Product guide

Humidity and Dew-Point Instruments
© Michell Instruments 2009-2011
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NOTE: In chemical engineering the preferred term for
this concept is ‘volumetric absolute humidity’, while
‘absolute humidity’ is used to denote the quantity referred
to in this document as ‘mixing ratio’. In meteorology the
preferred term is ‘vapour concentration’. Other terms such
as ‘vapour density’, ‘mass concentration’ and ‘moisture
content by volume’ are also sometimes used to mean the
same thing.
USAGE: It is important not to confuse the particular
quantity ‘absolute humidity’ with the general category of
‘absolute measurements of humidity’.
Dew point (or dew-point temperature) – The
temperature at which dew, or condensation, forms,
on cooling a gas. This is, in effect, the temperature at
which air becomes saturated in equilibrium with water.
Expressed in degrees Celsius (°C) or occasionally in other
units of temperature (e.g. in degrees Fahrenheit (°F) in
USA). See also frost point.
USAGE: Negative dew points, with respect to super-
cooled water below 0°C, are always shown with a minus
(-) sign. Where there is any risk of ambiguity, a plus (+)
sign may also be used for positive dew points:
e.g. ‘a range of dew points between -5 °C and +5 °C’
The term ‘dew point’ is often used generally to include
‘frost point’ (sec below). However in the range just below
0 °C, where either frost or dew (super-cooled water) can
form, the values of dew point and frost point differ.
The use of initials (e.g. ‘dp’) is not a recognized
abbreviation, but it occurs widely, and is used to
distinguish clearly between dew-point temperatures and
other values of (air) temperature. For example a dew-
point value might be expressed in the form ‘1.0°C dp’
Enthalpy (of humid gas) – Measure of the total
energy in a humid gas. Enthalpy is a function of the gas
temperature and pressure, and of the moisture content,
since water absorbs energy on changing from condensed
state to vapour. Enthalpy is a useful concept in air
conditioning, where it is important to know how much of
the ‘stored’ energy will be consumed, or released, when
the temperature or water content is raised, or lowered.
Enthalpy of a gas can be defi ned as the sum of ‘sensible’
and ‘latent’ heat for each component in the gas. (See
below for defi nitions of sensible heat and latent heat.)
Values of enthalpy are conventionally expressed relative
to a datum point (i.e. a zero or base line). For a dry gas,
this is normally the heat content at 0°C. For water vapour,
the enthalpy is usually expressed relative to the heat
content of liquid water at 0.01°C.
Probe – the part of an instrument that houses the
sensor remotely from the main body of the instrument,
e.g. at the end of a connecting electrical lead. In some
situations the word ‘probe’ may be used to refer to an
entire hygrometer. Also loosely used interchangeably
with ‘sensor’ and ‘transmitter’. ‘Probe’ may also refer to
a tube used to extract gas for measurement.
Sensor – the active or sensing part of a measuring
instrument. There are some cases where a whole
hygrometer is referred to as a ‘sensor’. Also loosely
used interchangeably with ‘probe’ and ‘transmitter’.
Transmitter – instrument which normally gives an
electrical output (analog or digital) rather than a
displayed result. The sensing head may be an integral
part of the transmitter or may be connected via an
external cable. Also loosely used interchangeably with
‘probe’ and ‘sensor’.
Wet-bulb temperature – temperature indicated by a
thermometer sheathed in wet wicking, and infl uenced
by the rate of evaporation from the wicking. Usually
paired with a ‘dry-bulb’ temperature to derive a value of
relative humidity.
1.2 Defi nitions of measured quantities
Many of the following defi nitions are based on those
in the British Standard BS 1339: 1965 (confi rmed
1981, under revision in 1996), ‘Defi nitions, formulae
and constants relating to the humidity of the air’. As
given below, the defi nitions are explanatory rather than
rigorous. For rigorous defi nitions, BS 1339 should be
consulted. In practice, the usage of some terms varies
according to the context: for example the terms in the
fi eld of air-conditioning are sometimes different from
the terms used in meteorology for the same quantities.
In each case a preferred term is given below, but
qualifying notes indicate where there are common
alternatives in use.
Units of measurement for expressing the quantities are
given, and may have alternative forms, e.g. ‘grams per
cubic metre’ is given by ‘g m
-3
’, alternatively written
‘g/m
3
’.
Absolute humidity – The mass of water vapour
present in unit volume of moist air of a given
temperature and pressure. SI (metric) units are grams
of water per cubic metre of air (g m
-3
). Older references
may be in terms of pounds per million cubic feet (lb/
MMscf) or in grains per cubic foot (gr ft
-3
). (One grain
0.0648 gram.).
A Guide to the Measurement of Humidity