DETERMINING THE DEW POINT

WITH A SLING PSYCHROMETER

 

GENERAL

 

This test method describes how to determine the dew point using a sling psychrometer. A psychrometer measures the ambient air temperature (dry bulb temperature) and the wet bulb temperature. From the dry bulb and wet bulb temperature the dew point is calculated. The dew point is the temperature at which moisture in the air will start condensing.

 

When painting structural steel, specifications may require the dew point to be measured. This is to ensure that no moisture is condensed on the surface to be painted.

 

APPARATUS

 

1. Sling psychrometer

2. Replacement thermometer

3. Replacement wicks

 

INSTRUMENT OPERATION

 

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

 

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

 

3. Holding the tube, whirl the body two to three revolutions per second.

 

4. Continue whirling until temperatures stabilize (1 1/2 minutes is usually ample).

 

5. Immediately read the wet bulb thermometer and then the dry bulb thermometer.

 

6. Figure the difference between the dry bulb and wet bulb temperature. On Table 1, locate the column, which is headed with this temperature difference. Then locate the row labeled with the appropriate dry bulb temperature. The value at the intersection of the column and row gives the dew point.

 

7. The dew point should be at least 5F less than the surface temperature of the steel being painted.

 

EXAMPLE

 

If the dry bulb temperature is 71F and the wet bulb temperature is 64F, the difference between the dry bulb and wet bulb is 7F. At the top of Table 1, locate the column headed with 7. Follow this column down to the row headed with the dry bulb temperature of 71F. The intersection of the column and row gives the number 60, which is the dew point in degrees Fahrenheit. Add 5 degrees to the dew point to get 65F. Sixty-five is the minimum surface temperature of the steel before painting should begin to ensure that there is no condensation on the surface.

 

NOTES

 

1. The surface and air temperature will vary depending on the location of the temperature measurements. For instance, a beam exposed to direct sunlight will be much warmer than a beam in the shade. Therefore if painting work is done in the shade, temperature measurements should also be done in the shade.

 

2. Moisture problems are most likely to be encountered in the morning, so temperature measurements should be made in the morning before painting begins.

 

REPORTING RESULTS

 

The following information should be reported: Time, dry bulb temperature, wet bulb temperature, dew point, and surface temperature.


 

