When a small amount of oil is placed in a cup, for example, it exerts force against the materials with which it comes into contact: air and the cup. If the oil were forced into a tank, as more oil was forced in, the force exerted against the tanks walls would increase. That force, or pressure, is an expression of quantity and concentration, and its measurement indicates both.
Oil pressure gauges are used to measure the quantity of oil in tanks, engine lubrication systems and in a wide variety of other settings. One of their most common uses is in automobiles; oil pressure gauges are among the most important indicators on a vehicle, as halting sustained low-oil operation of a motor can prevent serious engine damage. Oil pressure gauges can also be used to measure oil pressure in the lubrication systems of heavy machinery like tractors and other farm equipment. They are an essential part in the continuing proper operation of an extensive variety of mechanical systems.
In engines, oil does not merely sit in a tank. Oil is distributed throughout an engine to many of the working parts through a pressurized oil line. The oil pressure gauge in a vehicle or machine measures the oil pressure in the oil transmission line. Depending on the age of the equipment, the gauge may be completely mechanical, a combination of mechanical and electric or it can be digital.
Very old engines and some sport and enthusiast engine constructions still feature mechanical oil gauges. Vehicles of moderate age are generally equipped with electric oil gauges that involve the transmission of an electrical signal to the gauge indicator hardware. The most advanced systems involve the transmission of the pressure reading from a sensor to a computer, which in turn displays the reading as well as other readings like fuel level and engine temperature.
Many recently manufactured vehicles feature fully computerized gauge clusters, often on a single display. Different configurations are appropriate for different uses. For example, custom car builders tend to include mechanical gauges for aesthetic or for performance purposes, while computerized gauges are appropriate for casual motorists. In each case, though, the oil pressure gauge performs the essential function of preventing damage to engines and engine parts.