Air Humidity Measurement
12.3.6.1 Parameters of Moist Air
Several quantities are used to describe air humidity (see Section 4.2). The most common are humidity ratio and relative humidity, shown also on the psychrometric chart.
Some air humidity meters measure the relative humidity directly. Others measure either the wet-bulb temperature, the dewpoint temperature, or the absolute water vapor mass in a sample of air. The measured wet-bulb temperature is not the thermodynamic wet-bulb temperature, but an equilibrium temperature of a wet wick. In this equilibrium state the heat flow by convection (and conduction and radiation) is equal to the heat flow due to evaporation of the water from the wick.
The partial pressure of water vapor can be calculated as a function of the dry-bulb and wet-bulb temperatures, Eq. (12.23), and the relative humidity from its definition:
<n = —— ! 12.19)
Where Pw is the partial pressure of water vapor and pws is the saturation pressure of water vapor at the air dry-bulb temperature Tdb — The saturation pressure as a function of the temperature can be determined from tables or equations;20 see Table 12.4. Using the water vapor pressure, the humidity ratio is
W= 0.622, (12.20)
Where P is the total pressure of moist air. If, on the other hand, the dewpoint temperature Td is measured, the saturation pressure of water vapor at the dewpoint temperature is equal to the partial pressure of water vapor Pw = Pws(Td). By substituting this partial pressure into Eq. (12.19) or (12.20), the relative humidity or the humidity ratio can be calculated. Some fundamental instruments used as references for calibration measure the mass of water in a sample of air. In this case, the humidity ratio can easily be computed on the basis of its definition:
W=^’, (12.21)
MA
Where Mw and Ma are the masses of water and dry air, respectively. The rela tive humidity can further be determined using the equation
Tp =———————- P——————— . (12.22)
V (1 +0.622/W)pwsTdb 1
12.3.6.2 Electrical Hygrometers
Capacitive Sensors
The majority of modern, compact air humidity meters are based on electrical measurement principles. The capacitive sensor is an electrical capacitor having a moisture-dependent capacitance. The probe contains electrodes with a hygroscopic insulation material in between. The insulation material is chosen to have a small dielectric constant, whereas the dielectric constant of water is high. As a consequence, the absorbed water has a strong influence on the sensor’s capacitance. The electronics of the instrument determine the probe capacitance and convert it into a relative humidity reading. Because of the small sensor capacitance, electronic processing has to be completed close to the sensor. The capacitive sensors are usually manufactured using thin-film technology using polymers deposited on a glass or silicon substrate then coated with a porous metal electrode layer to protect the sensor as the insulating layer material.
Capacitive sensors are small and rapidly respond to changes in air humidity. The measurement range is 0-100% RH. Due to the electrical principle, they can
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TABLE 12.4 Saturation Pressure (Pa) over Liquid Water for the Temperature Range 0-40 °C
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TABLE 12.4 (continued)
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Be applied to different control and automation systems. They suffer to some extent from drift and require repeated calibration to maintain good accuracy.
Resistive Sensors
The resistive measurement principle is based on a humidity-dependent electrical resistance. The early probes used lithium chloride as the hygroscopic resistive material. Such probes are still available under the name Dunmore sensors. The measurement range of such devices is quite narrow, and the resistance versus humidity relationship is extremely nonlinear.
Recent developments are leading toward other materials like silica gel or polymers. Certain types of semiconductors are also used as resistive probes. The measurement range of resistive sensors varies depending on materials used. It can be as wide as 0-99% RH. The dynamics are fast enough for normal ventilation applications and the stability of good resistive sensors is high. This does not reduce the need for calibration, but the intervals of successive calibrations can be extended.
12.3.6.3 Mechanical Hygrometers11
Mechanical hygrometers are the oldest type of humidity-measuring instruments. They are based on the change of length of a stretched strip, bundle, or membrane of some hygroscopic material such as natural hair or, for example, cellulose butyrate. The length of the material increases when water is absorbed from the surrounding moist air. On the other hand, the effect of temperature changes is small. As a consequence the instrument responds practically only to air humidity and is calibrated to indicate relative humidity. The change in the probe length is transmitted to the movement of a pointer or a pen. A strain gauge can be used to provide an electrical signal. The accuracy of mechanical hygrometers is low. They suffer from nonlinearity, hysteresis, and drift, which cause a need for frequent recalibration. However, they are relatively cheap instruments, often available with a simple mechanical continuous recording feature. Mechanical hygrometers have a slow response and should not be used in situations where the humidity is changing rapidly.
12.3.6.4 Psychrometers 21-14
A psychrometer measures the dry-bulb and wet-bulb temperatures simultaneously. The measurement of the wet-bulb temperature is achieved by means of a wet wick placed over the thermometer bulb. The thermometer can be practically of any type. A cylindrically shaped sensor is preferred. The wet-bulb temperature- sensing element, covered with the wick, and the dry-bulb temperature sensor, are placed in the airstream to be measured. The stream, generated by a small fan, should have a velocity of 3-5 m s-1 and can be either transverse or axial. The
Wick-covered sensor is cooled down by evaporation until ir reaches a thermal equilibrium state where the (almost only) convective heat transfer is covering the heat required for water vaporization from the w’ick.