Table 1 - Dew Point Temperature - F

Difference Between Dry Bulb and Wet Bulb - F

Dry Bulb F

0

1

2

3

4

5

6

7

8

9

10

12

14

16

18

20

40

40

38

35

33

--

--

--

--

--

--

--

--

--

--

--

--

41

41

39

36

34

--

--

--

--

--

--

--

--

--

--

--

--

42

42

40

38

35

33

--

--

--

--

--

--

--

--

--

--

--

43

43

41

39

36

34

--

--

--

--

--

--

--

--

--

--

--

44

44

42

40

37

35

--

--

--

--

--

--

--

--

--

--

--

45

45

43

41

38

36

34

--

--

--

--

--

--

--

--

--

--

46

46

44

42

40

37

35

--

--

--

--

--

--

--

--

--

--

47

47

45

43

41

38

36

33

--

--

--

--

--

--

--

--

--

48

48

46

44

42

40

38

35

--

--

--

--

--

--

--

--

--

49

49

47

45

43

41

38

36

33

--

--

--

--

--

--

--

--

50

50

48

46

44

42

40

37

34

--

--

--

--

--

--

--

--

51

51

49

47

45

43

41

38

36

33

--

--

--

--

--

--

--

52

52

50

48

46

44

42

40

37

34

--

--

--

--

--

--

--

53

53

51

49

47

45

43

41

38

36

33

--

--

--

--

--

--

54

54

52

50

48

46

44

42

40

37

34

--

--

--

--

--

--

55

55

53

51

50

48

45

43

41

38

36

33

--

--

--

--

--

56

56

54

53

51

49

47

44

42

40

37

34

--

--

--

--

--

57

57

55

54

52

50

48

46

43

41

39

36

--

--

--

--

--

58

58

56

55

53

51

49

47

45

42

40

37

--

--

--

--

--

59

59

57

56

54

52

50

48

46

44

41

39

33

--

--

--

--

60

60

58

57

55

53

51

49

47

45

43

40

35

--

--

--

--

61

61

59

58

56

54

52

50

48

46

44

42

36

--

--

--

--

62

62

60

59

57

55

53

52

50

47

45

43

38

--

--

--

--

63

63

61

60

58

56

55

53

51

49

47

44

39

34

--

--

--

64

64

62

61

59

57

56

54

52

51

48

46

41

35

--

--

--

65

65

63

62

60

59

57

55

53

51

49

47

42

37

--

--

--

66

66

64

63

61

60

58

56

54

52

50

48

44

38

--

--

--

67

67

65

64

62

61

59

57

55

53

52

49

45

40

34

--

--

68

68

67

65

63

62

60

58

57

55

53

51

46

42

36

--

--

69

69

68

66

64

63

61

59

58

56

54

52

48

43

37

--

--

70

70

69

67

65

64

62

61

59

57

55

53

49

44

39

33

--

71

71

70

68

67

65

63

62

60

58

56

54

50

46

41

35

--

72

72

71

69

68

66

64

63

61

59

58

56

52

47

42

37

--

73

73

72

70

69

67

66

64

62

60

59

57

53

49

44

38

--

74

74

73

71

70

68

67

65

63

62

60

58

54

50

45

40

34

75

75

74

72

71

69

68

66

64

63

61

59

55

51

47

42

36

76

76

75

73

72

70

69

67

66

64

62

60

56

53

48

43

38

77

77

76

74

73

71

70

68

67

65

63

62

57

54

50

45

39

78

78

77

75

74

72

71

69

68

66

64

63

58

55

51

46

41


Table 1 - Dew Point Temperature - F (cont.)

Difference Between Dry Bulb and Wet Bulb - F

Dry Bulb F

0

1

2

3

4

5

6

7

8

9

10

12

14

16

18

20

79

79

78

76

75

73

72

70

69

67

66

64

59

57

53

48

43

80

80

79

77

76

74

73

72

70

68

67

65

61

58

54

50

44

81

81

80

78

77

75

74

73

71

70

68

66

63

59

55

51

49

82

82

81

79

78

77

75

74

72

71

69

67

64

60

57

52

50

83

83

82

80

79

78

76

75

73

72

70

69

65

62

58

54

52

84

84

83

81

80

79

77

76

74

73

71

70

66

63

59

55

53

85

85

84

82

81

80

78

77

75

74

72

71

68

64

61

57

54

86

86

85

83

82

81

79

78

76

75

73

72

69

65

62

58

56

87

87

86

84

83

82

80

79

78

76

75

73

70

67

63

59

57

88

88

87

85

84

83

81

80

79

77

76

74

71

68

64

61

59

89

89

88

86

85

84

82

81

80

78

77

75

72

69

66

62

60

90

90

89

87

86

85

83

82

81

79

78

76

73

70

67

63

61

91

91

90

88

87

86

85

83

82

80

79

78

75

71

68

65

63

92

92

91

89

88

87

86

84

83

81

80

79

76

73

69

66

64

93

93

92

90

89

88

87

85

84

83

81

80

77

74

71

67

65

94

94

93

92

90

89

88

86

85

84

82

81

78

75

72

68

67

95

95

94

93

91

90

89

87

86

85

83

82

79

76

73

70

68

96

96

95

94

92

91

90

88

87

86

84

83

80

77

74

71

69

97

97

96

95

93

92

91

89

88

87

85

84

81

78

75

72

70

98

98

97

96

94

93

92

90

89

88

87

85

82

79

76

73

72

99

99

98

97

95

94

93

92

90

89

88

86

83

81

78

74

73

100

100

99

98

96

95

94

93

91

90

89

87

85

82

79

76

74

101

101

100

99

97

96

95

94

92

91

90

88

86

83

80

77

75

102

102

101

100

98

97

96

95

93

92

91

89

87

84

81

78

76

103

103

102

101

99

98

97

96

94

93

92

91

88

85

82

79

78

104

104

103

102

100

99

98

97

95

94

93

92

89

86

83

80

79

105

105

104

103

101

100

99

98

96

95

94

93

90

87

84

82

80

106

106

105

104

102

101

100

99

98

96

95

94

91

88

86

83

81

107

107

106

105

103

102

101

100

99

97

96

95

92

90

87

84

82

108

108

107

106

104

103

102

101

100

98

97

96

93

91

88

85

84

109

109

108

107

105

104

103

102

101

99

98

97

94

92

89

86

85

110

110

109

108

106

105

104

103

102

100

99

98

95

93

90

87

86