Product guide

87
Humidity and Dew-Point Instruments
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Expressed in terms of energy per quantity of dry gas,
i.e. kilojoules per kilogram (kJ kg
-1
) (or other units, for
example British thermal units per pound, Btu/lb).
Equilibrium relative humidity (ERH) (over a
substance) – The value of relative humidity of the air,
at which there is no net exchange of moisture with any
nearby substance. This is used for indirectly indicating
or controlling the condition of moisture-sensitive
substances such as paper. Expressed as a percentage
(%). (See also water activity.)
Frost point (or frost-point temperature) – The
temperature at which frost forms on cooling a gas. This
is, in effect, the temperature at which air is saturated
in equilibrium with ice. It is the exact counterpart
to dew point (though values differ). Expressed in
degrees Celsius, °C, or occasionally in other units of
temperature, i.e. in degrees Fahrenheit (°F), in USA.
(See also dew point.)
Humidity ratio – Mixing ratio.
Latent Heat – Heat stored in a substance but not
directly related to its temperature. For example, heat is
stored in a gas because this heat was originally supplied
to evaporate it. ‘Latent’ means ‘hidden’. Expressed
in energy per unit mass of substance, i.e. joules per
kilogram (J kg
-1
). (See also enthalpy, sensible heat.)
Mixing ratio – Mass of water vapour per unit mass of
dry air with which it is associated. It is a dimensionless
ratio, but is often expressed in grams of water per
kilogram of dry gas (g kg
-1
) or in other units of mass.
For low levels of moisture content, this may be
expressed in parts per million by weight, i.e. mass of
water vapour per million parts mass of dry gas (ppm
w
or
ppm(w)).
NOTE: In chemical engineering this quantity is normally
termed ‘absolute humidity’ – but must not be confused
with the defi nition of ‘absolute humidity’ given above.
Mixing ratio is also alternatively known as ‘humidity
ratio’.
Moisture content – A humidity term best reserved for
general descriptive or qualitative use only. Use of this
term to identify a measured quantity should be avoided,
as there is a risk of confusion because ‘moisture
content’ has been used in the past to mean both mixing
ratio and specifi c humidity. Moisture content is also
a term particularly used to refer to the proportion of
water held in liquids or solids.
Mole – Amount of substance, which contains as many
elementary entities as there are atoms in 12 grams of
carbon 12. Expressed in moles (symbol, mol).
A Guide to the Measurement of Humidity
NOTE: When the mole is used, the elementary entities
must be specifi ed as atoms, molecules, etc.
Mole fraction – The mole fraction of a component is the
ratio of the amount (number of moles) of that component
to the total amount of substance present. Expressed as a
dimensionless ratio.
Partial pressure (of water vapour) – The part of the
overall pressure exerted by the water vapour component
in a gas. Expressed in units of pressure such as pascals
(Pa) or in multiples; kilopascals (kPa) or megapascals
(MPa), with non-SI alternatives such as millibar (mbar)
or millimetres of mercury (mm Hg). 100 Pa = 1 mbar
≈
0.75mm Hg.
Parts per million – Abbreviated as ‘ppm’, it must always
be stated whether this is by mass (weight) or by volume,
and whether the fi gure is the ratio of water vapour to dry
gas, or to total (moist) gas.
Parts per million by volume (ppm
V
, ppm(v)) –
Volume of water vapour per total volume of gas, for
an ideal gas. Sometimes expressed relative to the total
volume of moist gas (mole fraction times one million)
or sometimes relative to the total dry gas. For small
numbers of parts per million, the two are almost identical;
at higher humidity they become signifi cantly different.
Parts per million by weight, or mass (ppm
W
,
ppm(w)) – Sometimes used to express the amount
(mass) of water vapour relative to the total dry gas
(mixing ratio times one million), but sometimes to
express the amount relative to the total moist gas
(specifi c humidity times one million). For small numbers
of parts per million, the two are almost identical; at
higher humidity they become signifi cantly different.
Percentage saturation – The ratio of the actual
mixing ratio to the saturation mixing ratio at the same
temperature, expressed as a percentage (%).
NOTE: Under ordinary climatic conditions the percentage
saturation is almost identical to the relative humidity.
Relative humidity – The ratio of the actual vapour
pressure to the saturation vapour pressure over a plane
liquid water surface at the same temperature, expressed
as a percentage. This is commonly understood when
the term ‘X percent relative humidity’ is used. For actual
vapour pressure, e, and saturation vapour pressure, e
s
relative humidity (in %) = ––– x 100 (1)
e
e
s