Product guide

Humidity and Dew-Point Instruments
© Michell Instruments 2009-2011
96
• Surface fi nish of pipework is important for very dry
gases. Even the tiny quantities of water adsorbed on
the surfaces of non-hygroscopic materials can have
signifi cant effect. Polished or electropolished steel is
recommended for the best results.
• Clean environments are always best for humidity
measurements, but this is especially critical at very
low humidity. Even fi ngerprints harbour water.
High purity cleaning agents are recommended:
Analytical Reagent (AR) quality solvents for oil-
based contaminants, and purifi ed water (distilled or
deionized) for salts. Cleaning should be followed by
thorough drying by a clean method.
• Sample tubing should be as short in length as
possible. The surface area should be minimised by
using the narrowest tubing that the fl ow conditions
will permit.
• Avoid leaks. Minimising the number of connections
(elbows, tees, valves, etc.) helps with this.
• Adequate fl ow of the gas sample should be ensured,
to minimise the infl uence of sources of stray water in
the fl ow path.
• ‘Dead ends’ should be avoided, as they cannot easily
be fl ushed.
• Back-diffusion of moisture should be minimised, e.g.
by fast fl ow rates of gas, long exhaust tubes after the
sensor, or by valves which isolate the low-humidity
region from ambient air.
6.6 Practical recommendations for
specifi c types of hygrometer
Relative humidity capacitive sensor
• Care should be taken to avoid mechanical shock
(impact) or thermal shock (sudden temperature
changes). Sensors should be protected from steam or
water sprays, and from direct sunlight.
• Where a sensor is at risk of exposure to dust,
droplets, or the occasional knock during handling,
the appropriate guard or fi lters for the sensor head
should be used.
• Any temptation to breathe on the sensor, or to wave
it over cups of tea, etc. should be resisted. Filters and
saturation guarding may protect the sensor, but these
actions carry a risk of damage by condensation or
other contamination.
• Protective fi lters can slow the response time of
sensors. This can be avoided by removing any fi lter,
but the benefi t must be weighed against the risk of
damage to the sensor.
• Sensors should not normally be submerged in liquids.
In the case of a resistive (electrolytic) sensor, water
or other liquids would certainly damage the sensor
beyond repair.
• Salt solutions are especially commonly used for
calibration of electrical sensors, and should be
provided with traceability directly or via a calibrated
hygrometer. Protection of sensors from direct
contact with salt or solution is most important as
contamination would destroy or seriously impair the
sensing element.
Impedance dew-point hygrometer
• Sensors constructed using aluminium oxide or
ceramics essentially respond to vapour pressure. They
are often used at high pressure, or to sample gas
originating from systems at high pressure. If so, care
must be taken to ensure that the sample pressure is
known, and a correction applied if necessary.
• As for other electrical sensors, fi ltration should be
used to protect from dust and droplets or mist.
• Regular calibration is particularly important for
ceramic hygrometers. The calibration intervals may be
dependent on usage.
• Aluminium oxide sensors normally exhibit a
temperature coeffi cient, which may be partially
compensated within the instrument, but not totally. If
the hygrometer must be used at a temperature, other
than that at which it was calibrated, ensure that any
necessary correction to the reading is made, and that
any resulting uncertainty is taken into account.
• When using the sensors at low levels of moisture, the
precautions listed above under ‘sampling’ and ‘low
humidity’ should be observed.
A Guide to the Measurement of Humidity