Document vVXKQE7OG584B1QgZk04YJoGb

American Society of Heating and Ventilating Engineers Guide, 1934 If three persons are assumed to an apartment the total daily, use of hot water should approximate 20 X 3 X 40 gal or 2400 gal and if the peak hour is 10 per cent of this amount, the peak hour by this method shows probable demand of one-tenth of 2400 gal which indicates that the values in Table are safe. j 568 Chapter 40 TEST METHODS AND INSTRUMENTS Pressure Measurement, Temperature Measurement, Air Move ment, Humidity Measurement, Carbon Dioxide Determination, Dust Determination, Flue Gas Analysis, Measurement of Smoke Density, Heat Transmission, Eupatheoscope PRESSURE MEASUREMENT ATMOSPHERIC pressure is usually measured by a mercurial barom eter which, in its simplest form, consists of a glass tube about 3 ft long, closed at the upper end, filled with mercury and inverted in a shallow bath of mercury. The atmosphere, pressing on the exposed top of the mercury in the cistern, supports a column of mercury in the tube to a height of about 30 in. Readings are taken of the height of the column between the levels of mercury in the tube and in the cistern. Atmospheric pressure is the same as the pressure exerted by this supported column of mercury, and, in pounds per square inch, is equal to its height in inches times 0.491, which is the weight in pounds of 1 cu in. of mercury. At latitude 45 deg and sea level, and at a temperature of 32 F, the atmos phere will support a column of mercury 29.921 in. in height. The pressure of 14.7 lb per square inch, derived by multiplying 29.921 by 0.491, is called standard or normal barometric pressure. An aneroid barometer con tains no liquid. Atmospheric pressure in bending the thin corrugated top of a partially exhausted metallic box, or in distorting a thin-walled bent tube of metal, is made to move a pointer. It is portable but less accurate than the mercurial barometer. Pressures above or below atmospheric are usually measured by means of gages which indicate the difference between the pressure being measured and atmospheric pressure at the same time and place. A gage which indicates pressures higher than atmospheric is known as a pressure gage, and a gage which indicates pressures lower than atmospheric is known as a vacuum gage. The most common type of these gages contains a flexible hollow brass tube of oval cross section, known as a Bourdon tube. When subjected to unequal inside and outside pressures, this tube tends to' straighten out, and a pointer motivated by this straightening indicates the pressure difference on a suitably graduated scale. A gage which indicates pressures slightly above or below atmospheric is known as a draft gage. It is essentially a U tube containing either water, kerosene, alcohol or mercury, with one leg exposed to the air and the other connected to a point where the pressure is to. be determined. When the pressure being read is equal to atmospheric,, the.level of the liquid in the legs will be the same, indicating a zero gage pressure. When a pres- 569