Low pressure gauges are instruments that measure the pressure of gases and liquids that are moving at low velocities. They generally measure pressures of 10 to 15 pounds per square inch (PSI). As opposed to high pressure gauges, which measure up to 10,000 PSI, low pressure gauges assess pressures close to atmospheric pressure, which is ~14.7 PSI.
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Gases and liquids that are present in systems or closed vessels inherently have a pressure; this pressure is increased as more fluid is added to the volume or when the temperature rises. Low pressure gauges are made from bronze, brass and stainless steel. The dial or screen is protected by strong glass. Gauges have threaded fittings so they may be easily added to a system or vessel. They also have intake ports - differential pressure gauges require two - and relief valves. Gauges designed for low pressure are extremely sensitive and accurate out of necessity; many processes depend on the correct pressure to function properly and safely. Gauges are used in environments where the pressure tends to fluctuate frequently and so requires constant monitoring. Typical applications include air flow indicators, liquid level indicators, leak testers, draft gauges, natural gas gauges and many other applications in hobbyist, construction, manufacturing, industrial and commercial processes.
There are two main techniques used to measure pressure. Hydrostatic gauges have few moving parts and rely on the level of liquid in a tube to evaluate pressure. They use a liquid to compare the measured pressure to the pressure exerted by gravity on a fluid at equilibrium. Liquid column instruments and piston gauges use this method. A manometer is also a pressure measuring instrument, usually limited to measuring low pressures. This term is often used to refer specifically to liquid column hydrostatic instruments. The other kinds of gauges are aneroid. They do not use liquid but instead have a thin flexible membrane that curls, bends or twists according to the pressure being exerted upon it. One example of an aneroid gauge is a Bourdon tube, many of which have coiled membranes that roll or uncoil in accordance with the amount of pressure. A diaphragm gauge has a membrane in between two sealed spaces that deflects as the pressure changes. The differences or changes in pressure are read by a sensor and then indicated on the instrument to provide a quick reading. Bourdon tube gauges are effective for pressures of 15 PSI and above; they are not used to measure pressures lower than 15 PSI because they cannot provide reliable measurements. Other aneroid gauges are preferred instead.
