DETERMINING RELATIVE HUMIDITY

WITH A SLING PSYCHROMETER


 

GENERAL

 

This method describes how to determine the relative humidity with a sling psychrometer. A psychrometer measures the ambient air temperature (dry bulb temperature) and the wet bulb temperature. From these temperature measurements, the percent relative humidity is determined.

 

APPARATUS

 

1.   Sling psychrometer

 

PROCEDURE

 

1.     Before using, take the end cap off the psychrometer and fill the reservoir with water. Be sure that the wick is saturated with water and covers the mercury reservoir on the wet bulb thermometer. Also, be sure that the mercury reservoir on the dry bulb thermometer is dry.

 

2.     Pull the tube clear of the body so that the body can swivel.

 

3.     Hold the tube and whirl the body two to three revolutions per second.

 

4.     Continue whirling until the temperatures stabilize (should take 1-2 minutes).

 

5.     Stop whirling and immediately read temperature of the wet bulb thermometer and then the dry thermometer.

 

6.     Figure the temperature difference between the two thermometers.

 

7.     Use the attached table to determine the relative humidity.

 

EXAMPLE

 

After following the above procedure, a dry bulb temperature of 86°F and a wet bulb temperature of 71°F were obtained. Thus, the temperature difference is 15°F.

 

From the “Relative Humidity” table, locate the column at the top headed with 15. Follow this column down to the row headed with dry bulb temperature of 86°F. The intersection of the column and the row gives the number 47. This number is the percent relative humidity.

 

REPORTING RESULTS

 

Record the time, date, dry bulb temperature, wet bulb temperature, and relative humidity.

 


 

 

Table 1 – Relative Humidity, %

Difference Between Readings of Wet & Dry Bulbs in °F

Dry

Bulb

°F

 

 

1

 

 

2

 

 

3

 

 

4

 

 

5

 

 

6

 

 

7

 

 

8

 

 

9

 

 

10

 

 

11

 

 

12

 

 

13

 

 

14

 

 

15

 

 

16

 

 

17

 

 

18

 

 

19

 

 

20

 

 

21

 

 

22

 

 

23

 

 

24

 

 

25

 

 

26

 

 

27

 

 

28

 

 

29

 

 

30

Dry

Bulb

°F

40

41

42

43

44

92

92

92

92

93

83

84

85

85

85

75

76

77

77

78

68

69

69

70

71

60

61

62

63

63

52

54

55

55

56

45

46

47

48

49

37

39

40

42

43

29

31

33

35

36

22

24

26

28

30

15

17

19

21

23

  7

10

12

14

16

  0

  3

  5

  8

10

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

40

41

42

43

44

45

46

47

48

49

93

93

93

93

93

86

86

86

86

86

78

79

79

79

80

71

72

72

73

73

64

65

66

66

67

57

58

59

60

61

51

52

53

54

54

44

45

46

47

48

38

39

40

41

42

31

32

34

35

36

25

26

28

29

31

18

20

22

23

25

12

14

16

18

19

  6

  8

10

12

14

 

  2

  5

  7

  9

 

 

 

  1

  3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

45

46

47

48

49

50

51

52

53

54

93

94

94

94

94

87

87

87

87

88

80

81

81

81

82

74

75

75

75

76

67

68

69

69

70

61

62

63

63

64

55

56

57

58

59

49

50

51

52

53

43

45

46

47

48

38

39

40

41

42

32

34

35

36

37

27

28

29

31

32

21

23

24

26

27

16

17

19

20

22

10

12

14

16

17

  5

  7

  9

10

12

  0

  2

  4

  6

  8

 

 

 

  1

  3

 

 

 

 

 

 

 

 

 

 

 

 

50

51

52

53

54

55

56

57

58

59

94

94

94

94

94

88

88

88

88

89

82

82

82

83

83

76

76

77

77

78

70

71

71

72

72

65

65

66

66

67

59

60

61

61

62

54

55

55

56

57

49

50

50

51

52

43

44

45

46

47

38

39

40

41

42

33

34

35

37

38

28

30

31

32

33

23

25

26

27

29

19

20

22

23

24

14

16

17

18

20

  9

11

13

14

16

  5

  7

  8

10

11

  0

  2

  4

  6

  7

 

 

  0

  1

  3

 

 

 

 

 

 

 

 

 

 

55

56

57

58

59

60

61

62

63

64

94

94

94

95

95

89

89

89

89

90

83

84

84

84

84

78

78

79

79

79

73

73

74

74

74

68

68

69

69

70

63

63

64

64

65

58

58

59

60

60

53

54

54

55

56

48

49

50

50

51

43

44

45

46

47

39

40

41

42

43

34

35

36

37

38

30

31

32

33

34

26

27

28

29

30

21

22

24

25

26

17

18

20

21

22

13

14

16

17

18

  9

10

12

13

15

  5

  7

  8

10

11

  1

  3

  4

  6

  7

 

 

  1

  2

  4

 

 

 

 

  0

 

 

 

 

 

 

 

60

61

62

63

61

65

66

67

68

69

95

95

95

95

95

90

90

90

90

90

85

85

85

85

85

80

80

80

80

81

75

75

75

76

76

70

71

71

71

72

66

66

66

67

67

61

61

62

62

63

56

57

58

58

59

52

53

53

54

55

48

48

49

50

51

44

44

45

46

47

39

40

41

42

43

35

36

37

38

39

31

32

33

34

35

27

29

30

31

32

24

25

26

27

28

20

21

22

23

24

16

17

19

20

21

12

14

15

16

18

  9

10

12

13

14

  5

  7

  8

10

11

  2

  3

  5

  6

  8

 

