Product guide

Humidity and Dew-Point Instruments
© Michell Instruments 2009-2011
88
USAGE: The phrase ‘relative humidity’ is commonly
abbreviated RH although this is not a recognized
abbreviation. Values of relative humidity are commonly
expressed in units of percent relative humidity (% RH).
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.
Saturation vapour pressure (of water) – Maximum
pressure of water vapour that can exist at a given
temperature. Expressed in units of pressure e.g. in
pascals (Pa), or in non-SI units such as millibars (mbar)
or millimetres of mercury (mm Hg).
Sensible heat (of a gas) – Energy that resides in a
gas according to its temperature. Expressed in terms of
energy per mass of gas, e.g. in joules per kilogram (J
kg
-1
), or equivalent units. (See also enthalpy, latent heat.)
Specifi c humidity – Mass of water vapour per unit mass
of humid air. May be expressed as a dimensionless ratio,
or in grams of water per kilograms of humid gas (g kg
-1
)
or in kilograms per kilogram (kg kg
-1
)
Vapour pressure – That part of the total pressure
contributed by the water vapour. Expressed in units of
pressure e.g. in pascals (Pa), or in non-metric units such
as millibars (mbar) or millimetres of mercury (mm Hg).
Water activity (of a substance) – Water activity (a)
is the relative humidity, which is eventually reached in a
closed space where a hygroscopic substance, such as a
foodstuff, has been placed. It is the same as equilibrium
relative humidity (ERH) except that it is expressed on a
scale of 0 to 1 (no units), instead of 0% to 100%. Water
activity is particularly used in connection with foodstuffs.
(See also equilibrium relative humidity.)
2 Signifi cance of temperature and
pressure for humidity measurement
2.1 The effects of temperature on
humidity measurement
The effect of temperature on humidity is highly
signifi cant. Failure to take this into account can
sometimes lead to errors so large that the measurement
is meaningless. In many situations, the largest single
source of uncertainty in a humidity measurement is the
effect of temperature differences from place to place
in the process, room or chamber. The importance of
considering the temperature effects carefully cannot be
overstated when relative humidity is the parameter of
interest.
Temperature and condensation
One common cause of error in humidity measurement is
the occurrence of unwanted condensation. Condensation
can occur at cold spots, which are below the dew point
of the gas. In sampling systems, any condensation
totally invalidates the sampling process, since it changes
the water vapour content of the gas. To prevent
condensation, sample systems should always be kept
at a temperature above the maximum dew point, by
heating them if necessary.
Temperature and absorption or desorption of
water
Many materials contain moisture as part of their
structure; particularly organic materials, salts, and
anything which has small pores. The quantity of water
in these materials depends on the humidity of the
surrounding gas, and on the temperature. When the
temperature changes, water migrates from the material
to the surrounding gas or vice versa. Like condensation,
this can cause changes in the measured humidity.
However, unlike condensation, there is not usually a
critical temperature: whenever the temperature changes,
water moves between the material and the gas.
When measuring low dew points, desorption or
absorption of water vapour as the temperature changes
can produce very large errors in measurement. Even
in normal ambient conditions, absorption or desorption
can begin to be signifi cant, depending on the type of
material.
Temperature and saturation vapour pressure of
water
The saturation vapour pressure of water depends
strongly on temperature. Near room temperature, the
air’s capacity to hold water vapour doubles for every
10°C increase in temperature. The steepness of this
variation gradually changes across the temperature
range. At 80°C the saturation vapour pressure doubles
for every 20°C rise. At -60°C the saturation vapour
pressure doubles for only a 5°C rise in temperature.
Temperature and relative humidity
Relative humidity is highly dependent on temperature –
especially so because vapour pressure appears twice in
the formula for relative humidity,
relative humidity (in %) = ––– x 100 (2)
(e is the water vapour pressure, and e
s
is the saturation
vapour pressure at the prevailing ambient temperature.)
e
e
s
A Guide to the Measurement of Humidity