Pressure gauges measure the difference in pressure between two points. It is common for one of these two points to be the atmosphere,22 in which case the measured pressure is based on the reference of atmospheric pressure. This is called gauge pressure. Pressure gauges in commercial HVAC systems are most commonly used to measure the gauge pressure of fluids in pipes, such as chilled water, heating water, and steam. However, pressure gauges can also measure the difference in the pressure of the fluid at two points in a piping system, such as the differential pressure between the inlet and outlet of a strainer. In this case, the differential pressure would provide an indication of the extent that the strainer is clogged with debris.
Pressure gauges are connected with ¼-in. brass tubing in threaded openings in large pipes or in pipe tees for smaller pipes. A pressure snubber should be installed between the pipe and the pressure gauge for pipes with liquid flow, and a pressure siphon should be installed between the pipe and the pressure gauge for pipes with steam flow. Pressure snubbers consist of a porous metal disc that dampens the fluctuations in the fluid pressure and reduces wear on the gauge mechanism. Pressure siphons consist of a single coil of brass tubing that is filled with water to prevent live steam from entering the pressure gauge Bourdon tube. A gauge cock, which is typically a needle-type shutoff
valve, should also be installed between the pipe and the pressure gauge to enable the pressure gauge to be isolated from the fluid in the pipe