ABSOLUTE AND GAUGE PRESSEURE

 

For gases, pressure is sometimes measured not as an absolute pressure, but relative to atmospheric pressure; such measurements are called gauge pressure. An example of this is the air pressure in an automobile tire, which might be said to be "220 kPa", but is actually 220 kPa above atmospheric pressure. Since atmospheric pressure at sea level is about 100 kPa, the absolute pressure in the tire is therefore about 320 kPa. In technical work, this is written "a gauge pressure of 220 kPa". Where space is limited, such as on pressure gauges, name plates, graph labels, and table headings, the use of a modifier in parentheses, such as "kPa (gauge)" or "kPa (absolute)", is permitted.

Gauge pressure is a critical measure of pressure wherever one is interested in the stress on storage vessels and the plumbing components of fluid systems. However, whenever equation-of-state properties such as densities or changes in densities must be calculated, pressures must be expressed in terms of their absolute values.