The hygrometer (from ancient Greek ὑγρός hygrós 'moist', 'wet' and μέτρον métron ' measure ', 'measure') is a measuring instrument for determining air humidity . The air temperature can be used to determine the water vapor content of the air from the humidity .
Measuring devices and measuring methods
Absorption hygrometers contain a hygroscopic (water-attracting) material, the properties of which change with moisture. The best known is the hair hygrometer . It contains mostly human hair that expands with increasing moisture. The difference in length between completely dry air (0% relative humidity (RH)) and saturated air (100% RH) is around 2.5%. In the past, various natural hairs from humans, sheep or horses were used; today only human hair and synthetic fibers are used. The traditional weather houses are basically hair hygrometers as well.
In the case of inexpensive hygrometers, rolled-up metal strips coated with plastic are sometimes used, which also react with linear expansion (similar to bimetal thermometers ). However, these so-called spiral hygrometers are usually not as precise as hair hygrometers.
Another possibility is to measure the weight gain of a material when it stores water.
Modern electronic absorption hygrometers, on the other hand, are based on changing the electrical properties of a sensor:
- Capacitive sensors: This type of sensor uses a hygroscopic layer as a dielectric between the two electrodes of a capacitor . The absorption of moisture in the dielectric changes its properties and consequently the electrical capacitance of the sensor. All non-conductors (weakly or non-conductive, non-metallic substances) with hygroscopic properties (plastic polymers , ceramic materials, etc.) come into consideration as dielectric .
- Impedance sensors (also resistive hygrometers): This type of sensor uses a hygroscopic layer between two direct current electrodes, the ohmic resistance of which changes due to the absorption of moisture. For example, aluminum oxide or hygroscopic plastic polymers are used for this. A special feature are impedance sensors that determine the alternating current resistance. Such sensors measure both the change in the ohmic resistance of a weakly conductive hygroscopic material and its capacitive and inductive properties.
Psychrometers consist of two thermometers of the same type, the mercury vessel or the temperature sensor of one of which is covered with a continuously moistened gauze stocking. Heat is removed from the wet thermometer by evaporation and, as a result, it shows a lower temperature than the dry thermometer. The temperature difference between the two thermometers is a measure of the relative humidity. With the help of the Sprung formula, all relevant moisture measurements can be calculated. With the help of graphic psychrometer boards, the relative humidity can also be determined directly on site without an invoice. As a rule, a distinction is made between mechanically working ( aspiration psychrometer , sling psychrometer ) and electronic devices. The theoretical basis for the device is provided by the mixing theory according to Dr. Sunday. It is assumed that both thermometers are continuously ventilated at approx. 3 m / s. Few psychrometers still work according to the diffusion theory, which assumes that there is no ventilation at the thermometer vessel.
Dew point mirror hygrometer
The determination of the dew point is a fundamental, comparatively easy to understand and precise measuring method. It is used today as the most precise method for defining national humidity standards. With the dew point mirror hygrometer , a mirror is cooled down until the humidity is reflected on it. The moment of condensation is determined with a light source and a photosensor . The dew point is always understood as a value pair consisting of the dew point temperature and the associated pressure (pressure that prevailed at the time of measurement), so the dew point can easily be converted into absolute humidity. The temperature of the sample at the time of the dew point measurement is only required for conversion into relative humidity values.
- With chemical hygrometers , the properties of a material change due to the chemical reaction with water, for example in the form of color changes that can be read on test strips. In the desiccant silica gel, cobalt (II) chloride is / was used as a moisture indicator : blue = dried, light pink = moist.
- Coulometric hygrometer A coulometric humidity sensor is a humidity sensor that measures the water vapor content of the air ( air humidity ) in a very low concentration ( trace humidity ) . The measurement is based on the property of diphosphorus pentoxide to absorb the water vapor contained in the environment.
- Optical hygrometers determine the air humidity by measuring the absorption of light with a certain wavelength characteristic of water vapor or by measuring the change in the refractive index .
In addition, there are a number of other options for determining air humidity, which are used comparatively rarely, for example the resistive method (determination of the impedance of the alternating current resistance of a hygroscopic element), the lithium chloride dew point hygrometer (measuring method based on the hygroscopic properties of lithium chloride based) or the measurement of the neutron deceleration (neutrons slow down when they hit hydrogen nuclei). All procedures have their advantages and disadvantages, there is no such thing as the ideal hygrometer for all applications.
Special methods are used to determine the soil moisture . A particular problem with this determination results from the soil's content of various salts, which can affect most measurement methods and often also have a corrosive effect on sensors.
- A capacitive moisture meter is inserted into the ground and measures the water content without contact. The measuring principle is the very large difference in the relative permittivity ε r of earth and water.
- Tensiometers contain a porous ceramic cell which is connected airtight to a pipe filled with water. This cell is inserted into the ground, whereby the water sucked out of the cell from the ground creates a negative pressure in the filled pipe. This negative pressure is directly related to the soil water content.
- Time Domain Transmission Sensors (TDT) work on the basis of electrical fields. With the TDT principle, an electrical impulse is sent through a cable embedded in the sensor. The resulting electrical field allows conclusions to be drawn about the moisture in the soil.
