SF52 User’s Manual
18 97224 Issue 5, June 2017
APPENDIX C
• 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 de-ionised) 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.
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.
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