chilled mirror hygrometer
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A hair tension dial hygrometer with a nonlinear scale. A hygrometer is an instrument used to measure the amount of
water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous p ...
in air, in soil, or in confined spaces. Humidity measurement instruments usually rely on measurements of some other quantities such as temperature, pressure, mass, 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 the temperature of condensation (called the
dew point The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will cond ...
), or they sense changes in electrical
capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized ar ...
or resistance to measure humidity differences. A crude hygrometer was invented by
Leonardo da Vinci Leonardo di ser Piero da Vinci (15 April 14522 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, Drawing, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially res ...
in 1480. Major leaps came forward during the 1600s; Francesco Folli invented a more practical version of the device, while Robert Hooke improved a number of meteorological devices including the hygrometer. A more modern version was created by Swiss polymath
Johann Heinrich Lambert Johann Heinrich Lambert (, ''Jean-Henri Lambert'' in French; 26 or 28 August 1728 – 25 September 1777) was a polymath from the Republic of Mulhouse, generally referred to as either Swiss or French, who made important contributions to the subject ...
in 1755. Later, in the year 1783, Swiss physicist and Geologist
Horace Bénédict de Saussure Horace Bénédict de Saussure (17 February 1740 – 22 January 1799) was a Genevan geologist, meteorologist, physicist, mountaineer and Alpine explorer, often called the founder of alpinism and modern meteorology, and considered to be the firs ...
invented the first hygrometer using human hair to measure humidity. The maximum amount of water vapor that can be held in a given volume of air (
saturation Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds **Saturated and unsaturated compounds ** Degree of unsaturation **Saturated fat or fatty aci ...
) varies greatly by temperature; cold air can hold less mass of water per unit volume than hot air. Temperature can change humidity.


Classical hygrometer


Ancient hygrometers

Prototype hygrometers were devised and developed during the
Shang dynasty The Shang dynasty (), also known as the Yin dynasty (), was a Chinese royal dynasty founded by Tang of Shang (Cheng Tang) that ruled in the Yellow River valley in the second millennium BC, traditionally succeeding the Xia dynasty and ...
in Ancient China to study weather. The Chinese used a bar of charcoal and a lump of earth: its dry weight was taken, then compared with its damp weight after being exposed in the air. The differences in weight were used to tally the humidity level. Other techniques were applied using mass to measure humidity, such as when the air was dry, the bar of charcoal would be light, while when the air was humid, the bar of charcoal would be heavy. By hanging a lump of earth and a bar of charcoal on the two ends of a staff separately and adding a fixed lifting string on the middle point to make the staff horizontal in dry air, an ancient hygrometer was made.


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 inexpensive devices, and its accuracy is limited, with variations of 10% or more. In these devices, water vapor is absorbed by a salt-impregnated paper strip attached to a metal coil, causing the coil to change shape. These changes (analogous to those in a bimetallic thermometer) cause an indication on a dial. There is usually a metal needle on the front of the gauge that will change where it points to.


Hair tension hygrometers

These devices use a human or animal hair under some tension. The hair is
hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substan ...
(tending toward retaining moisture); its length changes with humidity, and the length change may be magnified by a mechanism and indicated on a dial or scale. In the late 17th century, such devices were called by some scientists ''hygroscopes''; that word is no longer in use, but ''hygroscopic'' and ''
hygroscopy Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance' ...
'', which derive from it, still are. The traditional folk art device known as a weather house works on this principle. Whale bone and other materials may be used in place of hair. In 1783, Swiss physicist and geologist
Horace Bénédict de Saussure Horace Bénédict de Saussure (17 February 1740 – 22 January 1799) was a Genevan geologist, meteorologist, physicist, mountaineer and Alpine explorer, often called the founder of alpinism and modern meteorology, and considered to be the firs ...
built the first hair-tension hygrometer using human hair. The pulley is connected to an index which moves over a graduated scale (e). The instrument can be made more sensitive by removing oils from the hair, such as by first soaking the hair in
diethyl ether Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable li ...
.


