FACTOID # 13: New York has America's lowest percentage of residents who are veterans.
 
 Home   Encyclopedia   Statistics   States A-Z   Flags   Maps   FAQ   About 
   
 
WHAT'S NEW
RELATED ARTICLES
People who viewed "Hygrometer" also viewed:
 

SEARCH ALL

FACTS & STATISTICS    Advanced view

Search encyclopedia, statistics and forums:

 

 

(* = Graphable)

 

 


Encyclopedia > Hygrometer
The interior of a Stevenson screen showing a motorized psychrometer
The interior of a Stevenson screen showing a motorized psychrometer

Hygrometers are instruments used for measuring humidity. A simple form of a hygrometer is specifically known as a "psychrometer" and consists of two thermometers, one of which includes a dry bulb and the other of which includes a bulb that is kept wet to measure wet-bulb temperature. Evaporation from the wet bulb lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer, which measures dry-bulb temperature. When the air temperature is below freezing, however, the wet bulb is covered with a thin coating of ice and yet 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. One device that uses the wet/dry bulb method is 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. Image File history File linksMetadata Download high resolution version (1632x1232, 660 KB) Interior of a Stevenson screen showing (from left to right) the dewcell, wet bulb with wick attached to water supply, dry bulb and dry thermistor. ... Image File history File linksMetadata Download high resolution version (1632x1232, 660 KB) Interior of a Stevenson screen showing (from left to right) the dewcell, wet bulb with wick attached to water supply, dry bulb and dry thermistor. ... Exterior of a Stevenson screen Interior of a Stevenson screen A Stevenson screen or Instrument shelter is a meteorological screen to shield instruments against precipitation and direct heat radiation from outside sources, while still allowing air to circulate freely around them. ... The term humidity is usually taken in daily language to refer to relative humidity. ... A clinical mercury thermometer A thermometer is a device that measures temperature or temperature gradient, using a variety of different principles. ... Wet-bulb temperature ... Vaporization redirects here. ... For other uses, see Temperature (disambiguation). ... The dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. ... A hygrometer used to measure the humidity of air. ... For the parapsychology phenomenon of distance knowledge, see psychometry. ...

Contents

Calibration

Accurate calibration of the thermometers used is of course fundamental to precise humidity determination by the wet-dry method; it is also important for the most accurate results to protect the thermometers from radiant heat and ensure a sufficiently high speed of airflow over the wet bulb. One of the most precise types of wet-dry bulb psychrometer was invented in the late 19th century by Adolph Richard Aßmann (1845-1918);[1] 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).[2] According to Middleton, 1966, "an essential point is that air is drawn between the concentric tubes, as well as through the inner one." [3]


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, (3) a second thermometer to measure the dry-bulb temperature of the heated air, then finally (4) a wet-bulb thermometer. According to the WMO 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." [4]. 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.

A sling psychrometer for outdoor use
A sling psychrometer for outdoor use

Other types of hygrometers are also commonly used to determine the ambient humidity. Such devices frequently use a human or animal hair under tension. The traditional folk art device known as a "weather house" works on this principle. In order to see changes that occur over time, several hygrometers record the value of humidity on a piece of graduated paper so that the values can be read off the chart. Image File history File linksMetadata Download high resolution version (1632x1232, 737 KB) A sling psychrometer, showing the wet bulb on top. ... Image File history File linksMetadata Download high resolution version (1632x1232, 737 KB) A sling psychrometer, showing the wet bulb on top. ... Hygrometers are instruments used for measuring humidity. ... A weather house is depicted on this New Years Day greeting card by the Czech artist Tavik Frantisek Simon. ...

Difficulty of accurate humidity measurement

Humidity measurement is among the more difficult problems in basic metrology. 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 driftso need regular recalibration. A further difficulty is that most hygrometers sense relative humidity rather than the absolute amount of water present, but RH is a function of both temperature and absolute moisture content, so small temperature variations within the air in a test chamber will translate into RH variations.


