# Air Humidity Measurement

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

TABLE 12.4 Saturation Pressure (Pa) over Liquid Water for the Temperature Range 0-40 °C

 Degrees C Tenths of degrees C 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 611 616 620 625 629 6.34 638 643 648 652 1 657 662 667 671 676 681 686 691 696 701 2 706 711 716 721 726 732 737 742 747 753 3 758 763 769 774 780 785 791 796 802 808 4 813 819 825 831 837 843 848 854 860 866 5 872 879 885 891 897 903 910 916 922 929 6 935 942 948 955 961 968 975 982 988 995 7 1002 1009 1016 1023 1030 1037 1.044 1051 1058 1066 S 1073 1080 1088 1095 1102 1110 1117 1125 1133 1140 9 1148 1156 1164 1172 1179 1187 1195 1204 1212 1220 10 1228 1236 1245 1253 1261 1270 1278 1287 1295 1304 11 1313 1321 1330 1339 1348 1357 1366 1375 1384 13.9.3 12 1403 1412 1421 1431 1440 1450 1459 1469 1478 1488 13 1498 1508 1518 1527 1537 1548 1558 1568 1578 1588 14 1599 1609 1620 1630 1641 1651 1662 1673 1684 1694 15 1705 1716 1728 .1739 1750 1761 1772 1784 1795 1807 16 1818 1830 1842 1854 1865 1877 1889 1901 1.914 1926 17 1938 1950 1963 1975 1988 2000 2013 2026 2038 2051 IS 2064 2077 2090 2104 2117 2130 2144 2157 2171 2184 1.9 2198 2212 2225 2239 2253 2267 2281 2296 2310 2324 20 2339 2353 2368 2383 2397 2412 2427 2442 2457 2472 21 2488 2503 2518 2534 2549 2565 2581 2597 2613 2629 22 2645 2661 2677 2694 2710 2726 2743 2760 2777 2763 23 2810 2827 2845 2862 2879 2897 2914 2932 2949 2967 24 2985 3003 3021 3039 3058 3076 3094 3113 3132 3150 25 3169 3188 3207 3226 3246 3265 3284 3304 3324 3343 26 3363 3383 3403 3423 3444 3464 3484 3505 .3526 3546 27 3567 3588 3609 3631 3652 3673 3695 3717 3738 3760 28 3782 3804 3827 3849 3871 3894 3916 3939 3962 3985 29 4008 4032 4055 4078 4102 4126 4150 4173 4198 4222 30 4246 4270 4295 4320 4345 4369 43.94 4420 4445 4470 31 4496 4522 4547 4573 4599 4626 4652 4678 4705 4732 32 4759 4786 4813 4840 4867 4895 4922 4950 4978 5006 33 5034 5063 5091 5120 5148 5177 5206 5236 5265 52.94 34 5324 5354 5384 5414 5444 5474 5505 5535 5566 5.597 35 5628 5659 5690 5722 5754 5785 5817 5849 5882 5914 36 5947 5979 6012 6045 6078 6112 6145 6179 6213 6247 37 6281 6315 6350 6384 6419 6454 6489 6525 6560 6596

TABLE 12.4 (continued)

 Degrees C Tenths of degrees C 0 0.1 0.2 0.3 0.4 I 0.5 0.6 0.7 0.8 0.9 38 6631 6667 6704 6740 6776 6813 6850 6887 6924 6961 39 6999 7036 7074 7112 7150 7189 7227 7266 7305 7344 40 7383 7423 7463 7502 7542 7583 7623 7664 7704 7745

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”

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.

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

TABLE 12.5 Relative Humidity of Air over Various Saturated Solutions of Salts21

Temperature, °C

 Saturated salt solution 0 10 20 30 40 50 60 Potassium chloride 89 88 86 84 82 81 80 Sodium chloride 76 76 76 75 75 75 75 Ammonium nitrate 77 72 65 59 53 47 42 Magnesium nitrate 60 57 55 52 49 46 43 Potassium carbonate — 47 44 43 42 — — Magnesium chloride 34 34 33 33 32 31 30 Potassium acetate 25 24 23 22 20 — — Lithium chloride 15 13 12 12 11 11 11

The partial pressure of water vapor constant by saturating the air with water at a specified temperature can be used.27