PRESSURE GAUGE, a term commonly applied to an in strument for measuring pressure, as distinct from a barometer, a manometer (qq.v.), or a gauge employed in vacuum practice. The following article deals with the type of gauge which registers pressure by the distortion of a metal tube or diaphragm, and which may be used to indicate the pressure of steam, gas, am monia, water, brine, petrol, or oil. The lowest pressure require ments are those for blast pressures, or gas or air, working up to about 2 lb. per sq.in., and the highest the hydraulic gauges up to 12 tons per square inch. For very low pressures a diaphragm arrangement is employed. In the Schaffer gauges a corrugated diaphragm is acted upon by the pressure and its deflection com municates the movement through a link and toothed quadrant to the pinion, which turns the pointer around the gauge dial. Such gauges will withstand rough usage, as on road vehicles and portable engines.
The Bourdon type has a bronze tube of elliptical cross-section bent into circular form. One end is soldered to a central block through which the fluid enters (see fig.) and the other end is sealed, and coupled by a link to a pivoted quadrant with teeth meshing with those of a pinion on the pointer spindle. Backlash between the teeth is absorbed by a hair-spring exerting constant pressure on the pinion. Pressure within the tube tends to change its cross-section from elliptical to circular, and the tube conse quently uncoils to a slight degree, so turning the pointer.
In a vacuum gauge the reduction of pressure has an opposite effect, and the mechanism must be arranged in reverse fashion to cause the pointer to move clockwise. In a compound gauge, read ing for both pressure and vacuum, the latter figures are placed to the left of the pressure figures. For high pressures a steel tube is fitted instead of bronze, being more elastic and less affected by high temperatures. Special construction or some suitable safe guard is necessary in dealing with some gases, such as oxygen, or where corrosive action is set up.
Special types include the differential pressure gauge for regis tering the difference only in pressure at any two points. There are two inlet tubes and two hands, and if the pressures are equal there is no reading on the dial. The electric alarm gauge possesses electric contact devices to ring a bell at a definite minimum or maximum pressure. The electric control gauge closes a circuit at a definite pressure, and operates a relay in the circuit of an electric motor; e.g., an electrically-driven air-compressor may be started and stopped automatically in accordance with the de mands for air. The self-recording pressure-gauge traces a perma nent record of varying pressures upon a chart. See also VACUUM and VACUUM-PUMP.