From Condensed Handbook of Measurement and Control, 3rd Edition


Pressure is measured as a force per unit area. Pressure measurements are important not only for the monitoring and control of pressure itself but also for measuring other parameters, such as level and flow (through differential pressure). Pressure measurement is one of the most common measurements made in process control. It is also one of the simplest in terms of which measuring device to select. One of the key items to consider is the primary element (i.e., strain gage, Bourdon tube, spiral, etc.). Primary-element materials should be selected to provide sufficient immunity from the process fluids and at the same time the required measured accuracy under the process conditions they will encounter.

This chapter provides some of the basic knowledge plant personnel will require to select the correct pressure-measuring device. However, it is essential that the instrument selector take into consideration the users' experiences.

Pressure-measuring instruments are really pressure transducers that convert the pressure energy into a measurable mechanical or electrical energy. Pressure measurement is always made with respect to a reference point. There are basically three types of pressure-sensing configurations (see figure 6-1).

  1. Gage pressure, where the reference is atmospheric pressure

  2. Absolute pressure, where the reference is complete vacuum

  3. Differential pressure, which represents the difference between two pressure levels (note that gage pressure is a differential pressure between a value and atmospheric pressure)

Figure 6-1: Absolute and gage pressure.

In certain cases, pressure devices must conform to specific requirements. For example, on pressures greater than 15 psig (103...

Products & Services
Vacuum Sensors
Vacuum sensors are devices for measuring vacuum or sub-atmospheric pressures.
Pressure Sensors
Pressure sensors include all sensors, transducers and elements that produce an electrical signal proportional to pressure or changes in pressure.
Pressure Gauges
Pressure gauges are used for a variety of industrial and application-specific pressure monitoring applications.  Uses include visual monitoring of air and gas pressure for compressors, vacuum equipment, process lines and specialty tank applications such as medical gas cylinders and fire extinguishers.
Mechanical Pressure Gauges
Analog pressure gauges are mechanical devices that include bellows, Bourdon tubes, capsule elements and diaphragm element gauges.
Pressure Switches
Pressure switches are actuated by a change in the pressure of a liquid or gas.  They activate electromechanical or solid-state switches upon reaching a specific pressure level.

Topics of Interest

Overview Temperature is a widely used measurement. Galileo is credited with inventing the first ther-mometer in 1595. Over the years thermometer technology has evolved, and measuring principles are...

Pressure Transmitters Theory of Operation Pressure is defined as force per unit area. If a weight of 100 pounds acts on an area of one square inch, the pressure would be stated as 100 pounds/1...

Glossary A Absolute position measurement Position measured from a fixed point. Absolute pressure Pressure measured with reference to a perfect vacuum. Accelerometer A sensor for measuring...

The conventional units for defining pressure are SPI, pounds per square inch. However, in all pressure measuring devices, a reference pressure has to be established and the PSI qualified to indicate...

Principles of Pressure Pressure is the force per unit area a fluid or gas exerts on its surroundings. A container of gas contains numerous atoms and molecules constantly bouncing of its walls. The...