|WikiProject Physics||(Rated C-class, Mid-importance)|
|WikiProject Meteorology||(Rated C-class, Mid-importance)|
this company sells about 9 different hygrometers. Can anyone account for why the different units show somewhat different humidity percents? The readings generally diverge from each other by approximately 5 percent points. I've seen 7% difference once. Nastajus 07:14, 19 August 2006 (UTC)
Humidity measurement is one of the more challenging technical problems in basic physics, mainly because small microclimate variations from place to place make it nearly impossible to maintain a consistent RH throughout the environmental chamber used for calibrating the devices. Two thermometers can be compared by immersing them in an insulated vessel of water and stirring vigorously to minimize temperature variations within the container, but air is many orders of magnitude less efficient as a heat-transfer medium (just compare the cross-sectional area of the pipes used to distribute heat from the furnace in a house with hot-water heating to the cross-sectional area of the ductwork used to distribute heat in a house with forced-air heating!). Unless one wishes to purchase highly-specialized equipment costing thousands of dollars, the most accurate means available to most of us for humidity measurement is a good-quality wet-dry psychrometer. Actually if a bunch of cheap digital devices are all within 5 percentage points of each other they are doing rather better than is typical of such devices! 220.127.116.11 11 August 2007
It's possible to directly measure changes in the permittivity of air due to humidity changes with a pair of closely spaced electrodes as a capacitor. It's not necessary to have the vapor absorb into some media used as a dielectric. Changes in humidity are seen almost instantly. And the changes are a fair amount larger than those from inserting a 1 mil plastic film between the electrodes. --Aflafla1 (talk) 03:53, 28 July 2013 (UTC)
Does anyone have a picture of a chart recording Hygrometer? Preferably the kind with two pens: red for temperature and blue for humidity? RJFJR 18:59, 3 October 2005 (UTC)
- There are already three images of hygrometers on this page. How many do we really need? :-) Tim Pierce 05:19, 7 December 2005 (UTC)
Info on the metal(s) in a coil hygrometer would be good (there's already a picture of one). I bought some (non-adjustable) coil hygrometers and they appear to be extremely inaccurate, but I recall an accurate one from childhood.
I added a bit of info. One tip for gaining an accurate one that (surprisingly) worked for me: I went to a store when the temperature and humidity was fairly low (dewpoint in the upper 40's Fahrenheit) in the fall and selected one based on the known humidity that day, assuming that indoor and outdoor dewpoint were the same. I have it in my basement and I also have an electronic sensor down there and they are usually within ~3% of each other. The electronic sensor was checked in salt chambers at 33% and 75% and it was within 2% at those points. These two in my basement are usually closer than two identical electronic ones I have, so I might have just got lucky ;) --Ron E (talk) 15:10, 26 August 2011 (UTC)
Is this calibration info good? If folks think it is, let's add a link or adapt it for use in the article.
How humidity is measured The humidity is one of the important measurements that weather observers make because the amount of water vapor in the air determines whether clouds or fog are likely and whether it's going to rain or snow. Humidity refers to the amount of invisible water vapor in the air and various measurements are used to define it. If you don't understand terms such as "dew point," "relative humidity' and "saturation," you should read the USATODAY.com Understanding humidity page before continuing on this page. A dewpoint, or condensation, hygrometer is a common way to measure the humidity. Such an instrument has a mirror that exposed to the open air, and which can be cooled as precise track is kept of the mirror's temperature. As the mirror cools, dew (or frost if the temperature is below freezing) will form on It. The temperature at which this happens is the air's dew point (or frost point in below-freezing air). An observer could watch the mirror to see when dew or frost forms, but a more precise way is to use photoelectric cells to detect when dew fogs the mirror. Other kinds of hygrometers use materials with electrical resistance that varies with the amount of moisture absorbed. With such hygrometers, a measurement of electrical resistance can be calibrated as a humidity measurement. Human (or animal) hair turns out to be a pretty good way to measure the humidity, as anyone who's ever complained about a "bad hair day," can tell you. The graphic below shows how this works. Hygrometer hygrometer is a instrument used to measure the moisture content of a gas, as in determining