A hygrometer UK is an instrument used for measuring the moisture content in the atmosphere.
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 differences. The first practical
hygrometer was invented by polymath Johann Heinrich Lambert in 1755.
Types Metal-paper coil type
The metal-paper coil hygrometer is very useful for giving a dial indication of humidity changes.
It appears most often in very inexpensive devices, and its accuracy is limited, with
variations of 10% or more. In these devices, water vapour is absorbed by a salt-impregnated
paper strip attached to a metal coil, causing the coil to change shape. These changes 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.
Whale bone may be used in place of hair. In 1783, Swiss physicist and geologist, Horace
Bénédict de Saussure built the first hair-tension hygrometer, using human hair.
It consists of a human hair eight to ten inches long, b c, Fig. 37, fastened at one extremity
to a screw, a, and at the other passing over a pulley, c, being strained tight by a silk
thread and weight, d.
The pulley is connected to an index which moves over a graduated scale. The instrument
can be made more sensitive by removing oils from the hair, such as by first soaking the
hair in diethyl ether. Psychrometers
A psychrometer consists of two thermometers, one which is dry and one which is kept moist
with distilled water on a sock or wick. The two thermometers are thus called the dry-bulb
and the wet-bulb. At temperatures above the freezing point of water, evaporation of water
from the wick lowers the temperature, so that the wet-bulb thermometer usually shows a lower
temperature than that of the dry-bulb thermometer. When the air temperature is below freezing,
however, the wet-bulb is covered with a thin coating of ice and may be warmer than the
dry bulb. Relative humidity is computed from the ambient
temperature as shown by the dry-bulb thermometer and the difference in temperatures as shown
by the wet-bulb and dry-bulb thermometers. Relative humidity can also be determined by
locating the intersection of the wet and dry-bulb temperatures on a psychrometric chart. The
two thermometers coincide when the air is fully saturated, and the greater the difference
the drier the air. Psychrometers are commonly used in meteorology, and in the HVAC industry
for proper refrigerant charging of residential and commercial air conditioning systems.
Sling psychrometer
The sling psychrometer, where the thermometers are attached to a handle or length of rope
and spun around in the air for a few minutes, is sometimes used for field measurements,
but is being replaced by more convenient electronic sensors. Alternatively a whirling psychrometer
uses the same principle, however the two thermometers are fitted into a device that resembles a
ratchet or football rattle.
Chilled mirror dew point hygrometers Dewpoint is the temperature at which a sample
of moist air at constant pressure reaches water vapor saturation. At this saturation
temperature, further cooling results in condensation of water. Chilled mirror dewpoint hygrometers
are some 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 ±1.2%. These devices need
frequent cleaning, a skilled operator and periodic calibration to attain these levels
of accuracy. Even so, they are prone to heavy drifting in environments where smoke or otherwise
impure air may be present. Modern hygrometers
Capacitive humidity sensors For applications where cost, space, or fragility
are relevant, other types of electronic sensors are used, at the price of a lower accuracy.
In capacitive humidity sensors, the effect of humidity on the dielectric constant of
a polymer or metal oxide material is measured. With calibration, these sensors have an accuracy
of ±2% RH in the range 5–95% RH. Without calibration, the accuracy is 2 to 3 times
worse. Capacitive sensors are robust against effects such as condensation and temporary
high temperatures. Capacitive sensors are subject to contamination, drift and aging
effects, but are suitable for many applications. Resistive humidity sensors
In resistive humidity sensors, the change in electrical resistance of a material due
to humidity is measured. Typical materials are salts and conductive polymers. Resistive
sensors are less sensitive than capacitive sensors - the change in material properties
is less, so they require more complex circuitry. The material properties also tend to depend
both on humidity and temperature, which means in practice that the sensor must be combined
with a temperature sensor. The accuracy and robustness against condensation vary depending
on the chosen resistive material. Robust, condensation-resistant sensors exist with
an accuracy of up to ±3% RH. Thermal conductivity humidity sensors
In thermal conductivity humidity sensors, the change in thermal conductivity of air
due to humidity is measured. These sensors measure absolute humidity rather than relative
humidity. Gravimetric hygrometer
A gravimetric hygrometer measures the mass of an air sample compared to an equal volume
of dry air. This is considered the most accurate primary method to determine the moisture content
of the air. National standards based on this type of measurement have been developed in
US, UK, EU and Japan. The inconvenience of using this device means it is usually only
used to calibrate less accurate instruments, called transfer standards.