The humidity can be determined using either charts or equations provided by the psychrometer manufacturer. The partial pressure of water vapor provides a more general approach and can be calculated from the “psychrometer equation”
Pw = PwsiTdb) ~ MTdb ~ Twb)p, (12.23!
Where A is the psychrometer constant and Twh is the wet-bulb temperature, The psychrometer constant has values between 5.4 and 6.9 x 1 (V4 L K-1,22’23’2′ depending on the airstream velocity and some other factors. To reduce the radiative exchange in hot environments, radiation shields should be fitted to both sensors. The thermometers must be adequately spaced from each other to avoid the wetting of the dry bulb. The dry-bulb sensor should not be in the wake of the wet — bulb sensor to ensure that the correct temperature is measured. The water used in the wick should be pure distilled water to stop lime scale buildup on the wick.
A psychrometer fitted with a fan is called an aspirated psychrometer or Assmann hygrometer. Another variant is the sling or whirling hygrometer. In this case the wet — and dry-bulb thermometers are attached to a frame with a handle. When measuring the temperatures, the frame is whirled around like a football rattle. The measurement range is dependent on the range of the thermometers but is usually wide enough for ventilation measurements. The response of the psychrometer is slow, taking a few minutes to reach the wet-bulb equilibrium state. Rapidly changing humidity cannot be monitored. The advantage of an instrument of this kind is that its construction and the fundamental nature of the measurement are simple. For this reason, if handled with care, it is a cheap but reliable instrument.
The dewpoint hygrometer detects the dewpoint temperature of air by cooling a surface in contact with the air to the dewpoint temperature. There are several ways to achieve cooling and to observe the formation of condensate on the surface. The early dewpoint hygrometers were cooled simply by applying the vaporization of ether or some other suitable liquid. Condensate formation on the surface was determined visually. Other cooling methods are to use a refrigerant flow in direct or indirect contact with the back of the surface, or to use electricity with a (thermoelectric) Peltier element.
The observation may be by a lamp illuminating the surface and a photocell to detect the scattered light due to the water droplets on the surface. The accurate measurement of the surface temperature, which is the dewpoint temperature, is critical. If a coolant is used, a close approximation for the surface temperature is the fluid temperature; otherwise a small thermocouple or resistance sensor can be attached to or embedded into the surface.
The range of dewpoint hygrometers depends on the temperature range of the cooled surface. In principle a temperature range of air from -70 to +100 °C can be covered. The measurement of the frost point at low temperatures involves large measurement errors. Typical error sources are surface contamination, gases dissolving in the water, cold-spot errors, and pooling or flooding.22 These factors considerably reduce the accuracy of the measurement. The dynamical response is slow and cannot handle rapid fluctuations of temperature or humidity.
The dewpoint hygrometer is claimed to be the most accurate instrument for measuring air humidity. When properly calibrated, the inaccuracy can be ±0.5% RH.26 On the whole, the dewpoint hygrometer is a reliable fundamental instrument suitable for many ventilation applications, but is more expensive than other humidity instruments.
12.3.6.6 Calibration of Hygrometers
Most hygrometer types require constant calibration. Especially mechanical hygrometers may have a strong drift, causing a bias error during a short period of time. Electrical hygrometers also require constant calibration. The only type not requiring calibration is the psychrometer, if it is based on stable temperature measurement, such as high-quality liquid-in-glass thermometers. In fact, the psychrometer can be used as the reference meter in simple calibration procedures.
The simplest way to calibrate a hygrometer is to use ambient air. The humidity of ambient air is measured using both the calibrated meter and the reference meter. A simple psychrometer or an Assmann hygrometer can be used as a reference. Provided the air conditions are stable, this method is used to check hygrometers in field measurements and other not too demanding purposes.
If the one-point calibration in ambient air is not sufficient, the next best approach is to use the calibration box method.21 The air state is created in a closed box made of nonhygroscopic material, like metal or plastic. A controlled state of humidity is maintained by exposing the air in the box to a liquid surface of a saturated salt solution. In practice, a dish containing the saturated water solution of a salt is placed on supports at the bottom of the box. The air in the box is circulated by means of a small fan. The box should be airtight and positioned in a constant temperature environment. The calibrated instruments are placed in the box. A dewpoint hygrometer can be used as a reference. A wide range of humidity can be created by using solutions of different salts. Table 12.5 shows a few examples of equilibrium humidities achieved with different salt solutions.
If high calibration accuracy is required, the simple box to create and maintain the state of air is not sufficient. In such a case equipment that keeps
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TABLE 12.5 Relative Humidity of Air over Various Saturated Solutions of Salts21 Temperature, °C
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The partial pressure of water vapor constant by saturating the air with water at a specified temperature can be used.27
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