The concept of pressure is simple—it is a force. Typically measured in psi or pounds per square inch, pressure is the force applied on a specific area. However, there are other ways of expressing pressure and different units of pressure measurement. It is important to understand the differences so that the user can apply specific measurements and units properly.

Depending on the application, there are several types of pressure. For instance, there is absolute pressure. Engineers define the zero point of absolute pressure as that occurring in a perfect vacuum, which is the case for some applications. Absolute pressure readings typically include the pressure of the media added to the pressure of the atmosphere. One can use the absolute pressure sensor to rely on a specific pressure range for reference while eliminating instances of varying atmospheric pressure. Thermodynamic equations and relations typically use absolute pressure.

Then there is gauge pressure, which indicates the difference between the pressure of the media and a reference. While the pressure of the media can be that of the gas or fluid in a container, the reference can be the local atmospheric pressure. For instance, the gauge for measuring tire pressure will read zero when disconnected from the tire. Which means, it will not read or register the atmospheric pressure. However, when connected to the tire, it can reveal the air pressure inside the tire.

Another type of pressure is the differential pressure. It is somewhat more complex compared to gauge or absolute pressure, as it is the difference in the pressures of two media. The gauge pressure can also be termed as a differential pressure sensor, as it measures the difference between the atmospheric and the media’s pressure. With a true differential pressure sensor, one can measure the difference between any two separate physical areas. For instance, by measuring the differential pressure, one can indicate the pressure drop or loss, from one side of a baffle to the other.

Compared to the above three, sealed pressure is less common. However, it is useful as a means of measurement. It measures the pressure of a media compared to a sample of atmospheric pressure that is sealed hermetically within a transducer. Exposing the pressure port of the sensor to the atmosphere will cause the transducer to indicate a reading close to zero. This is due to the presence of ambient atmospheric pressure on one side of the diaphragm and a fixed atmospheric pressure on the other. As they are nearly the same, the reading it indicates is close to zero. When they differ, the reading will be a net output other than zero.

The internal pressure can change due to differences in temperature. This may create errors exceeding the accuracy of the sensor. This is the main reason engineers use sealed sensors for measuring high pressures—the changes in the references cause only small errors that do not affect the readings much.

Engineers typically use several units when expressing measurements of pressure. They are easy to modify using the conventions of the International System of Units, even when they are not a part of that measurement system.