The basics of pressure gauges and pressure gauge measurement techniques
Pressure is a fundamental scalar quantity that finds application in nearly all sectors and industries. Pressure gauges are mechanical instruments used to measure pressure in fluid media. There are different pressure gauge types based on their unique design and applications. By measuring the pressure values of fluids in a system, pressure gauges ensure optimal performance and safety.
Pressure and pressure measurement
Pressure is simply defined as the force acting perpendicularly on a unit area. For stagnant liquid and gases, the pressure would be the amount of force acting on the wall of the container with the fluid. If the fluid is moving, there's an additional pressure called dynamic pressure acting in the fluid flow direction.
There are three forms of pressure: absolute pressure; gauge pressure; and differential pressure.
Absolute pressure represents the total amount of force acting on a unit area. The earth's atmosphere at sea level, for instance, has an absolute pressure of one atm, while a vacuum has an absolute pressure of zero.
Gauge pressure is the pressure of the fluid media relative to the atmospheric pressure. In other words, gauge pressure is calculated by subtracting atmospheric pressure from absolute pressure. A pressure gauge designed to measure vacuum pressure is called a vacuum gauge and will have the negative sign omitted. Gauge pressure values are denoted by the letter 'g,' e.g., 100 psi g.
Differential pressure is the pressure difference between any two points in a system. Some pressure gauges have an inbuilt feature to measure differential pressure between points, i.e., without the need of manually subtracting pressure values.
Pressure measurement techniques
There are several techniques used in measuring pressure, but the most common one is the use of aneroid or mechanical gauges. This technique isn't only effective, but it's also widely adopted and has seen key improvements over the decades.
Mechanical gauges use metallic, pressure-responsive elements to measure pressure values. The element could take any form, but its function is to flex elastically when it senses some pressure differential. When the element flexes, it deforms, and its displacement can be captured and converted into a rotating motion displayed on a pointer scale. There are three main types of aneroid/mechanical gauges widely used today.
These are the bourdon tube, capsule element and diaphragm. These pressure gauges have varying designs, measurement accuracy, and unique use cases.
Bourdon tube
This is a flat, thin-walled tube with one closed end, formed into a helix or C-shape. The tube also has a hole through where fluid pressure is applied. The oval cross-section becomes more circular when this happens, and the tube becomes straight. Once the fluid pressure decreases, the tube regains its original shape. This change in the tube's shape creates some displacement at the end of the tube, which is then converted into pointer rotation with gears and links.
Bourdon tubes measure gauge pressure and are among the most commonly used pressure gauges, thanks to their excellent sensitivity, accuracy, and linearity. They come in various designs to serve a wide range of applications. Bourdon tube pressure gauges have a measuring range of 0 – 0.6 bar to 0 – 1600 bar with an accuracy class of 0.1 – 4.0.
Diaphragm
This pressure gauge uses the displacement of a flexible membrane between two environments to measure the pressure values. The diaphragm can be metallic or ceramic with one side exposed to the atmosphere or sealed against a vacuum where absolute pressure is measured. The diaphragm flexes when pressure builds up, causing the gears and linkages to turn the displacement into dial measurements. This pressure gauge is best used for corrosive fluids and highly viscous liquids. The measuring range lies between 0 – 25 mbar and 0 – 25 bar and has an accuracy class of 0.6 – 2.5.
Capsule element
Capsule element pressure gauges are designed to measure air and dry gases at relatively low pressures. It consists of two circular membranes joined on the outer edge, where one diaphragm has an orifice in the middle that lets in the fluid media. When there's a pressure difference between the inner and outer fluid media, the chamber expands or contracts, giving some displacement and allowing for pressure measurement.
The capsule element pressure gauge finds applications in low-pressure pneumatic systems, vacuum pumps, breather valves, and overpressure & filter monitoring. They measure pressure values within the range of 0 – 1 mbar to 0 – 600 mbar. The accuracy class of this pressure gauge is 0.1 – 2.5.
Summary
Pressure gauges are widely used in different industries, and they come in varying designs. When choosing a pressure gauge, you should pay attention to its unique features and their suitability in your desired deployments. Some of the factors to consider are the pressure units that the device displays (could be in bar, atm, torr, etc.), the pressure ranges, and the accuracy class.
A pressure gauge, for example, with a maximum reading of 10 bar and accuracy class 3 may deviate from the actual pressure reading by 3%, i.e., 0.3 bar. The more pressure-sensitive your applications are, the fewer pressure deviations are required. The other things you should also keep in mind are material compatibility with the fluid media, safety and service life, and the additional accessories needed for optimal performance.
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