The word “fluid” technically can be used to describe a liquid or gas, and pressure gages used to measure the concentration of liquid in an enclosure closely resemble gas pressure gauges in terms of their construction and operating principles.
While the term “air pressure gauge” is not descriptive only of pressure gauges that measure air concentrations, it is limited to the measurement of gas concentration and excludes the measurement of liquid pressure. Air pressure gauges are important to industry, commerce and even in consumer products markets. They are used to determine gas concentration levels in gas storage tanks used for industrial processes, they are used to prevent explosions in boilers and they can be used to measure tire air pressure. Gas pressure is an expression of quantity, and measuring that pressure is the means by which the fullness or emptiness of a gas container can be determined.
There are many contexts in which air pressure gauges are employed, and the number of different gauge configurations is equally large. Barometers are examples of simple air pressure gauges; they are used to measure atmospheric pressure. A firm understanding of how a barometer works lends itself to a richer understanding of how more complicated pressure gauges operate. A barometer features a fluid in a container that is partially open to air exposure.
The level of the fluid in the barometer indicates the level of atmospheric pressure in the barometer’s immediate area. When the pressure increases, the level of the fluid in the barometer rises; when pressure falls, the fluid level also falls. All air pressure gauges accomplish this same task, though each configuration varies in the way in which it is accomplished. Bourdon gauges, for example, involve a strip of metal that changes shape based on differences in fluid pressure.
Bourdon gauges are used to measure pressures relative to ambient atmospheric pressure. This means that the idle needle position on a Bourdon gauge indicates current atmospheric pressure; any needle position relative to the idle position indicates a pressure relative to current atmospheric pressure. Other gauge configurations are capable of measuring pressure regardless of atmospheric pressure. These configurations are called absolute pressure gauges. Different configurations are appropriate for different applications.