  0

  2

  3

  5

 

 

 

 

  1

 

 

 

 

 

65

66

67

68

69

70

71

72

73

74

95

95

95

95

95

90

90

91

91

91

86

86

86

86

86

81

81

82

82

82

77

77

77

78

78

72

72

73

73

74

68

68

69

69

69

64

64

65

65

65

59

60

61

61

61

55

56

57

57

58

51

52

53

53

54

48

48

49

50

50

44

45

45

46

47

40

41

42

42

43

36

37

38

39

39

33

33

34

35

36

29

30

31

32

33

25

27

28

29

29

22

23

24

25

26

19

20

21

22

23

15

17

18

19

20

12

13

15

16

17

  9

10

12

13

14

  6

  7

  9

10

11

  3

  4

  6

  7

  8

 

  1

  3

  4

  5

 

 

 

  1

  3

 

 

 

70

71

72

73

74

75

76

77

78

79

96

96

96

96

96

91

91

91

91

91

86

87

87

87

87

82

82

83

83

83

78

78

79

79

79

74

74

74

75

75

70

70

71

71

71

66

66

67

67

68

62

62

63

63

64

58

59

59

60

60

54

55

56

56

57

51

51

52

53

53

47

48

48

49

50

44

44

45

46

46

40

41

42

43

43

37

38

39

39

40

34

34

35

36

37

30

31

32

33

34

27

28

29

30

31

24

25

26

27

28

21

22

23

24

25

18

19

20

21

22

15

16

17

18

19

12

13

14

16

17

  9

11

12

13

14

  7

  8

  9

10

11

  4

  5

  6

  8

  9

  1

  3

  4

  5

  6

 

 

75

76

77

78

79

80

82

84

86

88

96

96

96

96

96

91

92

92

92

92

87

88

88

88

88

83

84

84

84

85

79

80

80

81

81

75

76

76

77

77

72

72

73

73

74

68

69

69

70

70

64

65

66

66

67

61

61

62

63

64

57

58

59

60

61

54

55

56

57

57

50

51

52

53

54

47

48

49

50

51

44

45

46

47

48

41

42

43

44

46

38

39

40

42

43

35

36

37

39

40

32

33

35

36

37

29

30

32

33

35

26

28

29

31

32

23

25

26

28

30

20

22

24

26

27

18

20

21

23

25

15

17

19

21

22

12

14

16

18

20

10

12

14

16

18

  7

10

12

14

15

  5

  7

  9

11

13

  3

  5

  7

  9

11

80

82

84

86

88

90

92

94

96

98

96

96

96

96

96

92

92

93

93

93

89

89

89

89

89

85

85

85

86

86

81

82

82

82

83

78

78

79

79

79

74

75

75

76

76

71

72

72

73

73

68

68

69

69

70

65

65

66

66

67

61

62

63

63

64

58

59

60

61

61

55

56

57

58

58

52

53

54

55

56

49

50

51

52

53

47

48

49

50

50

44

45

46

47

48

41

42

43

44

45

39

40

41

42

43

36

37

38

39

40

34

35

36

37

38

31

32

33

35

36

29

30

31

32

34

26

28

29

30

32

24

25

27

28

29

22

23

24

26

27

19

21

22

24

25

17

19

20

22

23

15

17

18

20

21

13

15

16

18

19

90

92

94

96

98

100

102

104

106

108

96

96

97

97

97

93

93

93

93

93

89

90

90

90

90

86

86

87

87

87

83

83

83

84

84

80

80

80

81

81

77

77

77

78

78

73

74

74

75

75

70

71

71

72

72

68

68

69

69

70

65

65

66

66

67

62

62

63

64

64

59

60

60

61

62

56

57

58

58

59

54

55

55

56

57

51

52

53

53

54

49

49

50

51

52

46

47

48

49

49

44

45

46

46

47

41

42

43

44

45

39

40

41

42

43

37

38

39

40

41

35

36

37

38

39

33

34

35

36

37

30

32

33

34

35

28

30

31

32

33

26

28

29

30

31

24

26

27

28

29

22

24

25

26

27

21

22

23

24

25

100

102

104

106

108

110

112

114

116

118

97

97

97

97

97

93

94

94

94

94

90

90

91

91

91

87

87

88

88

88

84

84

85

85

85

81

81

82

82

82

78

79

79

79

79

75

76

76

76

77

73

73

74

74

74

70

70

71

71

72

67

68

68

69

69

65

65

66

66

67

62

63

63

64

64

60

60

61

61

62

57

58

58

59

59

55

55

56

57

57

52

53

54

54

55

50

51

52

52

53

48

49

49

50

51

46

47

47

48

49

44

44

45

46

47

42

42

43

44

45

40

40

41

42

43

38

38

39

40

41

36

36

37

38

39

34

35

35

36

37

32

33

34

34

35

30

31

32

33

34

28

29

30

31

32

26

27

28

29

30

110

112

114

116

118

120

97

94

91

88

85

82

80

77

74

72

69

67

65

62

60

58

55

53

51

49

47

45

43

41

40

38

36

34

33

31

120