Dewpoint Hygrometers

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. Cooled mirror dewpoint hygrometers are the most precise instruments available. They 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 dewpoint temperature.


Modern instruments use electronic means of recording the information. The two most common electronic sensors are capacitive or resistive. The capacitive sensors sense water by applying an AC signal between two plates and measuring the change in capacitance caused by the amount of water present. The resistive sensors use a polymer membrane which changes conductivity according to absorbed water. Recently, an unbalanced AC Bridge approach was adapted for low power/energy operation and has shown to provide better measurement performance over a wide operating range. To further increase accuracy in this same device, which combines a sensor in data logging instrument, a calibration method utilizing a large memory array was developed to maximize performance.In most instruments, resistive sensors can be read by common meters or data acquisition boards. Temperature must also be measured, as it affects the calibration of all these sensors.


Besides greenhouses and industrial spaces, hygrometers are also used in some saunas, humidors and museums. In residential settings, hygrometers are used to aid humidity control (too low humidity damages human skin and body, while too high humidity favours growth of mildew and dust mite). The sling or motorized psychrometer is used in meteorology. For the music festival in Finland, see Sauna Open Air Metal Festival. ... A humidor is being prepared for use An Elie Bleu Medaille in blue A humidor is any kind of box or room with constant humidity (and often temperature as well) used to store cigars or pipe tobacco. ... Mildew is a grey, mold-like growth caused by one of two different types of micro-organisms. ... Binomial name Dermatophagoides pteronyssinus The house dust mite (Dermatophagoides pteronyssinus), sometimes abbreviated by allergists to HDM, is a cosmopolitan guest in human habitation. ... // Meteorology (from Greek: μετέωρον, meteoron, high in the sky; and λόγος, logos, knowledge) is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting. ...

Wikimedia Commons has media related to:
Hygrometer

See also

ASOS sensors, located at Salinas, CA Automated airport weather stations are automated sensor suites which are designed to serve aviation and meteorological observing needs for safe and efficient aviation operations and weather forecasting. ... Moisture analysis is the method of measuring moisture content for industrial applications. ... Dew cells are instruments used for determining the dew point. ...

References

  1. ^ "Aßmann, Adolph Richard" by Guido Heinrich
  2. ^ "Smithsonian Catalog of Meteorological Instruments in the Museum of History and Technology" Prepared by W. E. Knowles Middleton
  3. ^ A History of the Thermometer ISBN:0801871530 by W. E. Knowles Middleton, Johns Hopkins Press 1966
  4. ^ " http://www.wmo.ch/pages/prog/www/IMOP/publications/CIMO-Guide/Draft-7-edition.html WMO Guide To Meteorological Instruments And Methods Of Observation (Draft Seventh edition, 2006), Chapter 4: Humidity, section 4.2.5: Heated psychrometer." World Meteorological Organization

Image File history File links Question_book-3. ...

