A widespread instrument for measuring the humidity of air (and other gases) is a condensation hygrometer. Its principle of operation is to measure the temperature, called the dew point, at which the condensation of moisture from the air begins.
What is air humidity
The hygrometer measures the moisture content in the air, which can be represented by an absolute or relative value. The first of them gives just a mass of water vapor in 1 cubic meter. m of air at a given temperature. But the second one shows how close the water vapor in the air is to the state of saturation, that is, to dynamic equilibrium with its liquid phase - when there is neither evaporation nor condensation. It is equal to the ratio of the measured absolute humidity to its absolute humidity in a state of saturation. When water vapor in the air is saturated (again, at a given temperature), the relative humidity of such air is 100%. In air, with unsaturated water vapor, it is correspondingly smaller.
How does a condensing hygrometer work
The principle of operation of any device for determining air humidity, as a rule, is to measure some other quantity, such as temperature, pressure, mass, or mechanical and electrical changes in a substance that absorbs moisture. By appropriate calibration and calculation, these measured values can lead to the determination of absolute or relative humidity. The temperature at which vapor saturation occurs, called the dew point, plays a very important role in this process. As a rule, modern electronic devices for determining air humidity measure precisely this temperature or changes in the electrical capacitance or resistance of various substances that absorb moisture, which are then converted (automatically) into humidity indicators.
Condensing Hygrometer: Device
His work is based specifically on measuring the water vapor content of air using the dew point method. This method involves cooling the surface, usually a metal mirror, to a temperature at which the water on the mirror surface is in equilibrium with the vapor pressure of the water in the gas sample above the surface. At this temperature, the mass of water on the surface of the mirror neither increases (when the surface is too cold) nor decreases (when the surface is too warm), i.e., the vapor above the mirror is in dynamic equilibrium with water condensate on the mirror (the vapor is saturated).
Such a mirror is made of a material with good thermal conductivity (like silver or copper) and is coated with a layer of an inert metal such as iridium, rubidium, nickel or gold to prevent tarnishing and oxidation. The mirror is cooled using a thermoelectric cooler (Peltier effect) before the formation of condensate. A light beam, as a rule, from a solid-state broadband light-emitting diode, is directed to a mirror surface, and a photodetector monitors reflected light, the flux of which is maximum in the complete absence of condensate on the mirror.
The method of operation of the hygrometer with a cooled mirror
When dew drops form on the mirror surface of the mirror, the reflected light is scattered. At the same time, its flow entering the photodetector decreases, which leads to a change in the output signal of the latter. This, in turn, is controlled by an analog or digital thermoelectric cooler control system, which maintains a stable mirror temperature at the dew point. With a properly designed system, the mirror is maintained at a temperature at which the condensation rate is exactly equal to the evaporation rate of the dew layer. An accurate miniature platinum resistance thermometer (PRT) mounted in a mirror measures its temperature at this point, which is automatically converted to a humidity reading.
The hygrometer for measuring air humidity of the considered design also includes a vacuum pump to pump the analyzed portion of gas, and additional filtering elements in dirty conditions.
The advantages of the hygrometers in question
Similar devices based on a simple principle of operation, with a wide range of measurements, high accuracy and stability of readings, are widely used in industry and in scientific research. A typical condensation hygrometer, unlike many other humidity sensors, can be made very stable, virtually wear-free, which minimizes the need for re-calibration. A humidity hygrometer at the dew point is capable of measuring it in the temperature range from 100 ° C to a minimum of -70 ° C. Moreover, the measurement accuracy is tenths of a degree.
Many hygrometers of this design are equipped with microprocessor control and, in combination with a resistive temperature sensor, are able to calculate and display on the external indicator any desired humidity parameters in addition to or instead of the dew point. In addition, these devices allow the transfer of results using wireless technology. Naturally, such devices are widely used as part of various industrial systems for automated data collection and control of relevant technological processes.
How much can a hygrometer cost? Its price, of course, is determined mainly by a set of implemented functions, depending on the availability and complexity of the electronic control system of the device. So, a stationary condensation hygrometer that looks like a digital oscilloscope costs at least $ 4,000. Particularly "advanced" models can cost more than $ 10,000. On the market you can find a fully functional portable hygrometer. Its price is from 1 to 2 thousand dollars.
