environment to be measured; or the
gas may be ‘sampled’ by extracting it to the instrument
through a tube. Some immersion sensors benefi t from
being in still air, some from being in moving air, and for
some the fl ow rate of air is critical. When measuring
gases with very low water content, the quality of the
sampling pathway is critical; hygroscopic materials
in the pathway can change the water content of
the sample. Also, care must be taken with certain
types of hygrometer, which may themselves affect
the environment, by giving off either water or heat.
(Indeed any instrument introduced at the wrong
temperature will infl uence its environment, affecting
the measurement result.) Conversely, moisture and
heat from the operator may affect measurements using
hand-held hygrometers.
Type of gas – In gases other than air, chemical
compatibility may be an issue. Calibration specifi c to the
gas in question may be required.
Materials of construction – Aside from considering
the hygroscopic nature of materials, other aspects of
suitability may need to be addressed. Some materials
may be chemically incompatible with the environment
of interest. Materials may be unacceptable for other
reasons, such as avoidance of Molded polymers and
glass for reasons of hygiene or safety.
Ease of use – Some hygrometers are straightforward
to use. Others require some skill.
Use for control or monitoring – In some gases the
user simply needs to measure humidity. In others, the
humidity measurement is fed into a process of control
of humidity. If so, then access to the electrical output
in some form is usually necessary. For use in control
of humidity, the response time and the degree of
hysteresis are usually important.
Hazardous areas – In areas where there is some
hazard, such as an explosive atmosphere, suitable
precautions must be taken. For example, extraction of
sample gas to remote probes may reduce the risk in
some cases, while other sensors can be designed to be
intrinsically safe, by limiting the electrical power to a
level low enough that it cannot ignite the atmosphere.
Where this is a concern, an appropriate certifi cation of
intrinsic safety should be sought.
End use – Humidity itself may be of interest, or
humidity may be used as an indication of the moisture
content of other solid or liquid substances. If so, it
may give no more than a very indirect indication of
the actual content, though ERH and water activity are
proper indications, in a different way, of the condition or
equilibrium of materials with the environment.
Calibration – Method of calibration, and ease of doing
so, should be considered. even when there does not
appear to be a call for highly accurate results.
Robustness – Both hygrometer sensors and casings
vary in their robustness against condensation, drying
out, temperature extremes, dust, chemical or other
contamination, vibration, or even simple handling.
Versatility – If more than one kind of use is envisaged,
the adaptability of the hygrometer should be borne in
mind.
Interchangeability – It may be desirable to have a
set of hygrometers which can be used interchangeably,
or to substitute other probes for use with the main
body of the hygrometer. It may be necessary to
replace components of the sensor which might fail.
In any of these cases, it is important to consider how
interchangeable the instruments or components really
are. Interchangeability is best ensured by calibration
against a consistent reference. However, replacing a key
component of an instrument may invalidate its previous
calibration.
Maintenance – Some types of hygrometers need
routine cleaning, regeneration, or replacement of parts.
Availability of spares – It may be worth knowing
about spare parts; whether they can be fi tted by the
user, and whether they are freely available locally and
quickly. Any consumable supplies, such as charts, should
be available and clearly coded or identifi ed, as they are
not always interchangeable.
After-sales service – The availability of a warranty,
or of a good maintenance or service contract may be a
factor in the decision.
4 How to interpret the data sheet
information?
Detailed below are technical terms used to make
statements about instrument performance, as typically
found in manufacturers’ data sheets. They are:
A Guide to the Measurement of Humidity