- Thermal determination , with this measuring method a hygroscopic body (e.g. made of clay ) is placed in the ground. After a while, the moisture content of the measuring body corresponds to that of the soil. The body is now heated, and the time is measured that is needed to heat the body by means of a glow wire, for example by 5 ° C. This period is directly related to the moisture content of the measuring body and thus to the soil water content. As an alternative to the filament, Peltier elements or microwaves can also be used.
Adjustment and calibration procedures
The calibration and adjustment are similar, but not identical processes. The aim of the adjustment is to obtain a device setting with minimal display errors. However, an adjustment does not say anything about deviations in the measurement results (setpoint to actual value); that is the job of calibration. The aim of calibration is to create a protocol (calibration certificate or calibration certificate), in this protocol the display deviation from a higher-order standard (setpoint to actual value) is documented - no changes (or adjustments) are made to the device during calibration, see here Calibrate versus justify
- A classic adjustment option for a hygrometer is the table salt method: To do this, place a small bowl of moist table salt in an airtight vessel and place the hygrometer next to the bowl with the salt. After about eight hours, a humidity equilibrium of 75% is established. The device can then be adjusted to this value .
- Another option for adjusting a hygrometer is to put some water in a sealable, transparent container (e.g. a fresh food container with a lid) and place the hygrometer on a small elevation (so that it does not get wet). the latter closes and waits a few hours for the water to condense on the walls of the vessel. This can be accelerated by placing the vessel in the sun and then (important!) Again in the shade for some time. So you wait until the air is saturated with water vapor. Now you quickly remove the hygrometer and set it to 100%. In order to finally check the setting again, use pure anhydrous glycerine instead of water (possibly other hygroscopic substances as well). This is put together with the hygrometer (increased) in the same, dry vessel (without water), closed and again waited a few hours. The hygrometer should then show 0%.
- The exact calibration takes place via the dew point , the most precise humidity measurements are carried out with dew point mirror hygrometers . Precise calibrations are therefore carried out by comparing them with measurement setups based on dew-point mirrors from the National Physico-Technical Institutions such as B. Physikalisch-Technische Bundesanstalt , National Physical Laboratory or their calibration laboratories, the German Calibration Service or the United Kingdom Accreditation Service .
- The humidity sensors used in industry are calibrated with the aid of humidity standards (humidity calibration standards). These are saturated salt solutions that create a specific equilibrium moisture content for the salt in a small airtight container. The container is manufactured with airtight bushings to match the respective humidity sensors. Various salts enable the production of reference humidities over the entire range from 0% RH to 100% RH. These humidity standards are also supplied with certificates that guarantee a certain level of accuracy. With careful work, accuracies of ± 1% RH to ± 2% RH can be achieved.
- Electronic humidity generators are available for standard calibration of a large number of humidity sensors. They generate the humidity set on the control panel in a small airtight chamber. The humidity sensors are inserted into the chamber with the aid of airtight bushings and can then be adjusted to the specified humidity value.
To the usual maintenance and adjustment at Haarhygrometern heard the regeneration . Since these hygrometers are based on natural fibers, which can lose their function if they dry out too long, this effect can be counteracted by occasional regeneration. Instruments that have already become inactive can be reactivated in this way. To do this, the device is wrapped in a damp cloth for about an hour or the measuring element, if directly accessible, is moistened with distilled water. The device should then display around 94–98% relative humidity. If this value is not displayed, it must be adjusted to this value. For this purpose, hair hygrometers have an adjustment screw that enables adjustment.
Architecture and construction
In the construction sector, various methods are used to determine moisture in order to check the moisture in the building structure:
- Inexpensive devices for resistance measurement have two tips that are inserted into the item to be tested (wall). Here the value partly depends on the force with which the tips are pressed in.
- Capacitive sensors measure the moisture in the wall using the capacitance of a capacitor. The devices usually have a penetration depth of 4–5 cm. The measured values depend on the material, so that the interpretation of the values requires knowledge of the building fabric. For example, an empty plastic pipe in the detection area also leads to a change in the measured capacitance.
In addition to locating penetrating or escaping moisture, such measurements are used, for example, to determine the degree of dryness of the screed for subsequent work.
Different methods of measurement were used as early as the Middle Ages, but they only provided inadequate results. Even today, the inflorescences of dried daisy family (Asteraceae), such as the acanthus-leaved boarwort , are used to predict rainy weather in southern France . An explanation can be found in the article silver thistle .
The first hair hygrometer was demonstrated by Horace-Bénédict de Saussure in 1783 . He used a blonde woman's hair for this. Around 1820 John Frederic Daniell succeeded in measuring the humidity above the dew point , in 1877 the astronomer Wilhelm Klinkerfues received a patent for a bifilar hygrometer with two parallel human hairs, and in 1887 Richard Assmann constructed the first psychrometer .
- Example of a humidity generator , accessed on February 28, 2012
- Dieter Weber: Technical moisture measurement: in gases and solids . Vulkan-Verlag, Essen 2002.