Psychrometer (wet-and-dry-bulb thermometer)

A psychrometer, or a wet and dry-bulb thermometer, consists of two calibrated thermometers, one that is dry and one that is kept moist with distilled water on a sock or wick. At temperatures above the freezing point of water, evaporation of water from the wick lowers the
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
, such that the wet-bulb thermometer will be at a lower temperature than that of the dry-bulb thermometer. When the air temperature is below freezing, however, the wet-bulb must be covered with a thin coating of ice, in order to be accurate. As a result of the heat of sublimation, the wet-bulb temperature will eventually be lower than the dry bulb, although this may take many minutes of continued use of the psychrometer. Relative humidity (RH) is computed from the ambient temperature, 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 dry and wet thermometers coincide when the air is fully saturated, and the greater the difference the drier the air. Psychrometers are commonly used in
meteorology Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...
, and in the HVAC industry for proper refrigerant charging of residential and commercial air conditioning systems.


Sling psychrometer

A sling psychrometer, which uses thermometers attached to a handle is manually spun in free air flow until both temperatures stabilize. This is sometimes used for field measurements, but is being replaced by more convenient electronic sensors. A whirling psychrometer uses the same principle, but the two thermometers are fitted into a device that resembles a ratchet or football rattle.


Chilled mirror dew point hygrometer

Dew point is the temperature at which a sample of moist air (or any other
water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous p ...
) 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. They use a chilled mirror and optoelectronic mechanism to detect condensation on the mirror's surface. The temperature of the mirror is controlled by electronic feedback to maintain a dynamic equilibrium between evaporation and condensation, 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. More recently, spectroscopic chilled-mirrors have been introduced. Using this method, the dew point is determined with spectroscopic light detection which ascertains the nature of the condensation. This method avoids many of the pitfalls of the previous chilled-mirrors and is capable of operating drift free.


Modern hygrometers


Capacitive

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 hygrometers, the effect of humidity on the dielectric constant of a
polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
or
metal oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
material is measured. With
calibration In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of kno ...
, these sensors have an accuracy of ±2% RH in the range 5–95% RH. Without
calibration In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of kno ...
, the accuracy is 2 to 3 times worse. Capacitive sensors are
robust Robustness is the property of being strong and healthy in constitution. When it is transposed into a system, it refers to the ability of tolerating perturbations that might affect the system’s functional body. In the same line ''robustness'' ca ...
against effects such as condensation and temporary high temperatures.D.K. Roveti
Choosing a Humidity Sensor: A Review of Three Technologies
Sensors Magazine (2001).
Capacitive sensors are subject to
contamination Contamination is the presence of a constituent, impurity, or some other undesirable element that spoils, corrupts, infects, makes unfit, or makes inferior a material, physical body, natural environment, workplace, etc. Types of contamination ...
, drift and aging effects, but they are suitable for many applications.


Resistive

In resistive hygrometers, the change in
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paralle ...
of a material due to humidity is measured. Typical materials are
salts In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...
and
conductive polymer Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers ...
s. 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 (
relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...
).


Thermal

In thermal hygrometers, the change in
thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
of air due to humidity is measured. These sensors measure absolute humidity rather than relative humidity.


Gravimetric

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 that it is usually only used to calibrate less accurate instruments, called Transfer Standards.


Optical

An optical hygrometer measures the absorption of light by water in the air. A
light emitter A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
and a light detector are arranged with a volume of air between them. The attenuation of the light, as seen by the detector, indicates the
humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...
, according to the
Beer–Lambert law The Beer–Lambert law, also known as Beer's law, the Lambert–Beer law, or the Beer–Lambert–Bouguer law relates the attenuation of light to the properties of the material through which the light is travelling. The law is commonly applied t ...
. Types include the Lyman-alpha hygrometer (using Lyman-alpha light emitted by hydrogen), the krypton hygrometer (using 123.58 nm light emitted by
krypton Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
), and the differential absorption hygrometer (using light emitted by two lasers operating at different wavelengths, one absorbed by humidity and the other not).


