analytical & laboratory EquipmEnt 21
evaporates from the lithium chloride solution at a rate
that is controlled by the vapour pressure of water in
the surrounding air. When the reel begins to dry, the
resistance of the lithium chloride solution increases, and
less current flows through the winding. This allows the
reel to cool. This heating and cooling of the reel reaches
an equilibrium point where it neither takes on nor gives
off water, and the equilibrium temperature is directly
proportional to the dewpoint of the surrounding air.
4
A resistance type hygrometer utilises the principle
that electrical resistance varies in a material that
absorbs moisture. Special sensors are used to measure
the resistance to a current passing between wires. This
type of sensor is suitable for mass production and seems
to be most used for home appliances and consumer
Table 1. Typical calibration intervals for measurement
products. However, it may not measure accurately in very
equipment.
low or very high humidity environments.
Humidity and dewpoint sensors include:
1. Psychrometer 5
A capacitance type hygrometer measures humidity
by detecting the change in capacitance of a thin
2. Mechanical hygrometer polymer film. This type of sensor can easily achieve
3. Lithium chloride dewpoint indicator sufficient accuracy, and is mostly used in industry. The
4. Resistance type hygrometer patented HUMICAP humidity sensors manufactured by
5. Capacitance type hygrometer (dew indicator) Vaisala use this technology.
6. Mirror dewpoint indicator
Sensors 1–6 can measure general humidity levels. 6
A mirror dewpoint indicator utilises the occurrence
of dew at dewpoint temperature when air
Sensors 5 and 6 are also used for low dewpoint containing water vapour is cooled. A mirror is cooled
measurement. The principle of each technology is until it reaches the dewpoint of the gas in question. As
described briefly in the following. dew condensation forms, it changes the light reflected
1
A psychrometer is a simple form of a hygrometer, from the mirror. When the mirror surface reaches an
which consists of two thermometers. One has a dry equilibrium state whereby evaporation and condensation
bulb and the other a bulb that is kept wet to measure are occurring at the same rate, the temperature of the
wet-bulb temperature. The wet bulb cools by evaporation mirror is equal to the dewpoint temperature of the tested
of the water. The amount of evaporation, as well as gas.
cooling of the thermometer, depends on the humidity of Sensors mostly used in cleanrooms include the
the atmosphere. This data, together with humidity tables resistance type hygrometer, capacitance type hygrometer
or calculations, is used to determine the vapour pressure (dew indicator) and mirror dewpoint indicator. When
of water in the surrounding air, and relative humidity. selecting a suitable instrument, it is important not only
This is a method often used in laboratories and humidity to pay attention to the price and product specifications,
and temperature test chambers. but also to consider the measurement accuracy,
➠
2
A mechanical hygrometer measures and
records humidity using an instrument
International
Sl-Units Pressure Sl-Units Te ture
that expands and contracts with humidity
level
➞ ➞
mpera
changes, such as human hair. This type of
measurement has been used for a long time.
National Measurement
The accuracy of the method is not very good.
Standards Laboratory
National Standard
➞
National Standard
➞
3
A lithium chloride dewpoint indicator
is a measurement principle based on
In-house Laboratory
Primary Standard
➞
Primar
➞
y Standard
the hygroscopic characteristic (ability of
a substance to attract water molecules) of
Primary Humidity Genera
lithium chloride. The sensor consists of a
➞
tor
reel covered with an absorbent fabric and
a bifilar winding (two insulated wires, with Working Standard
➞
Working Standard
➞
currents travelling through them in opposite
directions) of inert electrodes. The reel is
Customer
coated with lithium chloride. An alternating
Calibration for Humidity and Temperature Instruments
current is passed through the winding
and the lithium chloride solution, causing Table 2. Example of a traceability chain for installed humidity and temperature
resistive heating. As the reel heats, water measurement units.
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