Dew point mirror hygrometer
The dew point hygrometer is a device for determining the dew point of water vapor in air or process gases . The dew point is the temperature of moist air, which must be fallen below with unchanged pressure so that water vapor can separate as dew or mist . The relative humidity at the dew point is 100%. The air is then saturated with water vapor . The dew point mirror hygrometer uses a temperature-controlled mirror to measure the dew point . During the measurement, the reflectivity of the mirror is measured, which then begins to decrease as the temperature drops, when the dew point is reached and the mirror steams up.
Measuring process
The chilled mirror hygrometer must be well ventilated (or flooded with process gas). If necessary, the mirror is first heated until it is free of condensation and therefore no longer fogged up. The mirror is then slowly cooled down again until it steams up. This is detected by measuring optics and the associated mirror temperature is then the dew point temperature.
Low moisture contents and thus low dew point temperatures require longer measuring times. While a measurement typically only takes a few seconds at high dew point temperatures, with trace moisture (e.g. at very low dew point temperatures) a measurement can take longer than an hour. Such long measurement times are not due to the fact that it takes a long time to reach the low temperatures, but rather to the fact that the gas then contains very few water molecules . Therefore you have to wait or let enough process gas pass the mirror so that enough molecules can deposit to become visible as condensate.
Physical basics
Humidity measuring devices ( hygrometers ) that work according to the condensation process use the formation of liquid water or ice (through sublimation).
The dew point, as the temperature of a moist gas mixture, at which the condensation and evaporation of the moist component are exactly balanced, is reached when the gas is just saturated with the vapor . The vapor partial pressure that prevails in this equilibrium state is the saturation vapor pressure . How high the saturation vapor pressure is at a given temperature can be seen from the phase diagram of the water or the substances in other gas mixtures. Since the non-condensable parts of the gas mixture have almost no effect on the behavior of the steam, the dew point of moist air depends almost exclusively on the partial pressure of the water it contains. Each such state of equilibrium is determined by a point in the pT diagram. If you connect all these points with each other, you get the dew point curve as a phase boundary line .
If the energy flow can be regulated to lower the temperature, the physical dependencies described also apply to open systems such as flow processes.
overview
Daniell dew point hygrometer
In 1820, John Frederic Daniell succeeded in measuring the humidity using a dew point hygrometer he had developed.
Layout and function
A U-shaped glass tube with legs pointing downwards is attached to a column that carries a thermometer to determine the air temperature. The two legs of the U-tube are formed into balls at the ends.
The left sphere A is partially filled with diethyl ether . In leg B there is a thermometer, the tip of which is immersed in the ether. In the interior of the entire U-tube C and also of the right-hand sphere D there is thus ether vapor with a partial pressure corresponding to the ambient temperature .
The right sphere is covered with fabric on the outside and ether is drizzled on the outside. Due to the evaporation of the outside applied ether, the temperature of the right sphere drops and the inner ether vapor condenses within this sphere. This reduces the partial pressure in the interior of the glass construction. However, since the partial pressure has to rise again according to the outside temperature, ether evaporates within the left sphere. This in turn leads to a drop in temperature in the left sphere.
If the temperature drops below the condensation temperature of the ambient air on the surface of the sphere, water condensate is formed on the left surface of the sphere. A stripe is gold-plated on the left ball for better recognition of the formation of condensation . When condensation sets in, the temperature is read on the inner left thermometer.
By tilting the apparatus, the ether can flow from the left sphere into the right one. The left sphere absorbs heat from the environment and the formation of condensation disappears again. The temperature is also read off the left thermometer. The dew point temperature is determined from the mean of the two measured temperature values.
Regnault dew point caps
Henri Victor Regnault , a Franco-German physicist and chemist, developed the dew point cap in 1845.
Layout and function
A partially mirrored glass vessel is filled with ether in the area of the mirror. The glass vessel is closed at the top after filling. Glass tubes are inserted through two openings, one tube dipping into the ether. The thermometer is also inserted through the cap into the ether.
Air is blown through the tube, which protrudes into the ether. The ether evaporates in the space through the absorption of heat from this air flow. The ether vapor emerges from the second opening.
The evaporation of the ether leads to a drop in temperature of the mirrored wall, on which a layer of condensate from the water from the ambient air then forms. The formation of deposits is perceived with the eye and the temperature of the surface, which is equal to the temperature of the ether, is measured with the thermometer.
This surface temperature is the dew point temperature.
Application area
The area of application of the dew point caps for measuring dew point temperatures extended a few degrees below the ambient temperature.
The measurement uncertainty was high because, on the one hand, the point in time of condensation depends on the sensitivity of the human eye to differentiation and, on the other hand, the reading uncertainty of the thermometer by humans is included.
Dew point box according to Lambrecht
Wilhelm Lambrecht significantly improved the ability to detect condensation.
The greatest uncertainty when measuring with the dew point cap is the determination of the time of condensation. The human eye is very sluggish when it comes to creeping change. Lambrecht introduced a split wall of mirrors.
Layout and function
A metal plate A, polished brightly on the front, carries a round metal box B on the back, into which a thermometer Th protrudes from above. The metal can is half filled with ether.
Air is blown through the ether through a pipe C by means of a hand blower. This evaporates and cools the whole vessel and also the air in its immediate vicinity.
