Hygrometer

The hygrometer ( from Ancient Greek ὑγρός hygrostat " moist, wet " and μέτρον métron " measure, measure " ) is a measuring instrument for determining the humidity. With the air temperature can determine the water vapor content of the air from the humidity.

Measuring instruments and methods

Hygrometer used to measure the humidity. Depending on the product model, determine the various moisture degree.

Absorptionshygrometer

Absorptionshygrometer contain a hygroscopic (water- attracting ) material whose properties change due to the humidity. Most famous is the hair hygrometer. It usually contains a human hair, which expands when wet. The difference in length when completely dry air 0 % relative humidity ( RH) and air saturated with 100% RH amounts to approximately 2.5%. Earlier this different natural hair of humans, sheep or horses were used, today human hair and synthetic fibers only ( synthetic fibers ) are used. The traditional weather house are in principle also hygrometer.

With inexpensive hygrometers are also partly coated with plastics, used rolled metal strip, which also react with a linear expansion (similar bimetallic thermometer ). However, this so-called Spiralhygrometer are usually not as precise as hair hygrometer.

Another possibility is to measure the weight gain in the storage of water. However, modern electronic Absorptionshygrometer based on the change in the electrical properties of the sensor:

• Capacitive sensor: This type of sensor uses a hygroscopic layer as a dielectric between the two electrodes of a capacitor. Due to the absorption of moisture in the dielectric to its properties and as a result, the electrical capacitance of the sensor to change. As a dielectric, are, inter alia, all non-conductor (weakly or non-conductive, non-metallic substances ) with hygroscopic properties considered ( plastic polymers, ceramics, etc.).
• Impedance sensors ( also Resistive Hygrometer ): This type of sensor uses a hygroscopic layer between two DC electrodes, the ohmic resistance changes due to the absorption of moisture. For example, for this purpose are aluminum oxide or hygroscopic plastic polymers. A special feature impedance sensors that determine the resistance with alternating current, thus such sensors measure both, both the change in resistance of a weakly conductive hygroscopic material, as well as its capacitive properties.

Psychrometer

Psychrometers consist of two identical thermometers with the mercury vessel and the temperature sensor of the one with a continuously moistened Mullstrumpf is covered. The wet thermometer is removed by heat of vaporization, and as a result shows a lower temperature than the dry thermometer. The temperature difference between two thermometers, is a measure of the relative humidity. Use the Sprung'schen formula can thus calculate all relevant humidity measurements. Using graphical psychrometric can the relative humidity determine directly without invoice on the spot. In general, a distinction is made between mechanical working ( Aspirationspsychrometer, Schleuderpsychrometer ) and electronic equipment. Provides the theoretical basis for the device, the mixture theory by Dr. Sunday. In this case, it is assumed that both the thermometer continuously aerated with about 3 m / s. Few psychrometer still work after diffusion theory, which assumes that the thermometer bulb there is no ventilation.

Hygrometer

The determination of the dew point is a fundamental, relatively easy to understand and precise measurement method. It is now used as the most accurate method for defining the national calibration standards. In the hygrometer, a mirror is cooled down until the humidity is reflected on it. With a light source and a photo sensor, the moment of the condensation is determined. The dew point is understood always as a pair of values ​​of dew point temperature and corresponding pressure ( pressure of the one existing at the time of measurement ), so can the dew point easily just convert to absolute humidity. Only for conversion into relative humidity values ​​are needed in addition the temperature of the sample at the time of dew point measurement.

Other methods

• Hygrometers in chemical properties is due to the chemical reaction of a material with a change of water, for example in the form of color changes which can be read on test strips. In desiccant silica gel is cobalt (II ) chloride / was used as a moisture indicator: blue = dry, light pink = wet.

• Coulometric hygrometer as coulometric moisture sensor is defined as a humidity sensor that measures the water vapor content of the air ( humidity) in very low concentrations ( trace moisture ). The measurement is based on the property of phosphorus pentoxide to absorb the water vapor contained in the ambient.

• Optical hygrometer to determine the relative humidity by measuring the absorption of light having a particular characteristic wavelength, or to water vapor, by measuring the change in refractive index.

There are also a number of other ways to determine the humidity, but these are comparatively rarely used, such as the Resistive method ( determining the impedance of the AC resistance of a hygroscopic element ), the lithium chloride dew point hygrometer (measuring method based on the hygroscopic property of the lithium chloride based ), or the measurement of the neutron braking ( neutrons are slower when hitting hydrogen nuclei ). All methods have their advantages and disadvantages, the optimal Hygrometer for all applications, there are not.

See also: coulometric moisture sensor, Hygrograph

Soil moisture

To determine the soil moisture special methods are used. A particular problem of this provision results from the content of the soil at various salts which can interfere with most methods of measurement and often corrosive to sensors.

