Tables of physical and chemical constants and some mathematical functions / by G.W.C. Kaye and T.H. Laby.

  • G. W. C. Kaye
Date:
[1941]
Catalogue details

Licence: Public Domain Mark

Credit: Tables of physical and chemical constants and some mathematical functions / by G.W.C. Kaye and T.H. Laby. Source: Wellcome Collection.

    59/200 (page 47)
    HVGROMETRY RELATIVE HUMIDITY AND DEW-POINT The relative humidity is the ratio (expressed as a percentage) of the water vapour actually present in unit volume, to that which the air would contain if saturated at the air temperature t. For all practical purposes, this is equal to the ratio of the pressure (p) of the vapour actually present (i.e. the saturation pressure at the dew¬ point) to the saturation pressure at air temperature. For a table of saturation pressures, see p. 49. CHEMICAL HYGROMETER The values below are grams of water contained in a cubic metre (io6 c.c.) of saturated air at 760 mm. total pressure. Calculated from Regnault’s observations. Temp. 0 1 2 3 4 5 6 7 3 9 0° C. 4-84 5-18 5*54 5-92 6*33 6-76 7-22 7-70 8-21 8-76 10 9*33 9-93 10-57 11-25 11*96 12-71 13-50 14*34 15-22 16-14 20 17-12 18-14 19-22 20-35 21*54 22-80 24-11 25-49 26-93 28-45 30 30-04 31-70 33-45 35*27 37*i8 39-i8 4i*3 43-5 45-8 48-2 WET- AND DRY-BULB HYGROMETER Apjohn (1835), August (1825), and others, by making various assumptions (some of doubtful legitimacy) have derived formulae of the type : pw —p — AH{t — A)[i -f- B(kt — /„)] Where tw is the wet-bulb temperature, p the actual pressure, and pw the saturation pressure at temperature tw, H is the barometric pressure and A, B and k are constants. Thus the relative humidity is 100 p\ps where ps is the saturation pressure at the dry-bulb temperature. (See Love and Sineat, Proc. Boy. Soc., Victoria, 10, 1, 1911 ; Whipple, Proc. Phys. Soc.. 1933; Arnold, Phys. Rev., 1932.) The values of A in this formula depend on the speed of the air passing the wet bulb, appropriate values being shown below for the case where H is measured in mm. and t, tw in Centigrade degrees. A = 0-00068 for moving air, as in Assmann ventilated psychrometer. A = 0-00075 in a Stevenson screen as used by Meteorological Office. A = 0-0008 in open air with slight wind. A = 0-00084 in open air with no wind. A = o-ooi in a small closed room. The values below are based on tables issued by the Prussian Meteorological Office and by the National Physical Laboratory, both of which are for use with ventilated instruments. WET- AND DRY-BULB HUMIDITY VALUES Dry-bulb Wet-bulb depression. Temperature. 0°*5C. 1°0 1°*5 © ■ O ea 2° 5 3°*0 3° 5 4°*0 5° 0 6°‘0 -9° C. t . 85% 7i% — 8 t . 87 73 59% 46% -6 + . 88 76 64 52 40% 29% —4 t . 89 78 67 57 46 36 -2 t . 90 80 70 61 52 42 33% 25% 0 . . 91 82 73 65 56 48 39 3i 2 . • 92 84 76 68 60 52 45 37 22% 4 . . 92 85 78 70 63 56 49 42 29 6 . . 93 86 79 73 66 60 53 47 35 23% 8 . . 94 87 81 75 69 63 57 5i 40 29 10 . . 94 88 82 76 7i 65 60 54 44 34 7°0 18% 24 + Super-cooled water (not ice) on wet-bulb. _