MANOMETER (Greek, ((rarefaction meas., an instrument for measuring the pres sure exerted by gas or liquid. It may have many forms, of which the mercurial barometer is one. (See One of the com monest designs, for the measurement of pres sures not greatly different from that of the at mosphere, consists of a U-tube, one of whose legs is open to the air, while the other is in communication with the gas or liquid whose pressure is to be measured. The lower part of the U is filled with some non-volatile liquid of known density, and the difference between the pressure of the fluid under examination and that of the atmosphere is found by observing the difference between the levels of the manometric fluid in the two branches of the U-tube. If the absolute pressure of the fluid is desired, it is necessary to add the atmospheric pressure to the differential pressure as read from the ma nometer. In rough work it may be sufficient to assume the atmospheric pressure to be 14.7 pounds per square inch; but in more refined ob servations the atmospheric pressure must be determined by reading the barometer, simul taneously with the manometer. Mercury is commonly the liquid that is used in the U-tube, but when the di erences in pressure that are to be read are very small, some less dense liquid may be used with advantage. Sulphuric acid is often employed in such cases; and where (as in the measurement of chimney draft) a slight amount of evaporation from the mano metric fluid is uniiniortant, water may be em ployed. When the pressure to be measured materially exceeds one atmosphere, the siphon manometer, as lust described, is modified by sealing one of the ends of the U-tube, instead of leaving it open to the air. In this case the pressure is determined by observing the amount of compression that it produces in the air that is confined in the sealed arm of the. siphon, by the manometric fluid; for it is known, by Boyle's law, that the volume of the air in this arm is sensibly proportional to the recip rocal of the absolute pressure, so long as the temperature remains constant. Boyle's law is not rigorously exact, however, and when a high degree of precision is required from the com pression manometer, it is necessary to make allowances for its error. Data for this purpose
have been given by Amagat, up to 85 atmos pheres, when the temperature of the manome ter is maintained at 16° C. (60.8° F.), Consult Amagat,'Comptes Rendus,' Vol. XCIX, p. 1153; Preston, 'Theory of Heat,' p. 403). In steam engineering the commonest form of manometer is the "Bourdon gauge," which depends for its action upon the elastic deformation of a flat tened metallic tube when exposed to an internal pressure. In practice the flattened tube is bent into a circular form, one end of it being fixed while the other communicates, by means of a multiplying gear, with an index hand which travels over the face of a graduated dial. A tube so constructed straightens out slightly when subjected to an internal pressure, return ing again to its original form when the pressure is removed. The deformation is approximately proportional to the magnitude of the pressure (so long as the tube is not strained beyond its elastic limit), and hence the dial may be gradu ated, without difficulty, so as to indicate true pressures, at least to a degree of precision quite sufficient for the purposes of steam engi neering. All such gauges should be carefully compared • with a standard mercury column, however, before great reliance is placed upon them; for it is found that they are sometimes seriously in error in some parts of the scale, even when sensibly correct in other parts. In using them in connection with steam boilers, care should also be taken to prevent steam or highly heated water from coming into direct contact with the curved tube, since the elastic properties of the tube are injured by overheat ing. To ensure the proper protection of the gauge, a siphon, or a complete circular bend, should be placed in the pipe between the gauge and the boiler. The trap so formed will fill with water of condensation the first time the boiler is fired up, and thereafter it will be im possible for steam to enter the gauge directly.