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)

PvsTdb

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)

P~Pw

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”

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.

12.3.6.5 Dewpoint Hygrometers

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

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

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