Most pressure gauges, like those on scuba tanks, are calibrated to read zero at atmospheric pressure. Readings from such gauges are called the gauge pressure, which is the pressure relative to atmospheric pressure. When the pressure inside the tank exceeds atmospheric pressure, the gauge reports a positive value. Some gauges are designed to measure negative pressure. For example, many physics experiments must take place in a vacuum chamber, a rigid chamber from which some of the air is pumped out. The pressure inside the vacuum chamber is lower than the ambient atmospheric pressure, so the pressure gauge on the chamber reads a negative value.
The absolute pressure is the sum of gauge pressure and atmospheric pressure. The absolute pressure in fluids cannot be negative. There are various devices that are used for measuring pressure, ranging from tire gauges to blood pressure monitors. Many other types of pressure gauges, such as manometers and barometers, are commonly used to test the pressure of fluids. Some other types of pressure gauges include strain gauges, capacitance pressure gauges, piezoelectric pressure gauges, and ion gauges, which measure pressure by ionizing molecules in highly evacuated chambers. Different pressure gauges are useful in various pressure ranges and under different physical situations.
