Perret Opticians
 
We have been opticians for three generations in our family, and our activity is targeted on three areas, optometry, contact lenses and optical instruments.

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INSTRUMENTS

 

METEOROLOGY  

Hygrometers

 

 

.about the humidity of air

Of the three main elements of the air, nitrogen and oxygen can only be found in gaseous condition, while water occurs in all three states of aggregation. In the solid state of aggregation water can mainly be found in form of ice clouds, hail, snow pellets and snow. Liquid are the small and minute drops of water, which floating in the atmosphere form gouds, fog and haze. The gaseous water, the vapor, can be found almost all the time and everywhere in the atmosphere. The vapor is invisible, odourless and tasteless and can't be perceived directly by human sense organs. One knows only as much as that: not only the air which is too dry but also the air which is too moist can be unbearable and can cause harms.

The hygrometers measure the moisture of the air. The moisture is characterized by absolute moisture , which is the exact water quantity in one m3, or by the relative moisture, which is relevant of the temperature of the air. For a precise water quantity, if the air temperature drops, the relative moisture will be higher.

...about the measuring of the humidly of air

The psychrometers offer the best method to determine the humidity of air. Those instruments consist of two thermometers. The first one indicates the temperature of air; the conservator tank of the second one is surrounded by a moist muslin envelope.

If the air is saturated with vapor, both thermometers indicate the same temperature. If the air is unsaturated, the . water at the moist thermometer vaporizes. The evaporation requires some heat, the so-called evaporation heat. This heat is taken from the.ambience of the moist wick, above all from the tank of the moist thermometer, so that the temperature at the moist thermometer is falling. This difference between the dry and the moist thermometer depends on the humidity of the air; the greater the difference, the lower the relative humidity of air.

There is hygrometers made with synthetic fibers or with hairs, or with bi-hygrometric, just like the thermometer. They are use to measure the relative moisture. The absolute moisture is measures with the psychrometer, which is made with two thermometers, one is dry and the other wet , the differences of temperature between the two thermometers depends of the rate of the water evaporation, which itself depends on the water quantity in the air. With the means of calculation the absolute moisture can be established. The hair hygrometers are more sensitive and the psychrometer is the most precise.

...about the history of the psychrometer

      

Psychrometers do,already exist since 1825, when August, for the first time, used such an instrument for the measuring of the humidity of air. Whenever humidity of air is measured with the help of psyehrometers, this requires not only two thermometers but also a psychrometric chart. After having read the temperatures and having calculated the difference, you can determine the humidity of air in the point of intersection of the dry temperature and the difference in temperature in the psychrometer chart. The following figure 2 shows a part of such a psychrometer chart:

Dry temperature

Difference in temperature
  1 2 3 4 5 6 7 °C
21°C 91 83 75 67 60 53 46 %
20°C 91 83 74 66 59 51 44 %
19°C 91 82 74 66 58 50 43 %
18°C 90 82 73 65 57 49 41 %

 

Psychrometric chart

Difference in temperature

Tw = wet temperature

Tw (en °C)

0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

0

100

90

81

72

64

56

50

42

36

30

25

20

16

1

100

91

82

74

66

58

52

45

39

34

28

23

18

2

100

91

83

75

67

60

54

48

42

36

31

26

22

3

100

92

84

76

69

62

56

50

44

39

34

29

25

4

100

92

84

77

70

64

57

52

47

41

36

32

28

5

100

93

85

78

71

65

59

54

48

43

39

34

30

6

100

93

85

79

72

66

61

55

50

45

41

36

33

7

100

93

86

79

73

67

62

57

52

47

43

39

35

8

100

93

87

80

74

69

63

58

54

49

45

41

37

9

100

94

87

81

75

70

65

60

55

51

47

43

39

10

100

94

87

82

76

71

66

61

57

53

49

45

41

11

100

95

89

83

77

72

67

62

58

54

50

47

43

12

100

94

89

83

78

73

68

63

59

56

52

48

44

13

100

95

90

84

78

74

69

65

61

57

53

50

46

14

100

95

89

84

79

74

70

66

62

58

54

51

47

15

100

94

89

84

80

75

71

67

63

59

55

52

49

16

100

95

90

85

80

76

72

68

64

60

57

54

50

17

100

95

90

85

81

77

72

69

65

62

58

55

52

18

100

95

90

86

81

77

74

70

66

63

59

56

53

19

100

95

91

86

82

78

74

70

66

63

60

57

54

20

100

96

91

87

82

78

74

71

67

64

61

58

55

21

100

96

91

87

83

79

75

72

68

65

62

59

56

22

100

95

91

87

83

80

76

72

69

66

63

60

57

      23

100

96

91

87

84

80

76

73

69

67

63

61

58

 

 

...about the humidity measuring in the meteorology

For the extremely exact measurings of humidity in the meteorology psychrometers are used as well. For that purpose a permanent air current of 2,4 m/sec is produced with the help of.a supplementary fan for the measuring of the psyehrometric difference. As a result of the draft the difference in temperature increases a little bit.. In addition, the air pressure and its variations is taken in consideration for the calculation of the humidity of air.

