Michell Instruments PC33, Руководство пользователя

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PC33 & 52 User’s Manual
18 97238 Issue 2, October 2013
appendix d
• Cleanliness of the environment will affect different hygrometers in different
ways. Dust and airborne droplets should be avoided or filtered 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 confirmed 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.
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 vapor 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 vapor 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.
• Condensation in a sampling process can invalidate humidity measurements by
reducing the water content of the gas being measured. What is more, condensed
liquid may alter the humidity elsewhere by dripping or running to other locations
and evaporating there. In these circumstances, measurement results may be
misleading if hygrometer location is not considered carefully.
• Water droplets or mist must be avoided. These can result in overestimates of the
humidity of the air between the droplets. Such results may exceed 100% RH,
or may be impossible to interpret meaningfully. Droplets of liquid also damage
some electrical types of humidity sensor. Filtering the air sample can eliminate
droplets.
• If pumps are used for sampling gas, these should be located after the hygrometer,
to avoid contaminating the measurement environment. Where possible, oil
free pumps should be used, or filters employed. Oscillations in pressure due
to pumping can sometimes be reduced or buffered using a needle valve or a
reservoir of large volume.
• Special treatments such as filtration can change the amount of moisture in a gas.
Some drying agents take out other gases too.
• When sealing any sensor or probe into a port or manifold in a duct or chamber,
leaks through the probe or electrical cable should be considered. These are not
always sealed against passage of ambient air.
• Where sampling involves a step change in temperature, pressure or gas flow
rate, relative to the process being sampled, results may need to be converted
or interpreted. For example ‘pressure dew point’ will differ from the value found
after expanding the gas sample to atmospheric pressure. Care should be taken
to distinguish between ‘gauge’ and absolute values of pressure.