External links

A hemispherical cup anemometer of the type invented in 2000 by John Thomas Romney Robinson An anemometer is a device for measuring the velocity or the pressure of the wind, and is one instrument used in a weather station. ... A barograph is a recording aneroid barometer. ... A barometer is an instrument used to measure atmospheric pressure. ... A ceiling balloon cabinet A fully inflated ceiling balloon Diagram of a filler stand Regulator valve and pressure guages attached to helium cylinder A ceiling balloon is used by meteorologists to determine the height of the base of clouds above ground level during daylight hours. ... Ceiling projector exterior Interior of a ceiling projector Alidade for use with a ceiling projector The ceiling projector or cloud searchlight is used to measure the height of the base of clouds (called the ceiling) above the ground. ... Laser Ceilometer A ceilometer is a device that uses a laser or other light source to determine the height of a cloud base. ... Dark adaptor goggles The Dark adaptor goggles are one of the lesser known tools in the field of meteorology. ... A disdrometer is an instrument used to measure the drop size distribution and velocity of falling precipitation. ... Field mill is a specialized instrument used for measuring the strength of electrical fields in the atmosphere near thunderstrom clouds. ... Standard Ice Accretion Indicator (upside down) The Ice Accretion Indicator is an L-shaped piece of aluminium 38 cm (14. ... A FASOR used at the Starfire Optical Range for LIDAR and laser guide star experiments is tuned to the sodium D2a line and used to excite sodium atoms in the upper atmosphere. ... Lightning detector at the Kennedy Space Center in Florida. ... A nephelometer is an instrument for measuring suspended particulates in a liquid. ... Nephoscope is instrument for measuring the altitude, direction, and velocity of movement of clouds. ... Pan evaporation is a measurement that combines or integrates the effects of several climate elements: temperature, humidity, solar radiation, and wind. ... A pyranometer is a type of actinometer used to measure broadband solar irradiance on a planar surface. ... radiosonde with measuring instruments A radiosonde (Sonde is German for probe) is a unit for use in weather balloons that measures various atmospheric parameters and transmits them to a fixed receiver. ... Standard Rain Gauge Tipping Bucket Rain Gauge Recorder Close up of a Tipping Bucket Rain Gauge Recorder chart A rain gauge (also known as an udometer or a pluviometer[fluviograph] or a cup) is a type of instrument used by meteorologists and hydrologists to gather and measure the amount of... Snow Gauge A snow gauge is a meteorological instrument used to record the depth of snowfall at a specific location. ... SODAR (sonic detection and ranging) - meteorological instrument which priciple of operation is based on sound waves scattering by atmosheric turbulence. ... A solarimeter is a pyranometer used to measure combined direct and diffuse solar radiation. ... A sounding rocket, sometimes called an elevator research rocket, is an instrument-carrying suborbital rocket designed to take measurements and perform scientific experiments during its flight. ... Exterior of a Stevenson screen Interior of a Stevenson screen A Stevenson screen or Instrument shelter is a meteorological screen to shield instruments against precipitation and direct heat radiation from outside sources, while still allowing air to circulate freely around them. ... Sunshine Recorders are used to indicate the amount of sunshine at a given location. ... A thermograph is a recording thermometer. ... A clinical mercury thermometer A thermometer is a device that measures temperature or temperature gradient, using a variety of different principles. ... Rawinsonde weather balloon just after launch. ... Weather radar in Norman, Oklahoma with rainshaft (Source: NOAA) Environment Canada King City (CWKR) weather radar station. ... Weather vane Weather cock Aerovane A weather vane, also called a wind vane, is a movable device attached to an elevated object such as a roof for showing the direction of the wind. ... A windsock is a large, conical, open-ended tube designed to indicate wind direction and relative wind speed. ... A wind profiler is a piece of weather observing equipment that uses sound waves to detect the wind speed and direction at various elevations above the ground. ...

  Results from FactBites:
 
Hygrometer (379 words)
The hygrometer is a device for measuring the relative humidity of air.
The wick around the wet bulb was supplied with water from the bottom of a [missing] glass tube, which was held by the two clips in the center of the instrument.
The hygrometer in the middle column was purchased from Eimer and Amend in June 1894.
hygrometer: Definition and Much More from Answers.com (1232 words)
In an electrical hygrometer the change in the electrical resistance of a hygroscopic substance is measured and converted to percent relative humidity.
In a third group of hygrometers, commonly called dew-point apparatus, the dew-point temperature is determined; this is the temperature at which the moisture in the gas is at the point of saturation, or 100% relative humidity.
There are several types of hygrometers that are commonly used, one is specifically known as a "psychrometer" and consists of two thermometers, one of which has its bulb constantly kept wet which measures the wet-bulb temperature.
  More results at FactBites »

 
 

COMMENTARY     


Share your thoughts, questions and commentary here
Your name
Your comments

Want to know more?
Search encyclopedia, statistics and forums:

 


Press Releases |  Feeds | Contact
The Wikipedia article included on this page is licensed under the GFDL.
Images may be subject to relevant owners' copyright.
All other elements are (c) copyright NationMaster.com 2003-5. All Rights Reserved.
Usage implies agreement with terms, 1022, m