The humidity of a gas may be measured by a variety of techniques. Some are based on the determination of the partial pressure of vapor in the gas (e.g., dew point determination);
Measurements of wet and dry bulb temperatures involve the use of the psychometric chart. There are other methods which depend on the influence of humidity on the physical properties and behavior of various substances.
Measurement of Dew Point
This is the determination of the temperature at which the partial pressure of the water vapor is equal to its saturation value. It involves gradually reducing the temperature of a mirror, or other highly polished surface, within the gas and determining the temperature at which condensation is first observed. This temperature is known as the dew point. The partial pressure of water in the gas is, therefore, that of saturated gas at this temperature. Therefore, the required value of humidity can be obtained from the saturation curve on the psychrometric chart.
The reliability of the measurement depends on the accuracy with which the first formation of dew can be detected. This may be done photometrically using a photo-resistive optical-sensing bridge. When the humidity is so low that no condensation occurs until the surface temperature has been reduced below 0°C, the moisture is deposited as a frost; this may be detected by methods based on alpha particle attenuation. Good control of the cooling of the mirror may be achieved by using a thermoelectric module in which heat is transferred away from the polished surface by the Peltier mechanism.
As an alternative to reducing the temperature of the surface, this may be maintained constant (typically at 0°C) and the gas compressed until the partial pressure of the water vapor reaches the saturation value when moisture starts to be deposited on the surface. For a gas at a pressure p1 with a partial pressure of water of pv1, compression to a pressure p2 will be necessary for moisture deposition to occur, where p2/p1 = pvs/v1 , pvs being the saturation vapor pressure at the temperature of the surface.
Measurement of Wet and Dry Bulb Temperatures
If a stream of gas of known (dry bulb) temperature is passed rapidly over a small wet surface sufficiently rapidly for the condition of the air not to be significantly changed, a dynamic equilibrium is established with the heat transferred from the air being exactly balanced by the latent heat required to vaporize the water at the surface. If the air flow rate is such that forced convection is the dominant mechanism by which heat is transferred (conduction and radiation effects then negligible), the ratio of the coefficients of heat and mass transfer will be constant and the equilibrium temperature to which the surface falls will be the wet bulb temperature. For the air-water system, the psychrometric ratio is approximately unity and the wet bulb and adiabatic saturation temperatures are almost identical.
Therefore, the adiabatic cooling lines on the psychrometric chart represent the compositions of all air-water vapor mixtures, not only with the same adiabatic saturation temperatures, but also with the same wet bulb temperatures. Thus, if the adiabatic cooling line corresponding to the measured wet bulb temperature is selected, the point on this line corresponding to the dry bulb temperature will indicate the humidity of the gas.
A wet and dry bulb hygrometer incorporates two thermometers, one with a bare (dry) bulb and the other (the wet bulb) covered with a porous fabric which is maintained saturated with water. Air is drawn rapidly (>5ms−1) over the thermometer bulbs and, when the temperature of the wet bulb falls to an equilibrium value, both temperature readings are taken.
Chemical/Physical Absorption Methods
A metered volume of gas is passed over a suitable absorbent whose increase in mass is determined. The method is very accurate, but somewhat laborious. Suitable absorbents for water vapor include phosphorus pentoxide dispersed on pumice and concentrated sulfuric acid.
Electrical Conductivity Measurements
The electrical resistance of thin films of hygroscopic materials, such as lithium chloride, is a function of temperature and of moisture content which itself depends on the humidity of the atmosphere to which it is exposed. In a lithium chloride cell, a skein of very fine fibers is wound on a plastic frame carrying two electrodes in contact with the fibers. The current flowing for a constant voltage difference applied across the electrodes gives a measure of electrical resistance. The instrument can be calibrated to give direct readings of humidity.
An alternative system utilizes a sensor in the form of a hydroxyethyl-cellulose film incorporating a matrix of conducting carbon particles which expands and contracts according to the moisture content of the atmosphere to which it is exposed. The electrical resistance is a function of the degree of compression of the matrix and the system can be calibrated to give direct readings of humidity.
Many cellulose materials, including hair, wool and cotton, are hygroscopic and undergo changes in physical dimensions and shape according to the amount of moisture they have absorbed. In the hair hygrometer, the length of a hair or fiber is affected by the humidity of the surrounding atmosphere and this property can be exploited in instruments, which can be constructed to give direct readings. There is a need to calibrate at frequent intervals because of drifting of the zero. Problems are acute when the instrument is used over a wide humidity range.
Thermal Conductivity Meters
The rate of heat loss from a heated wire depends on the thermal conductivity of the surrounding gas which, in general, is a function of its moisture content. A hot wire element, or a thermistor, can be used to fulfil the dual functions of heat source and temperature sensor. Thus, for a constant applied voltage, the temperature, and, hence, resistance of the element, will be a function of the humidity of the gas with which it is in contact.
A variety of other methods is available and each has its own field of application. The principles of operation include:
- selective absorption of infrared electromagnetic radiation;
- measurement of heat of absorption on to a surface;
- electrolytic hygrometry where the quantity of electricity required to electrolyze water adsorbed from the atmosphere on to a thin film of dessicant is measured;
- piezoelectric hygrometry employing a quartz crystal with a hygroscopic coating on which moisture is alternately absorbed from a wet-gas and desorbed in a dry gas stream; the dynamics is a function of the gas humidity;
- capacitance meters in which the electrical capacitance is a function of the degree of deposition of moisture from the atmosphere;
- observation of color changes in active ingredients, such as cobaltous chloride.