Product guide

89
Humidity and Dew-Point Instruments
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Roughly speaking, at room temperature, a change in
dew point of 1°C corresponds to a change in relative
humidity of 6 percent of the relative humidity value.
For example at 50% RH, an uncertainty in dew point of
±1°C corresponds to an uncertainty of ±3% RH.
A change of 1°C in the measurement of the ambient
temperature has almost exactly the same signifi cance.
The size of the effect under different conditions is
illustrated in Table 1.
Overall, a useful rule of thumb is that ±1°C uncertainty
in either dew point or temperature leads to an
uncertainty of ±6 percent of the relative humidity value.
A note on temperature measurement in air
A thermometer indicates its own temperature. It is
important to note this because a thermometer may not
always be at the same temperature as its surroundings.
Thermometers can be infl uenced by the temperatures
of other objects nearby (not the ones which are
intended to be measured). Thermometers can also
suffer from time lags, and self-heating errors may affect
electrical resistance thermometers. All these effects are
at their worst when a measurement is undertaken in
air, as opposed to in liquid. Errors from these sources
can easily amount to several tenths of a degree, so the
effects on relative humidity can be signifi cant, as shown
above.
2.2 The effects of pressure on humidity
measurement
Since all measurements of humidity stem from the
measurement of a vapour pressure of water, it follows
that variations in overall pressure of the gas system
may have an effect an the measured humidity.
Throughout this document, the values of pressure are
given in absolute terms (atmospheric pressure being
101325 Pa, or 1013 mbar), and not in ‘gauge’ pressures
(where atmospheric pressure would have a value of
zero).
In a gas mixture such as room air, the total pressure
P(total air) of the system can be expressed as the sum of
partial pressures:
p
(total)
= p
(nitrogen)
+ p
(oxygen)
+ p
(water)
+ p
(others)
(3)
It therefore follows that if any of the partial pressures
of the component gases varies, the total pressure
p(total) will vary. Also, if the total system pressure is
changed, either by compression or expansion, each of the
component partial pressures will BE changed by a similar
factor to p(total).
This basic rule can be applied to any measure of humidity
to predict the effect of changes in either component
or overall pressures. Below are some simple examples
showing the effect of pressure change.
Effect of doubling pressure on a relative humidity
of 40% RH at constant temperature, without
changing composition
Relative humidity is expressed as a ratio of vapour
pressures (the actual vapour pressure relative to the
saturation vapour pressure). Doubling p(total) will yield
a similar doubling of p(water). If the saturation vapour
pressure remains unchanged (i.e. if temperature is
unchanged) then a relative humidity of 40% RH would be
doubled to 80% RH.
As a general approximate rule, the actual relative
humidity value can be multiplied by the fractional change
in total system pressure to give the resultant value of
relative humidity. NOTE: Where the result exceeds 100%
RH, condensation WILL occur.
This rule is similar for other measures of concentration in
terms of mass per unit volume, such as absolute humidity
(g m
-3
).
Effect of doubling system pressure on mixing ratio,
without changing composition
The mixing ratio of water in a gas system is simply the
ratio of the mass of the water vapour to the mass of the
dry gas.
Table 1. Effect of a temperature change of 1 °C at various levels of temperature and relative humidity. The change in the relative
humidity levels is not symmetric.
Relative
humidity
Temperature
10°C 20°C 30°C 40°C 50°C 60°C
10% RH ±0.7% RH ±0.6% RH ±0.6% RH ±0.6% RH ±0.5% RH ±0.5% RH
50% RH ±3.5% RH ±3.2% RH ±3.0% RH ±3.0% RH ±2.6% RH ±2.3% RH
90% RH ±6.3% RH ±5.7% RH ±5.7% RH ±5.4% RH ±4.6% RH ±4.1% RH
A Guide to the Measurement of Humidity