29. [ Specification of hygrometer. [Substituted by Notification No. 4(a) 4-91-16B, dated 6-4-1995.]
(1)Each hygrometer shall comprise two mercurial thermometers of wet-bulb and dry-bulb of similar construction, and equal in dimensions, scale and division of scale. They shall be mounted on a frame with a suitable reservoir containing water.(2)The wet-bulb shall be closely covered with a single layer of muslin, kept wet by means of wick attached to it and dropping into the water in the reservoir. The muslin covering and the wick shall be suitable for the purpose, clean and free from (size of) grease.(3)No part of the wet-bulb shall be within 76 millimetres from the dry-bulb or less than 28 millimetres from the surface of the water in the reservoir, and the water reservoir shall be below it, on the side of it away from the dry-bulb.(4)The bulb shall be spherical and of suitable dimensions, and shall be freely exposed on all sides to the air of the room.(5)The bores of the stems shall be such that the position of the top of the mercury column shall be readily distinguishable at a distance of 60 centimetres.(6)Each thermometer shall be graduated so that accurate readings may be taken between 10 and 50 degrees celsius.(7)Every degree from 10 degrees upto 50 degrees shall be clearly marked by horizontal lines on the stem, each fifth degree shall be marked by longer marks than the intermediate degrees and the temperature marked opposite each fifth degree, i.e. 10, 15, 20, 25, 30, 35, 40, 45, 50.(8)The markings as above shall be accurate, that is to say, at no temperature between 10 and 50 degrees shall be indicated readings, be in error by more than one ninth of a degree.(9)A distinctive number shall be indelibly marked upon the thermometer.(10)The accuracy of each thermometer shall be certified by the National Physical Laboratory, Delhi or some competent authority appointed by the Chief Inspector and such certificate shall be attached to the Humidity Register.]
