Document DMx4QQobxq1rpwE5kzQ2Jkz3B

184 CHAPTER 11 1948 Guide, well. Such barometers are equipped with ;an adjustment to compensate for change; in level of mercury in the well.. The reading should be taken at the top of the meniscus and is obtained on a Verniecseale. . ; Correction for variation of the density of the mercury column and :for expansion of the brass scale, which are usually calibrated for 32 F mercury and 62 F scale temperature, should be made by'subtracting from the observed height in inches the value of-.C determined by-Equatiori 2. = h (t -- 28.630) (1.1123 t- 10978) where, , C = correction to be subtracted, inches of mercury, , h = observed height, inches of mercury. t = observed temperature of the barometer, Fahrenheit degrees. . ' ' Standard atmospheric pressure at sea level is 29.921 in. Hgi Since normal atmospheric pressure decreases about 0.01 iri: Hg for-each'10 ft increase'in. elevation, it is important to make a correction-if the elevation of the barometer is not that of the test apparatus. In many cases the barometric reading-may be obtained from a--nearby Weather Bureau' Station, in which case inquiry should be made ns'to whether the value is as observed or corrected to sea level. - - . .. Atmospheric pressure may also be measured by . an aneroid barometer which is.easily portable. . In this type, variations: in atmospheric pressure bend the thin surface of a sealed box or tube.: The aneroid type is not as accurate as the mercurial and needs frequent calibration. . Most of the pressure:gages used in engineering work indicate the difference between the pressure being measured and-the atmospheric pressure. Such pres-. Sures are called gage pressures. Absolute pressure may be obtained .by adding-barometric pressure and gage pressure algebraically. Pressure Gages . . The Bourdon type gage is a widely used device for measuring pressures.The Bourdon tube is elliptical in. cross-section and circular in.form, and is connected.by.suitable linkage to a pointer which moves over a dial. An increase in pressure tends to straighten the tube and a decrease has the opposite effect. When used with high temperature steam, .the tube naust be protected by a water seal. When used with ammonia it must be made of,steel, or other material not attacked by this substance. When, used for sub-atmospheric pressure, the gage is known as a vacuum.gage,, and is usually graduated in- inches of mercury. For pressures above atmospheric, it is termed a pressure gage and is graduated-usually in pounds.per square inch. " Occasionally, gages are graduated in feet of water. 'Some, made to indicate either pressure or vacuum, are termed compound gages. , Cali bration is usually made by means of a dead weight,tester, consisting of a platform and weights resting on a piston acting on oil in a cylinder. The pressure at all points in the fluid is determined from the area of the piston and the total weight resting.on the. oil.. Adjustments are provided in the gage linkage to make-necessary corrections. A correction chart may be used for accurate work. Manometers. . For measuring low pressures or differences in pressure between two points as in a duct system, the. vertical U tube is a simple and accurate. Instruments and Measurements x 185 gage and is used for: test work.with, waterfor with.various fluids such .-as : mercury, kerosene, .or alcohol. ;Readings are usually in inches of one of these fluids; and must-be arithmetically converted into inches of water or other standard units. The glass tube used to form the manometer should, be chemically clean and the bore should not be less than ?{$ in. A modified form of the U tube manometer using mercury as the- fluid, can be con structed to measure absolute pressures directly 7. For-.measuring pressure differences, of-a few inches of water, -or; less, I/ gages are often set. at an angle for scale amplification. In commercial gages of this type, commonly termed .draft gages, only one tube ofr small bore, is used and- the other leg is replaced by a reservoir. Although the scale is calibrated to read in inches of water, a fluid having the density and characteristics of kerosene is often used. It is necessary, in such a gage, to use a fluid having the same gravity as that for which the gage was originally calibrated, or to apply a correction if another fluid is used. Such gages may be checked one against another to detect errors in gravity of fluid.. For more accurate calibration the gage may be checked against.a micromahometer. or(a calibrating device known as a hook'gage 8. The accuracy, of a draft, gage is dependent on the slope of the tubes and. consequently the base of the gage must'be leveled carefully. .' It is riot desirable to use a slope of. less.than 1 in. 10. Where pressures are read under, extreme conditions of temperature and calibration is possible only at normal temperature it is necessary to correct for the change in density of the liquid in- the manometer 9. . .. For measuring low pressure differences to within 0.001 in. of water very, sensitive micromanometers- areavailable, such as the' Illinois or Wahlen;