Applications

Aside from greenhouses and industrial spaces, hygrometers are also used in some incubators,
sauna A sauna (, ), or sudatory, is a small room or building designed as a place to experience dry or wet heat sessions, or an establishment with one or more of these facilities. The steam and high heat make the bathers perspire. A thermometer in a ...
s,
humidor A humidor is a humidity-controlled box or room used primarily for storing cigars, cigarettes, cannabis, or pipe tobacco. Either too much or too little humidity can be harmful to tobacco products; a humidor's primary function is to maintain a ste ...
s and
museum A museum ( ; plural museums or, rarely, musea) is a building or institution that cares for and displays a collection of artifacts and other objects of artistic, cultural, historical, or scientific importance. Many public museums make thes ...
s. They are also used in the care of wooden musical instruments such as pianos, guitars, violins, and harps which can be damaged by improper humidity conditions. Hygrometers play a big part in firefighting as the lower the relative humidity, the more vigorously fuels may burn. In residential settings, hygrometers are used to assist in humidity control (too low humidity can damage human skin and body, while too high humidity favors growth of
mildew Mildew is a form of fungus. It is distinguished from its closely related counterpart, mould, largely by its colour: moulds appear in shades of black, blue, red, and green, whereas mildew is white. It appears as a thin, superficial growth consi ...
and
dust mite House dust mites (HDM, or simply dust mites) are various species of acariform mites belonging to the family Pyroglyphidae that are found in association with dust in dwellings. They are known for causing allergies. Biology Species The curren ...
). Hygrometers are also used in the coating
industry Industry may refer to: Economics * Industry (economics), a generally categorized branch of economic activity * Industry (manufacturing), a specific branch of economic activity, typically in factories with machinery * The wider industrial sector ...
because the application of paint and other coatings may be very sensitive to humidity and
dew point The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will cond ...
.


Difficulty of accurate humidity measurement

Humidity measurement is among the more difficult problems in basic metrology. According to the WMO Guide, "The achievable accuracies or humidity determinationlisted 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 (or alcohol, for temperatures below the freezing point 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. In a cold and humid environment, sublimation of ice may occur on the sensor head, whether it is a hair, dew cell, mirror, capacitance sensing element, or dry-bulb thermometer of an aspiration psychrometer. The ice on the probe matches the reading to the saturation humidity with respect to ice at that temperature, i.e. the frost point. However, a conventional hygrometer is unable to measure properly under the frost point, and the only way to go around this fundamental problem is to use a heated humidity probe.


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 (1845–1918); 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 (some modern versions use an electric fan with electronic speed control).Smithsonian Catalog of Meteorological Instruments in the Museum of History and Technology
Prepared by W. E. Knowles Middleton
According to Middleton, 1966, "an essential point is that air is drawn between the concentric tubes, as well as through the inner one."A History of the Thermometer by W. E. Knowles Middleton, Johns Hopkins Press 1966 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 (1) a thermometer to measure the ambient dry-bulb temperature, (2) the
heating element A heating element converts electrical energy into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element. Unlike the Peltier effect, this process is indepen ...
, (3) a second thermometer to measure the dry-bulb temperature of the heated air, then finally (4) a wet-bulb thermometer. According to the
World Meteorological Organization The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics. The WMO originated from the Intern ...
Guide, "The principle of the heated psychrometer is that the water vapor 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."." url= archiveurl= WMO Guide To Meteorological Instruments And Methods Of Observation (Seventh edition, 2008), Chapter 4: Humidity, section 4.2.5: Heated psychrometer." World Meteorological Organization 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 researchersSalt Calibration of Hygrometers
/ref> 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 (Sodium Chloride) 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 Airport weather stations are automated sensor suites which are designed to serve aviation and meteorological operations, weather forecasting and climatology. Automated airport weather stations have become part of the backbone of weather observin ...
* Dewcell * Humidistat *
Moisture analysis Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...
* Soil moisture sensor


References


External links


IMA moisture measurement training site



NIST page on humidity calibration



Article on RH sensors

NOAA homepage for cryogenic chilled-mirror frostpoint hygrometers
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