A pressure test verifies the pressure capacity or tests the current pressure of a system. A pneumatic test is one type of pressure test. Common pneumatic measurement test equipment includes gauges that read pressure and a source of compressed gases. Pressure testing is critical in a number of industrial areas for safety reasons and in the proper functioning of the equipment.
In pressure testing, if the fluid used to pressurize the system is liquid, usually water, the test is called a hydrostatic test. A pneumatic test implies the fluid being used is a gas, usually air or an inert gas. In determining the pressure capacity of a system, all ports to the system are closed except one through which fluid is added until either the pressure rating for the system is achieved, the pressure cannot be achieved due to leaks in the system, or the system fails catastrophically by bursting.
A pneumatic test is inherently more dangerous than a hydrostatic test due to the higher energy content of a compressed gas, and this type of test is limited to lower pressures or smaller systems. A serious injury or death may occur due to the explosions during a pneumatic test. Safety procedures for pneumatic testing are extensively documented in engineering references and code books. The American Society of Mechanical Engineers (ASME) prints the Boiler and Pressure Vessel Code (BPVC), and provides training and certification programs for engineers around the world.
A disadvantage of hydrostatic testing is the introduction of water in systems that must be water free. To test large, high-pressure systems, a low-pressure pneumatic test might be used to detect leaks followed by a high-pressure hydrostatic test at the fabricators shop prior to installation. Alternatively, methanol or other hydrocarbon liquid may be used in place of a pneumatic test. Vacuum testing is limited to low pressures as well, but it is less dangerous than pneumatic testing and is often used in laboratories to check for leaks in glassware setups.
A common example of a pneumatic pressure measurement is the use of a tire pressure gauge to measure pressure in car tires. A mechanic also measures the compression on an automobile engine by running the vehicle and measuring the pressure at each of the pistons. Cryogenic systems, such as those used in surgery, and refrigeration are often pneumatically tested as well.
To measure pressure in systems with a pneumatic test, a pressure gauge with an internal deformable diaphragm is used. The pressure is the difference between system pressure and atmospheric pressure and is often reported in pounds per square inch gauge, psig (kPa/cm2 gauge). In reactor design, wherein the operating pressure may be necessary to know from a kinetic or thermodynamic perspective, or the use of gas law calculations is required, the atmospheric pressure, 14.7 psi (101.3 kPa) must be added to the gauge pressure to get total system pressure.