Disadvantages of Condensing Hygrometers
While the system of hygrometers under consideration is considered the most effective in the process of measurement, its drawback is the inevitable contamination of the parts of the measuring path during operation.
Hygrometers equipped with cooled mirrors tend to increase measurement inaccuracies due to the presence of soluble and insoluble contaminants deposited on the mirror. Insoluble particles affect the optical characteristics of the mirror. Moderate dustiness or the appearance of insoluble particles on the mirror ensures the presence of concentration centers on which dew or frost can form, thereby increasing the response time of the device. Soluble impurities affect the vapor pressure of condensed moisture on the mirror, which shifts the dew point. Modern measuring hygrometers (at least their more complex models) include “self-test” functions that allow the device to detect and respond to contamination by introducing appropriate amendments to the algorithms for calculating humidity indicators.
Regardless of the availability of such capabilities, almost all hygrometers in question need periodic inspection and cleaning.
Maintenance of hygrometer with a cooled mirror
What recommends the user of the device in this sense its instruction. A hygrometer that is sensitive to contamination must be periodically cleaned to ensure the stability of the measurement results, although this may increase the cost of its maintenance. Inspection of the instrument mirror is usually carried out using the built-in microscope, and its maintenance is done manually after opening the measurement compartment.
If the mirror surface is cleaned with the frequency required in the instruction manual for its operation, then the measurement accuracy can be maintained. Convenient access to the mirror surface for cleaning is usually provided by a hinge between the optical components and the mirror. On the market now you can find any condensing hygrometer that a consumer needs. The photo below shows an example of its execution.
The use of hygrometers in metrology
A properly designed and maintained hygrometer with a cooled mirror provides humidity measurements with an accuracy several orders of magnitude greater than other popular moisture meters allow. Their inherent accuracy of measurements, especially when equipped with a platinum resistance thermometer for measuring temperature, a mirror and a medium power microscope for monitoring the state of the mirror, makes it ideal for metrological measurements. The possibilities of transmitting information via wireless digital communication channels open up wide possibilities for using such hygrometers in global systems for collecting and processing meteorological information.
Use in factory laboratories and contaminated environments
These instruments for determining air humidity are ideal for measuring its absolute value in factory climate laboratories. They are often used as references to monitor the accuracy of other instruments, such as relative humidity sensors, used to control cameras for climatic testing.
The stability of the characteristics of the materials used in the construction of these hygrometers, as well as the possibility of their multiple cleaning, make the devices suitable for very long-term service in environments with the majority of pollutants without loss of calibration. This stability of characteristics makes them suitable for use in gas flows where high levels of contaminants in gas samples have irreversibly harmful effects on less stable types of humidity sensors. For example, this type of hygrometer is widely used to control the dew point during thermal hardening of the surfaces of metal products in air with special impurities. In such cases, providing easy access to the mirror for cleaning is particularly desirable.
Moisture-sensitive production
Specialized packaging processes needed in the manufacture of pharmaceuticals, films, coatings and other products are often monitored by refrigerated mirror hygrometers. Again, their choice in this case is affected by the stability of the accuracy of the measurements and the long service life. In addition, since these processes are usually less sensitive to instrument costs, the high cost of these hygrometers is not a determining factor in choosing a humidity monitoring scheme.
High temperature gases and their dew points
Hygrometers of this type are often chosen to measure dew point temperatures above ambient temperature. Instruments with cooled mirrors were used as early as 1966 to monitor Apollo hydrogen fuel cells operating at a temperature of 250 ° C and a pressure of 700 psi. With today's thermoelectric mirror cooling technologies, dew points up to 100 ° C (and higher, provided that the pressure is above atmospheric) are easily measured. In such cases, all surfaces of the measuring compartment of the hygrometer in contact with the gas sample must have a temperature above the highest expected dew point, otherwise condensation will occur on these surfaces, and the measurement will be erroneous.
In hygrometers designed to measure the dew point of high-temperature gases, it is common practice to use electric heaters with a temperature controller to maintain the temperature of the walls of the measurement compartment above the highest expected dew points. Solid-state optical components, such as LEDs and detectors, are maintained at their rated operating temperature (typically 85 ° C) to prevent their destruction and failure of the hygrometer. This can be achieved by thermally isolating these components from the heated measuring compartment.