Product guide

Humidity and Dew-Point Instruments
© Michell Instruments 2009-2011
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If any gas is compressed or expanded without adding or
removing components, then the mass of all components
is unchanged. Therefore the mixing ratio is unaltered by
the pressure change. This can also be deduced from the
fact that the defi nition of mixing ratio does not involve
pressure or temperature.
The rule is similar for other dimensionless measures of
concentration (those expressed in mass per unit mass,
or volume per unit volume), such as mole fraction
and specifi c humidity, including cases where these are
expressed in terms of parts per million.
Effect of doubling system pressure on dew-point
temperature, without changing composition
The dew-point temperature of a System is directly
related to the water vapour pressure of that system.
From equation (2) it can be seen that a doubling
of total pressure p(total) will yield a doubling of
the water vapour pressure, p(water). Reference to
vapour pressure tables will allow the new dew-point
temperature to be calculated.
For example, for a system of pure water vapour with
a dew point of +10°C, the water vapour pressure is
about 1228 Pa. If the system pressure is doubled, the
resultant water vapour pressure will be 2 x 1228 = 2456
Pa, which equates to a dew-point temperature of about
+20.8°C.
For dew points in the presence of air or other gases
(rather than pure water vapour alone), a correction may
need to be made for the water vapour enhancement
factor (see below).
Water vapour enhancement factor
The examples above assume that all the component
gases exhibit ‘ideal gas’ behaviour. In practice a small,
pressure-dependent correction may be required: the
‘water vapour enhancement factor’.
In air, this numerical correction is less than 1% of value
when pressure is doubled from atmospheric pressure.
For a tenfold increase from atmospheric pressure,
the effect is between about 1% and 10% of value,
depending on the dew point of the gas.
3 Selection of a type of hygrometer
3.1 Identifying measurement needs
To ensure ‘fi tness for purpose’ it must be clear fi rst of all
what is the purpose. Is a humidity measurement needed
at all? If so, what use is it to the business or process?
To be useful, how should the results be expressed? Is
the measurement to meet a practical need, or to comply
with a documented specifi cation, and is the specifi cation
meaningful and realistic? Once the need and aim of
making the measurement is clear, it is important to decide
what factors are relevant to achieving this aim. The user
should consider the following issues:
3.2 Measurements required
Relative or absolute – Which unit or scale of
measurement? The quantity of interest may be relative
humidity, dew point, or some other measure of the
concentration of water vapour.
It is best to select a method of measurement which
intrinsically detects the quantity of interest. Many
hygrometers display results in terms of two or more
humidity parameters. This is often useful, but it should
be understood that normally only one parameter is
being measured, and the other values are the result of
numerical conversions. Because of this, an instrument
might give a reliable indication of only one of the
parameters shown, while providing a less good indication
of the other parameters.
Range (humidity and other variables) – Different
types of measurement are suited to different ranges of
humidity. It is best to avoid using an instrument at the
upper or lower extreme of its range of measurement.
Ranges of temperature, fl ow rate, and pressure or
vacuum also need to be specifi ed in order to select the
best instrument.
For example, where use at elevated pressure is planned,
the construction of the instrument may be as important
as the operating principle. On the other hand, for some
types of measurement, a correction may have to be
applied to the results if the pressure varies.
Performance – It is worth deciding if any particular
performance characteristics are important for the
application in question. There may be a need to set
criteria for sensitivity, stability (in terms of repeatability
or reproducibility), uncertainty, response time, resolution,
linearity or hysteresis.
Output, readout – The humidity result may simply be
shown as a number on a digital display, or on a numbered
scale. It may be given as an electrical voltage or current
signal (analog output). Several parameters or units of
humidity may be shown, together with temperature.
Readings may be given continuously or at intervals.
A Guide to the Measurement of Humidity