13.7: Pressure Gauges

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.

When a soccer ball is deflating, gas escapes till the inside pressure equals the atmospheric pressure. At this point, the pressure gauge reads zero.

Most gauges are calibrated to measure zero at atmospheric pressure, and the pressure measured is called the gauge pressure.

The absolute pressure is the total pressure, including gauge pressure and atmospheric pressure.

There are two important classes of pressure gauges: the manometer, which measures gauge pressure, and the barometer, which measures atmospheric pressure.

A manometer is a U-shaped glass tube containing water or mercury. One end of the tube is exposed to atmospheric pressure and the other is connected to a container of unknown pressure.

The pressure at the bottom of both ends is compared, and the gauge pressure is estimated.

A mercury barometer consists of a single long tube, closed at one end. The tube is filled with mercury and inverted over a mercury dish.

The mercury level in the tube rises and falls, depending on the pressure outside the tube. From the mercury level, the atmospheric pressure is estimated.