Pressure is defined in physics as the amount of force in a given area as applied to an object, with the direction of the force perpendicular (at a 90 degree angle) to the surface. It is the effect when a force is exerted on all points of a surface.

Three kinds of pressure measurement

Pressure is measured against three different reference points.

• Absolute pressure is a measurement relative to a perfect vacuum.
• Gauge pressure is a measurement relative to ambient or atmospheric pressure.
• Differential pressure is the difference in pressure between two volumes.

A car’s tire pressure is gauge pressure, as it is the measurement of the amount of pressure inside the tire compared to the ambient or atmospheric pressure outside the tire. If a car’s tire pressure reads at 32 pounds per square inch or psi, this means that the air pressure inside the tire is 32 psi above the air pressure outside it. Ambient or atmospheric pressure is a commonly used reference point, because it is constant – 14.7 psi. Since a perfect vacuum is actually zero pressure, absolute pressure is the measurement of the total amount of pressure. In the case of a car’s tire, its absolute pressure would then be the gauge value plus the atmospheric value, or 46.7 psi. Differential pressure is a measurement more commonly used in industrial processes. Instruments that measure differential pressure have two access points, each one connected to the volumes that need to be compared. This simplifies the process of comparative measuring, because it takes away the need of having two instruments that use a constant, separately attached to each volume.

Units of pressure measurement

Several units of measurement are used for pressure. The unit of measurement used depends on the context or field of study. The most standard is ‘pascal’ or ‘Pa’ which is set by the International System of Units (SI). Pa is actually a shorter version of ‘newton per square meter’ or N/m2 where newton represents the amount of force and square meter represents the given area. Non-SI, or the old metric system, would use kg/cm2 where kilogram instead of newton is used to represent force and square centimeter for area. ‘Bar’ is an older unit of measurement and is commonly used for atmospheric air pressure. At sea level, air pressure is equal 1 bar. Meteorologists still use both Pa and bar. Since pressure is also commonly measured through its ability to displace a contained given amount of liquid, it can also be expressed in ‘millimeters of mercury’ or ‘mmHg’ or ‘centimeters of water’, mercury and water being the two most common fluids used in pressure measuring instruments that use this method. mmHg is still used for blood pressure and centimeters of water is still used for lung pressure. In the automotive industry, pounds per square inch is still the standard unit of measurement.

Measuring instruments for pressure

Some instruments used to measure pressure use a liquid column. The amount of pressure is determined by the amount of liquid displaced. These types of measuring instruments were collectively called manometers or more technically, liquid column hydrostatic instruments. Another class of pressure measuring instrument does not use liquids and are thus called aneroid (‘without liquid’). These instruments are mechanical and may use closed coiled tubes, which expand or uncoil as pressure increases or coil more tightly as it decreases. These are called Bourdon gauges, named after Eugene Bourdon who patented the design in 1849. Others use membranes that expand and contract in reaction to the amount of pressure. These types of instruments are sometimes called diaphragm or bellows gauges.

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