the relative humidity of air. The temperature at which dew or frost forms is a measure of the absolute humidity—the weight of water vapor per unit volume of air or other gas at the temperature before cooling. Knowing absolute humidity and air temperature, the observer can calculate relative humidity. The most common type of hygrometer is the dry- and wet-bulb psychrometer. It consists of two identical mercury or electrical thermometers, one of which has a wet cotton or linen wick around its bulb. Evaporating water from the wick absorbs heat from the thermometer bulb, causing the thermometer reading to drop. The differences between dry-bulb and wet-bulb temperatures are compared on psychrometric charts. The temperature at which dew forms, called the dew point, is determined by a dew-point, or condensation-type, hygrometer, which is basically a mirror, usually of polished metal, cooled until dew or frost forms on it. Various cooling methods employ compressed carbon dioxide, dry ice, liquid air, or mechanical refrigeration. When dew or frost forms on the mirror, the temperatures between when dew appeared and when it disappeared is the dew point and is read with optical, electrical, or nuclear techniques. Because human observation varies, photoelectric cells are sometimes used to register the time at which the mirror fogs. Another type of hygrometer works on the principle that electrical resistance varies in a material that absorbs moisture, with the resistance to a current passing between wires measured by special sensors. The most accurate way to measure humidity is with an electric hygrometer, in which a known volume of gas passes over a hygroscopic, or moisture-absorbing, material such as phosphorus pentoxide. It is weighed before and after to determine how much water it took out of the gas. The older types of mechanical hygrometer use human hair, which stretches as it absorbs moisture, with a linkage connecting the center of a bundle of hairs under tension to a pointer. Another uses goldbeater's skin, a membrane from the intestines of oxen. Some materials can determine relative changes in humidity because they change color as they absorb moisture. Hygrometer From Wikipedia, the free encyclopedia Jump to: navigation, search
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 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. —Preceding unsigned comment added by 18.104.22.168 (talk) 16:35, 25 September 2008 (UTC)
There's lots of cheap digital hygrometers now available from places that sell consumer electronics. Are they any good? Are they basically the capacitive type described in the article and do they mostly obsolete the other types? Should there be more space devoted to them in the article? 22.214.171.124 (talk) 06:42, 24 January 2009 (UTC)
- You would probably get a better answer at the WP:Reference desk. However, as a quick answer, we seem to have problems with our digitals at low temperatures, −20 °C (−4 °F) and colder, reading too low, and these would not be cheap ones. CambridgeBayWeather Have a gorilla 11:14, 24 January 2009 (UTC)
I propose adding something to section about psychrometers
How do people feel about my adding the following to that section of the article?
"The dry bulb constitutes one of the two thermometers in a sling psychrometer. The other is called a sock. The person using the sling psychrometer covers the sock with water before putting the instrument into motion. In order to get a reading, the user slings (swinging it causing the two thermometers to rotate) the sling psychrometer. During the slinging, the water on the sock evaporates."
Here is the source: p. 308, Barron's Earth Science The Easy Way by Alan D. Sills. Hauppauge, New York: Barron's Educational Series, Inc., 2003.
- There is a sentence about sling psychrometers already there. By the way does the book really call the wet-bulb a sock? I've never heard it called that before. We call it a wet-bulb, (see Wet-bulb temperature) and the muslin sleeve that goes over it at below freezing temperatures is the sock, until it gets too cold and then we just make ice. But getting a nice even ice coating on a sling is a real pain. Enter CambridgeBayWeather, waits for audience applause, not a sausage 07:08, 9 December 2009 (UTC)
Searching the term humidistat, a device analogous to a thermostat but which responds to humidity not temperature, redirects to humidifier. A humidistat should have its own entry, but at least it should redirect to here, not humidifier. Many modern microwaves utilize a humidistat in "smart cooking" applications such as microwavable popcorn. Darqcyde (talk) 15:12, 8 September 2011 (UTC)
There is a sentence at the end of the paragraph that states "These devices make measurements a lot faster but are often not allowed in explosive environments.". Does anyone have a reference to backup this information? DevRockinAZ (talk) 08:31, 18 December 2014 (UTC)