• A capacitive moisture meter is plugged into the soil and measures contact the water content. Measuring principle is the very large difference in the dielectric constant εr of earth and water.

• Tensiometer include a porous ceramic cell which is hermetically connected to a water-filled tube. This cell is introduced whereby the floor of the cell produced by the water sucked in the filled tube a negative pressure in the ground. This negative pressure is directly related to soil water content.

• Time Domain Transmission sensors ( TDT ) work on the basis of electric fields. TDT in principle, an electrical pulse is transmitted through a conduit embedded in the sensor. The resulting electric field allows to draw conclusions on the moisture of the soil.

• Thermal provision for this measuring method, a hygroscopic body is placed (eg, clay ) in the soil. After a time corresponding to the moisture content of the measuring body of the soil. Then the body is heated, thereby the time period is measured, which is needed by the body by filament to heat, for example, 5 ° C. This period of time is directly proportional to the moisture content of the measuring body and thus to soil water content. Alternatively, the filament can also work with Peltier elements or microwaves.

Adjustment and calibration

Calibrating and adjusting are similar, but not identical operations. The aim of adjusting it is to obtain a device setting with minimal indication of failure. About deviations of the measurement results (target to actual value), however, an adjustment does not say anything, which is the object of the calibration. The aim of the calibration is to establish a protocol ( calibration certificate or calibration ), in this protocol, the accuracy error is a normal higher order (target to actual value) documented - the device will be no changes during calibration ( or adjustments ) performed, see calibration compared to Adjust

• Another classic adjustment possibility of a hygrometer is the saline method: One is to a small bowl of moist saline in a hermetically closed vessel and sets the hygrometer next to the bowl with the salt. After about eight hours a humidity balance of 75% is established. To this value, the device can then be adjusted.

• Another way to adjust a hygrometer is that you (such as a food storage box with lid ) is some water in a sealable, transparent vessel and the hygrometer on a small increase ( so it does not get wet ) is in the vessel, the latter closes and waits for a few hours until the water condensed on the vessel walls. Speed ​​can be this by placing the jar in the sun and then (important!) again for some time in the shade. So you wait until the air is saturated with water vapor. Now one removes quickly the hygrometer and sets it to 100 %. To check the setting again in conclusion, you now used instead of water, glycerin (highly concentrated as possible, possibly other hygroscopic substances). This one gives together with hygrometer (increases ) in the same vessel dry (without water), closes it and waits a few hours. The hygrometer should be 0 %.

• The exact calibration is performed above the dew point, the most accurate humidity measurements are performed with Taupunktspiegelhygrometern. Therefore precise calibration is performed by comparison with measurement setups taupunktspiegelbasierten the National Metrology Institutions such as Physikalisch-Technische Bundesanstalt, the National Physical Laboratory or their calibration laboratories German Calibration Service or United Kingdom Accreditation Service.

• The humidity sensor used in industry are calibrated using calibration standards ( moisture measurement standards ). These are saturated salt solutions, which produce a specific equilibrium moisture of the salt in a small airtight container. The container is made ​​with airtight penetrations match the respective humidity sensors. Various salts permit the preparation of Reference Wet over the whole range of 0 % RH to 100% RH. This humidity standards are delivered with certificates guaranteeing a certain accuracy. With careful work accuracies of ± 1% RH be achieved to ± 2% RH.
• For the standard calibration of a large number of humidity sensors there are electronic humidity generators. They generate the set on the control panel moisture in a small airtight chamber. With the help of air-tight feedthroughs humidity sensors are placed in the chamber and can then be adjusted to the predetermined humidity value.

Among the usual maintenance and adjustment at Haarhygrometern heard regeneration. Since these hygrometer based on natural fibers that can lose too long drying out their function, acts to counter this effect by an occasional regeneration. Already inactive become instruments can be reactivated. That the device is about an hour wrapped in a damp cloth or the measuring element, if directly accessible, moistened with distilled water. Then the device should show about 95 % relative humidity. If this value is not displayed, must be set to this value. For this hygrometer have an adjustment screw that allows adjustment.

History

In the Middle Ages, different methods were used to measure, however, yielded only insufficient results. Even today, in southern France, the inflorescences dried daisy family ( Asteraceae), such as the Akanthusblättrige Eberwurz, used for the prediction of rainy weather. An explanation can be found here.

The first hair hygrometer was demonstrated in 1783 by Horace - Bénédict de Saussure. He used a blonde woman hair. Around 1820 John Frederic Daniell succeeded in measuring the humidity is above the dew point, 1877 was the astronomer Wilhelm Klinkerfues a patent for a bifilar - hygrometer with two parallel strained human hair, 1887 Richard Assmann constructed the first psychrometer.