PRESSURE GAUGE

instrument for measuring the condition of a fluid (liquid or gas) that is specified by the force that the fluid would exert, when at rest, on a unit area, such as pounds per square inch or newtons per square centimetre. The reading on a gauge, which is the difference between two pressures, is known as the gauge pressure. If the lower of the pressures is the pressure of the atmosphere, the total, or absolute, pressure is the sum of the gauge and atmospheric pressures. Pressure gauges (left) Manometer; (right) Bourdon tube The simplest device for measuring static pressures up to about 90 pounds per square inch (62 newtons per square centimetre) is a U-tube manometer like the one shown at the top on the left . The differential pressure is the difference in level (head) h, multiplied by the density d of the liquid and is indicated by the difference in level between the two columns of liquid. The manometer liquids most commonly used are mercury, oil, alcohol, and water. Pressure gauges (left) Manometer; (right) Bourdon tube The Bourdon-tube gauge, invented about 1850, is still one of the most widely used instruments for measuring the pressure of liquids and gases of all kinds, including steam, water, and air up to pressures of 100,000 pounds per square inch (70,000 newtons per square centimetre). As shown on the right in the Figure, a flattened circular tube (cross-section in C-c) is coiled into a circular arc; one end is soldered to the central block B and is open to the fluid the pressure of which is to be measured; the other end is sealed and coupled to the pointer spindle. If the pressure inside the tube is greater than the outside pressure, the tube tends to straighten out, thus turning the pointer; the pressure is read on a circular scale. Metal bellows and diaphragms are also used as pressure-sensing elements. Because of the large deflections for small pressure changes, bellows instruments are particularly suitable for pressures below atmospheric. Two corrugated diaphragms sealed at their edges to form a capsule, which is evacuated, are used in aneroid barometers to measure atmospheric pressure (see altimeter). These instruments employ mechanical linkages and so are primarily useful for measuring static pressures or pressures that change slowly. For rapidly changing pressures, electrical pressure transducers that convert pressure to an electrical signal are more suitable. These include strain gauges; moving contact resistance elements; and inductance, reluctance, capacitative, and piezoelectric devices. Electromechanical transducers, which are used in hydraulic controllers, where speed and power are needed, convert changes in pressure of fluid to electrical signals.