Product guide

Humidity and Dew-Point Instruments
© Michell Instruments 2009-2011
8
Calibration of in-service hygrometers should be verifi ed
on a 6 to 12 months schedule. Under normal operating
conditions, accuracy is excellent, and an adjustment is
normally not necessary. Saturated salt and unsaturated
salt RH reference elements are available for calibration
procedures, as well as more sophisticated automatic
calibrators such as the S503, S904 and OptiCal. Under
normal operating conditions, experience has shown that
the life expectancy of sensors is longer than 10 years.
Behaviour in corrosive environments
H5000 and H6000 sensors use a synthetic polymer as a
dielectric that is intrinsically resistant to corrosion and
contamination. However, many industrial applications
involve a large number of acid and oxidizing agents, and
the long-term sensor reactions to oxidizing environments
(such as S0
2
) with high RH levels are quite interesting.
The KESTERNICH test is used to simulate the unit
response under these conditions.
The test consists of immersing the products in an
aggressive gas. A single testing day corresponds to 1 year
under normal operating conditions, which accelerates
ageing by 365 times. The KESTERNICH test (name of its
inventor) meets the DIN 50.018 KF W 0.2 S standards.
Michell’s H5000 and H6000 sensors have been subjected
to the KESTERNICH test under the supervision of an
external laboratory - Laboratoire Central des Industries
Electriques (L.C.I.E., n0356502 dated 14 July 1991).
The test results showed:
Appearance: slight alteration after 10 years.
Operation: No measurable effect after 10 years.
Condensation and cleaning
Condensation or occasional wetting have no adverse
effects on H Series sensor reliability. H5000 and H6000
revert to their initial metrological features once the liquid
water has been thoroughly eliminated.
H5000: at humidities above 98% RH there is a high risk
of wetting. In this case the sensor will not be damaged,
but 2-3 hours may be necessary before the sensor reverts
to its normal condition.
H6000: wetting could occur in close-to-saturation
operating conditions but in such cases the sensor will not
be seriously damaged and it may take up to 30 minutes
before reverting to its initial measuring conditions.
It is recommended not to touch the surfaces of the
sensor with mechanical devices or your fi nger. Do not try
to remove particles or deposits as they do not modify the
characteristics of the sensor.
Humidity Sensor Technology
Cleaning: may be done by immersing the sensor in clean
water and drying naturally in air.
Infl uence of air velocity on the measurement
H5000 and H6000 sensing elements require some
elementary precautions in order to prevent the pollution
of the active surface. The H5100 & H6100 cover provides
mechanical protection.
H6000 hydrophobic layer increases dust protection in high
air fl ow and in case of frequent wetting.
In all cases, some form of mechanical protection, such as
a slotted, sintered or polymer guard is recommended. It
is generally not necessary to force fl uid circulation due to
the rapid response time of the sensor, however a moving
air or gas fl ow will aid equilibration times in the process
itself. Fluid fl ow velocity up to 20m/second will have no
adverse effects on sensors protected by a mechanical
cover.
Calibration recommendations
All types of RH generators can be used for short
calibration periods or for tests under 24 hours.
Nevertheless, Stable Saturated Salt Solutions (S.S.S.S.)
are suitable for calibrations and inspection but are not
recommended for long-term tests, especially at high
humidity.
A stabilization period of 5-15 minutes is recommended
for relative humidity sensor calibration or other tests,
depending upon the method of RH generation (e.g. a 15
minute period is recommended for S.S.S.S.). However, it
should be noted that the actual stabilization time of some
saturated and unsaturated mixtures can be signifi cantly
longer. Michell can advise on a calibration timing protocol
to suit your exact needs.