When the dew point is reached, the water vapor contained in the air condenses and steams up the bare plate A. So that the moment when the plate is fogged up can be clearly observed so that the thermometer can be read immediately, the middle part of the bare surface is through a slot D of the part below.
The condensation deposit then only appears on the upper middle part of the surface connected to the metal can and can be easily recognized against the non-fogged part.
Automatically working chilled mirror according to Harold E. Edgerton
In 1965, Harold E. Edgerton succeeded for the first time in building an automatically working, cooled chilled mirror.
Layout and function
Automatic dew point mirrors consist of a mirrored surface, which is usually brought to the condensation temperature using a Peltier element . When the dew point temperature is reached, the water condenses on the mirror, reducing the reflectivity of the mirror. This process is automatically detected with optics and the current temperature is determined.
An essential element of automatically operating chilled mirror instruments is the heating and cooling element. For this purpose, Peltier elements are usually used, which go back to an invention by the French physicist Jean Peltier in 1834. He recognized that a solder joint of two different types of metal through which an electric current flows, heats up or cools down depending on the direction of the current. As a result, by regulating the current electrically, a certain temperature can be generated at a connection point between two different metals. The Peltier element was largely ignored until the middle of the 20th century . It was not until the development of semiconductors that Peltier elements were also developed significantly, and in the 1960s it led to the technical use of Peltier elements in heating and cooling technology .
In 1965 Edgerton (founder of the company EG & G 1947) succeeded in making the observation of the condensation independent of the human eye by developing an electro-optical scanning combined with an automatic temperature measurement of the mirror surface. With this and with the use of temperature-dependent measuring resistors , the last important step towards the automated measurement of the dew point temperature in open systems was done. Since the 1970s, dew point mirrors have been used to automatically measure the dew point temperature in open systems.
Chilled mirror today
The technical design of the closed mixed gas volume and the way in which the temperature is lowered is implemented differently for the individual devices.
Modern chilled mirrors are usually constructed as follows:
A mirrored metallic surface is cooled down to the condensation temperature using a Peltier element . When the dew point temperature is reached, the water condenses on the mirror. This reduces the reflectivity of the mirror. If the mirror is illuminated by means of a light emitting diode and the reflected light is captured by means of a photodiode , an electric current is generated which depends on the reflectivity of the mirror.
A second light beam, which is not influenced by the condensation phenomena, serves as a reference value . Both light beams have the same intensity before condensation and generate the same current on two photodiodes. The comparison of the two currents, preferably in a bridge circuit , is used to control the Peltier current . If the reflected beam deviates from the original illuminance, which is recorded by the reference beam, due to the appearance of condensation on the mirror, a difference signal is generated at the bridge, which is used to control the Peltier current.
At a constant dew point temperature, the Peltier current is regulated within very narrow limits, so that the temperature of the mirror surface is stable to ± 0.05 ° C. Condensation and evaporation processes are therefore in equilibrium.
The dew point temperature is measured by means of a temperature-dependent platinum measuring resistor (Pt100, Pt1000) as the surface temperature (or body temperature) of the mirror, free of drift and hysteresis .
Dew point mirrors with Peltier cooling can be used for a measuring range of −100 ° C to 100 ° C with a minimum measurement uncertainty of ± 0.1 K. Instead of or in addition to Peltier cooling, chilled mirrors can have cryostatic cooling with liquid nitrogen. Such setups can measure dew point temperatures down to −115 ° C.
Dew point mirrors are sensitive to solid and liquid particles in the measurement gas, as particles of any kind can both scatter the measurement light beam and interfere with the reflectivity of the mirror. The measuring method cannot distinguish whether the scattering particle is condensed water or dirt.
In combination with a gas temperature sensor and a pressure sensor, some devices are able not only to measure and display all three relevant physical quantities, but also to calculate all other parameters of the humidity using a built-in computer . There are two different groups of devices: those that take the enhancement factors into account in the calculation and those that work with the simple Magnus formula .
Hydrocarbon dew point
In practice, the dew point of water is probably the most frequently measured dew point. However, hydrocarbons , e.g. B. Solvent , a so-called hydrocarbon dew point which, depending on the process, is significant and must be measured.
An unmodified dew point mirror hygrometer is not suitable for measuring hydrocarbon dew points, the condensate of the hydrocarbons changes the mirror properties too little. Therefore, with hydrocarbon dew point sensors , one measures the scattered light from roughened surfaces, which is reduced by the formation of condensation.
Individual evidence
- ↑ Josef Reiner: The meteorological instruments. Rudolf A. Lang Verlag, Pössneck, 1949.
- ^ F. Kohlrausch: Practical Physics. Volume 1, BG Teubner Verlag, Stuttgart 1986.
- ↑ H. Gobrecht: Bergmann Schaefer textbook of experimental physics. Volume 1, de Gruyter Verlag, Berlin 1974, ISBN 3-11-004366-1 .
- ↑ Dr. Ing.Bernhard Prümm: condensation measurement method . ( Page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. on: pruemmfeuchte.de
- ↑ Dew point mirror for extreme trace moisture measurement S4000 TRS
literature
- D. Weber: Technical moisture measurement in gases and solids. Vulkan-Verlag, Essen 2002, ISBN 3-8027-3201-4 .
- HD Baehr, S. Kabelac: Thermodynamics. 14th edition. Springer-Verlag, Berlin 2009, ISBN 978-3-642-00555-8 .