Applications Besides greenhouses and industrial spaces,
hygrometers are also used in some incubators, saunas, humidors 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. In residential settings, hygrometers are used to aid humidity
control. Hygrometers are also used in the coating industry because the application of
paint and other coatings may be very sensitive to humidity and dew point. With a growing
demand on the amount of measurements taken the psychrometer is now replaced by a dewpoint
gauge known as a dewcheck. These devices make measurements a lot faster but are often not
allowed in explosive environments. Difficulty of accurate humidity measurement
Humidity measurement is among the more difficult problems in basic meteorology. According to
the WMO Guide, "The achievable accuracies [for humidity determination] listed in the
table refer to good quality instruments that are well operated and maintained. In practice,
these are not easy to achieve." Two thermometers can be compared by immersing them both in
an insulated vessel of water and stirring vigorously to minimize temperature variations.
A high-quality liquid-in-glass thermometer if handled with care should remain stable
for some years. Hygrometers must be calibrated in air, which is a much less effective heat
transfer medium than is water, and many types are subject to drift so need regular recalibration.
A further difficulty is that most hygrometers sense relative humidity rather than the absolute
amount of water present, but relative humidity is a function of both temperature and absolute
moisture content, so small temperature variations within the air in a test chamber will translate
into relative humidity variations. Calibration standards
Psychrometer calibration Accurate calibration of the thermometers used
is fundamental to precise humidity determination by the wet-dry method. The thermometers must
be protected from radiant heat and must have a sufficiently high flow of air over the wet
bulb for the most accurate results. One of the most precise types of wet-dry bulb psychrometer
was invented in the late 19th century by Adolph Richard Aßmann; in English-language references
the device is usually spelled "Assmann psychrometer." In this device, each thermometer is suspended
within a vertical tube of polished metal, and that tube is in turn suspended within
a second metal tube of slightly larger diameter; these double tubes serve to isolate the thermometers
from radiant heating. Air is drawn through the tubes with a fan that is driven by a clockwork
mechanism to ensure a consistent speed. According to Middleton, 1966, "an essential point is
that air is drawn between the concentric tubes, as well as through the inner one."
It is very challenging, particularly at low relative humidity, to obtain the maximal theoretical
depression of the wet-bulb temperature; an Australian study in the late 1990s found that
liquid-in-glass wet-bulb thermometers were warmer than theory predicted even when considerable
precautions were taken; these could lead to RH value readings that are 2 to 5 percent
points too high. One solution sometimes used for accurate humidity
measurement when the air temperature is below freezing is to use a thermostatically-controlled
electric heater to raise the temperature of outside air to above freezing. In this arrangement,
a fan draws outside air past a thermometer to measure the ambient dry-bulb temperature,
the heating element, a second thermometer to measure the dry-bulb temperature of the
heated air, then finally a wet-bulb thermometer. According to the World Meteorological Organization
Guide, "The principle of the heated psychrometer is that the water vapour content of an air
mass does not change if it is heated. This property may be exploited to the advantage
of the psychrometer by avoiding the need to maintain an ice bulb under freezing conditions.".
Since the humidity of the ambient air is calculated indirectly from three temperature measurements,
in such a device accurate thermometer calibration is even more important than for a two-bulb
configuration. Saturated salt calibration
Various researchers have investigated the use of saturated salt solutions for calibrating
hygrometers. Slushy mixtures of certain pure salts and distilled water have the property
that they maintain an approximately constant humidity in a closed container. A saturated
table salt bath will eventually give a reading of approximately 75%. Other salts have other
equilibrium humidity levels: Lithium Chloride ~11%; Magnesium Chloride ~33%; Potassium Carbonate
~43%; Potassium Sulfate ~97%. Salt solutions will vary somewhat in humidity with temperature
and they can take relatively long times to come to equilibrium, but their ease of use
compensates somewhat for these disadvantages in low precision applications, such as checking
mechanical and electronic hygrometers. See also
Automated airport weather station Dewcell
Humidistat Moisture analysis
References
External links IMA moisture measurement training site
USATODAY.com: How a Sling Psychrometer Works NIST page on humidity calibration
Article on difficulty of humidity calibration Article on RH sensors
NOAA homepage for cryogenic chilled-mirror frostpoint hygrometers