A hygrometer is an instrument used for measuring the
moisture content in the environment.
Humidity measurement instruments usually rely on
measurements of some other quantity such as temperature, pressure, mass or a
mechanical or electrical change in a substance as moisture is absorbed
Hygrometers are used in greenhouses, however hygrometers are also used in some
incubators, saunas, humidors, industrial areas and museums.
They are also used in the care of wooden musical instruments
such as guitars and violins which can be damaged by improper humidity
conditions.
A hygrometer is used to aid humidity control (too low
humidity damages human skin and body, while too high humidity favours growth of
mildew and dust mites).
Humidity measurement instruments usually rely on
measurements of some other quantity such as temperature, pressure, mass or a
mechanical or electrical change in a substance as moisture is absorbed.
By calibration and calculation, these measured quantities
can lead to a measurement of humidity.
Modern electronic devices use temperature of condensation,
or changes in electrical capacitance or resistance to measure humidity changes.
Types of hygrometer:
Metal/pulp coil type
The familiar metal/paper coil hygrometer is useful for
giving a dial indication of humidity changes, but it appears most often in very
inexpensive devices and their accuracy is very limited.
A search through many identical units in a display might show differences in indicated humidity of 10% or more.
In these devices, humidity is absorbed by a salt-impregnated paper strip attached to a metal coil, causing it to change shape.
These changes in length (analogous to those in a bimetallic thermometer) cause an indication on a dial.
A search through many identical units in a display might show differences in indicated humidity of 10% or more.
In these devices, humidity is absorbed by a salt-impregnated paper strip attached to a metal coil, causing it to change shape.
These changes in length (analogous to those in a bimetallic thermometer) cause an indication on a dial.
Hair tension hygrometers
These devices use a human or animal hair under tension.
The length of the hair changes with humidity and the length change may be magnified by a mechanism and/or indicated on a dial or scale.
The traditional folk art device known as a "weather house" works on this principle.
The length of the hair changes with humidity and the length change may be magnified by a mechanism and/or indicated on a dial or scale.
The traditional folk art device known as a "weather house" works on this principle.
Electronic hygrometer
Dewpoint is the temperature at which a sample of moist air
(or any other water vapor) at constant pressure reaches water vapor saturation.
At this saturation temperature, further cooling results in condensation of water. Chilled mirror dewpoint hygrometers are one of the most precise instruments commonly available.
These use a chilled mirror and optoelectronic mechanism to detect condensation on the mirror surface. The temperature of the mirror is controlled by electronic feedback to maintain a dynamic equilibrium between evaporation and condensation on the mirror, thus closely measuring the dew point temperature.
An accuracy of 0.2 °C is attainable with these devices, which correlates at typical office environments to a relative humidity accuracy of about ±0.5%.
These devices need frequent cleaning, a skilled operator and periodic calibration to attain these levels of accuracy.
At this saturation temperature, further cooling results in condensation of water. Chilled mirror dewpoint hygrometers are one of the most precise instruments commonly available.
These use a chilled mirror and optoelectronic mechanism to detect condensation on the mirror surface. The temperature of the mirror is controlled by electronic feedback to maintain a dynamic equilibrium between evaporation and condensation on the mirror, thus closely measuring the dew point temperature.
An accuracy of 0.2 °C is attainable with these devices, which correlates at typical office environments to a relative humidity accuracy of about ±0.5%.
These devices need frequent cleaning, a skilled operator and periodic calibration to attain these levels of accuracy.