Product guide

95
Humidity and Dew-Point Instruments
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• 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 fi lters 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 fi ltration 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 fl ow 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.
6.3 Dew point in general
• The measuring environment and all parts of the
sampling pathway must be kept above the dew
point if condensation is to be avoided. Electrical
trace heating or other heating methods should be
used if necessary. An excess temperature of 10°C
above the dew point is usually a safe margin.
• For measurements in the region below 0°C it must be
clear whether the condensate is dew or frost. Failure
to distinguish between these can result in errors of
about 1°C for every 10 °C below zero.
6.4 Relative humidity in general
• Due care must be taken of temperature.
• Care must be taken when expressing uncertainties,
changes or fractional differences in relative humidity.
For example, the difference between 50% RH and
52% RH is 2% RH. This can also be expressed as a
difference of 4% of value. It is important to distinguish
clearly between these two kinds of statement.
6.5 Recommendations specifi c to
ranges of measurements
• Ambient humidity - Avoid using hygrometers near the
body, which is a source of heat and moisture. Do not
breathe close to the measurement.
• High humidity, above the ambient range - Ample lines
should be maintained above the dew point of the gas
being measured, to avoid condensation. Electrical
trace heating is often the most practical method.
• Low humidity, and very dry gases - If possible,
prepare for measurements by fl ushing sample lines
and hygrometers with dry gas, or by evacuating to
low pressure. Drive off stray residual water by baking
assemblies if possible (but not instruments – unless
designed for this!). The lower the moisture content
to be measured, the more dramatically the required
drying time multiplies.
• Avoid hygroscopic materials. At low humidity (anything
much below a dew point of 0°C) the amounts of
water given off by organic and porous materials can
dramatically affect the value of humidity. The lower
the level of moisture, the more signifi cant the effects.
• Choose impermeable materials, to avoid inward
diffusion of moisture through sampling tubes and
enclosures. Steel and other metals are practically
impermeable. PTFE (‘Tefl on’) is only slightly permeable
and will usually be satisfactory for dew points above
-20°C, and sometimes below this level. Materials
such as PVC and rubber are relatively permeable and
so totally unsuitable at low humidity, and not really
satisfactory in any humidity range.
A Guide to the Measurement of Humidity