For the determination of the humidity of air for the daily requirements such an accuracy isn’t necessary.

...about the absolute humidity

The vapor, which really is in the air, is called absolute humidity and is measured in gramme per cubic metre. At a particular temperature of the air the absolute humidity can’t rise at will. If more and more vapor is admitted to the air, the absolute humidity rises up to a point where the air is no longer able to accept any further vapor. For that reason there is a saturation maximum for every degree of temperature, as it is shown in the following figure 3:

Temperature

 Saturation maximum

0°C

4.8 g/m3

5°C

6.8 g/m3

10°C

9.4 g/m3

15°C

12.8 g/m3

20°C

17.3 g/m3

25°C

23.1 g/m3

30°C

30.3 g/m3

35°C

39.6 g/m3

40°C

51.1 g/m3

The saturation maximum rises together with the rising temperature. Hot air is able to accept much more vapor than cold air. As a result one gets the following curve in figure 4:

i_hyg_saturation.gif (8561 octets)

If, for instance, 30 g of. vapor are admitted to a cubic metre of air, only 23,1 g of the vapor are accepted as a maximum at 25· C (figure 3). The remaining 6,9 g of the vapor condense to water:

i_hyg_saturation_2.gif (6746 octets)

...about the relative humidity of air

The actual humidity of air is mostly lying under the saturation maximum. The portion of the actual humidity of air put into ratio to the saturation maximum results in the relative humidity of air. The relative humidity of air is expressed in per cent:

absolute humidity
--------------------------------
saturation maximum		 x100 = relative humidity

The optimal relative humidity depends on the given situation: for the human housing spaces 50 – 70 % of humidity are felt to be comfortable. For the sauna are 5 – 20% recommended, for the cold-storage depot 90 %.

...about the dew point

If the air.is cooled off at a constant absolute humidity of air, the moment will come where the air can’t hold the actual vapor anymore:

i_hyg_point_rosee.gif (11346 octets)

If the air contains too much vapor at the cooling, the excess vapor is given off as condensate. This condensation point is called dew point.

...about the frost warning

For the frost warning it’s the dew point line for O°C which is decisive. As everybody knows, the dew point at 0°C exists with 4,8 g/m3 of vapor = 100 per cent of relative humidity.

The maximum saturation at +5°C is given at 64 g/m3. In relation to the freezing point, the dew point has to be calculated in the ratio 4,8 g/m3 to 6,8 g/m3 = 71 %.

°C

max.g/m3

related to

dew point
0

4.8g

4.8g

100%
5

6.8g

4.8g

71%
10

9.4g

4.8g

51%
15

12.8g

4.8g

38%
20

17.3g

4.8g

28%

If these dew point values are reached or if they are fallen short of at the corresponding temperatures, the same danger of frost exists as at O° C.

i_hyg_point_gel.gif (8565 octets)

 

Polymeter

The polymeter is a combined hygrometer-thermometer for measuring air temperature, absolute and relative humidity of air, saturation vapour pressure, vapour pressure, dew point temperature and saturation deficit.

The polymeter has a special importance for the prediction of night frost. The prediction is based on the following physical laws:
At the night the earth radiates heat into the space strongly with clear sky. The cooling down of the earth is transmitted to the lower layers of air near the earth. But there is a limit in cooling down. With reaching the dew point temperature the water vapour contained in the air is condensating and latent heat becomes free. When the measured dew point temperature in the late evening is near 0 °C or below, then the danger of night frost is given.

 

 

 

 

 

 

The polymeter is a normal hair hygrometer with a pointer passing over a scale reading 0 – 100% humidity. Above this is another scale in degrees Celsius to be read in conjunction with the first scale.

A mercury thermometer reads the air temperature and the numerical difference between this and the value indicated by the hygrometer needle gives the temperature at which the onset of dew or fog will occur.

 

 

 

 

 

 

 

 

 

Encyclopedia, the hygrometer

 

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