A widely used 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 condensation of moisture from the air begins.
What is air humidity
A hygrometer measures the moisture content of the air, which can be represented as an absolute or relative value. The first of them gives simply the 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 a state of saturation, i.e. 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 of the air to its absolute humidity in the saturation state. When the water vapor in the air is saturated (again, at a given temperature), the relative humidity of that air is 100%. In air with unsaturated water vapor, it is, accordingly, less.
How a condensation hygrometer works
The principle of operation of any device for determining the humidity of the air, 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. A very important role in this process is played by the temperature at which vapor saturation occurs, called the dew point. As a rule, modern electronic devices for determining air humidity measure this temperature or changes in the electrical capacitance or resistance of various absorbent substances, which are then converted (automatically) into humidity indicators.
Condensation hygrometer device
His work is based precisely on the measurement of water vapor in the air by the dew point method. This method involves cooling a surface, typically a metal mirror, to a temperature at which the water on the surface of the mirror is in equilibrium with the vapor pressure of the water in the sample gas above the surface. At this temperature, the mass of water on the surface of the mirror neither increases (if the surface is too cold) nor decreases (if the surface is too warm), i.e. the vapor above the mirror is in dynamic equilibrium with the water condensate on the mirror (steam is saturated).
This mirror is made of a material with good thermal conductivity (like silver or copper) andplated with an inert metal such as iridium, rubidium, nickel or gold to prevent tarnishing and oxidation. The mirror is cooled by a thermoelectric cooler (Peltier effect) until condensation begins to form. A beam of light, usually from a solid-state broadband light emitting diode, is directed at a mirror surface, and a photodetector monitors the reflected light, the flow of which is maximum when there is no condensation on the mirror.
Chilled mirror hygrometer operation method
When dew drops form on the mirror surface of the mirror, the reflected light is scattered. In this case, its flux 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 that maintains a stable mirror temperature at the dew point. With a properly designed system, the mirror is maintained at a temperature where the rate of condensation is exactly equal to the rate of evaporation of the dew layer. An accurate miniature platinum resistance thermometer (PRT) mounted in the mirror measures its temperature at that point, which is automatically converted into a humidity reading.
The hygrometer for measuring air humidity of the considered design also includes a vacuum pump to pump in the analyzed portion of gas, and additional filtration elements in dirty conditions.
Advantages of the considered hygrometers
Such instruments, based on a simple principle of operation, with a wide measurement range, high accuracy and stable readings, are widely used in industry and scientific research. A typical dew point hygrometer, unlike many other humidity sensors, can be made very stable, virtually wear-resistant, minimizing the need for recalibration. The dew point humidity hygrometer is capable of measuring dew point in the temperature range from 100 °C to a minimum of -70 °C. In this case, the measurement accuracy is tenths of a degree.
Many hygrometers of the considered design are equipped with microprocessor control and, in combination with a resistive temperature sensor, are able to calculate and display on an 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 technical processes.
How much would a hygrometer like this cost? Its price, of course, is determined mainly by the 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 marketYou can also find a fully functional portable hygrometer. Its price is from 1 to 2 thousand dollars.
Disadvantages of condensation hygrometers
While the considered system of hygrometers is considered the most efficient in the measurement process, its disadvantage is the inevitable contamination of the parts of the measuring path during operation.
Hygrometers equipped with chilled 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 provides concentration centers on which dew or frost can form, thereby increasing the response time of the device. Soluble impurities affect the amount of vapor pressure from condensed moisture on the mirror, which shifts the dew point. Modern measuring hygrometers (at least their more sophisticated models) include "self-test" features that allow the device to detect and respond to contamination by making appropriate adjustments to the humidity calculation algorithms.
Regardless of these capabilities, virtually all hygrometers in question need to be checked and cleaned periodically.
Maintenance of Chilled Mirror Hygrometers
What does the instruction manual recommend to the user of the device in this sense. A hygrometer that is sensitive to dirt must bebe cleaned periodically to ensure the stability of the measurement results, although this may increase the cost of its maintenance. Inspection of the instrument's mirror is usually done with the built-in microscope, and maintenance is done manually after opening the measurement compartment.
If the cleaning of the mirror surface is performed at the frequency required in the instructions for its operation, then in this way it is possible to maintain the accuracy of measurements. Convenient access to the mirror surface for cleaning is usually provided by a hinge between the optical components and the mirror. You can now find any condensation hygrometer that the consumer needs on the market. The photo below shows an example of its execution.
Use of hygrometers in metrology
A properly designed and maintained chilled-mirror hygrometer provides humidity measurements with orders of magnitude greater accuracy than other popular moisture meters. Its inherent measurement accuracy, especially when equipped with a platinum resistance thermometer for temperature measurement, a mirror and a medium power microscope for mirror monitoring, 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 labs and contaminated environments
This air humidity meter is ideal for measuring its absolute value in factory climate laboratories. They are often used as references to control the accuracy of other instruments, such as relative humidity sensors used to control environmental test chambers.
The stability of the materials used in the construction of these hygrometers, as well as the ability to repeatedly clean them, make the instruments suitable for very long service life in environments with most contaminants without loss of calibration. This stability of performance makes them suitable for use in gas streams where high levels of contaminants in gas samples are irreversibly detrimental to less stable types of humidity sensors. For example, this type of hygrometer is widely used to control the dew point during heat hardening of the surfaces of metal products in an air environment with special impurities. In such cases, providing easy access to the mirror for cleaning is especially desirable.
Moisture sensitive production
Specialized packaging processes required in the production of pharmaceuticals, films, coatings and other products are often monitored by chilled mirror hygrometers. Again, their choice in this case is influenced by the stability of measurement accuracy and long service life. Moreover, since these processes tend to be less sensitive toinstrument 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
This type of hygrometer is often chosen to measure dew point temperatures above ambient temperature. Chilled mirror instruments were used as early as 1966 to monitor Apollo rocket hydrogen fuel cells operating at 250°C and 700 psi. With today's thermoelectric mirror cooling technologies, dew points up to 100 °C (and higher, assuming pressures above atmospheric pressure) are easily measured. In such cases, all surfaces of the hygrometer's measuring chamber that are 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 thermostatically controlled electric heaters to keep the walls of the measuring chamber above the highest expected dew points. Solid-state optical components such as LEDs and detectors are maintained at their nominal operating temperature (typically 85°C) to prevent degradation and damage to the hygrometer. This can be achieved by thermally insulating these components from the heated measurement chamber.
