Pressure gauges are specially designed instruments used to measure the pressure of a given fluid media. These instruments are classified into analog and digital pressure gauges. Under each category, different types of pressure gauges exist, which are used to measure fluid conditions in different settings. Before we look at the types of pressure gauges, let’s first understand the basics of pressure and pressure measurement.
What is Pressure and Why Measure It?
Pressure is the force applied perpendicularly to a surface per unit area. When dealing with fluid media like gas and liquid, the pressure of the fluid is the amount of force exerted on the wall of the container at any given point.
Pressure can be measured in three forms, namely:
- Absolute pressure – represents the total amount of force acting on a surface per unit area. The earth’s atmosphere has an absolute pressure of 1 atm, while a perfect vacuum has an absolute pressure of zero.
- Gauge pressure –is the pressure of a fluid media relative to the atmospheric pressure. In other words, the gauge pressure is absolute pressure minus atmospheric pressure. Most pressure gauges in the market fall under this category. Those designed to measure pressure less than the atmospheric pressure are called vacuum gauges, and the negative sign may be omitted.
- Differential pressure – this is the pressure difference between any two points. One point is often used as a reference when calculating the pressure of the second point.
One of the benefits of measuring the pressure of fluids is to ensure the optimal working of machines and systems. It also helps prevent undue accidents or system malfunctions due to pressure incompatibilities between system parts or fluid media.
How to Measure Pressure
Over the years, new techniques have been developed for measuring pressure, but most still borrow the classical technology used in mechanical gauges. Most mechanical gauges use a metallic pressure-sensitive element that takes different forms when applied pressure differential. Depending on the magnitude of the pressure differential, the deformation of the elastic component can be measured then converted into pointer rotation on a calibrated scale.
Here are three main types of aneroid or mechanical gauges available in the market:
The Bourdon Tube
This is a c-shape, thin-walled tube used to measure gauge pressure of fluids. When fluid pressure is applied inside the tube, the tube’s cross-section becomes more circular, and it tends to become straighter. Once the fluid pressure disappears, the tube regains its original shape. Any changes in the tube’s shape create some motion pattern converted into pointer rotation and transmitted with links and gears.
Due to the high sensitivity, accuracy, and linearity of the Bourdon gauge, this instrument finds applications in a wide range of industries. Its pressure gauge ranges from 0 – 0.6 bar to 0 – 1600 bar. The accuracy class of this pressure gauge falls between 0.1 and 4.0.
The Diaphragm
This pressure gauge is metallic or ceramic and is often welded or clamped between two flanges. One side of this pressure gauge is frequently exposed to the atmosphere, where the pressure is measured. When the pressure builds up, the diaphragm flexes, and the slight movements are transferred using gears and linkages to rotations displayed on a pointer scale.
The diaphragm pressure gauge is best used to measure pressures of highly viscous media and corrosive gases & liquids. The typical measuring range lies between 0 – 2.5 mbar and 0 – 25 bar, and the accuracy class of this pressure gauge is between 0.6 and 2.5.
Capsule Element
This pressure gauge measures air and dry gases at relatively lower pressures. They consist of two circular membranes that are joined on their outer edge. One diaphragm has a hole in the middle that lets in the fluid media. Any pressure difference between the inner and outer media causes expansion and contraction of the chamber, which allows for instant pressure measurement.
This type of pressure gauge is used in low-pressure pneumatic systems, vacuum pumps, breather valves, etc.
The typical range of the capsule element is 0 – 1 mbar and 0 – 600 mbar. The accuracy class falls between 0.1 and 2.5.
Pressure Gauge Selection Criteria
When choosing a pressure gauge for your unique applications, you need to consider a couple of factors. These include:
- Pressure units – there are different display units of pressure, and you always want to choose what’s easy to work with. Examples include the standard bar, atm, torr, pascal, etc. You should also know how to convert from one pressure display unit to another.
- Pressure ranges – pressure ranges are either in bar or mbar. The right range depends on the specific use case of the pressure gauge, i.e., whether high pressure or low-pressure applications.
- Nominal size – this is determined by the diameter of the pressure gauge. The nominal sizes are classified into seven: 40, 50, 63, 80, 100, 160 & 250 mm.
- Accuracy class – a pressure gauge with a maximum reading range of 10 bar and an accuracy class of 4 can deviate from the actual pressure reading by 4% or 0.4 bar.
- Materials – always consider the chemical properties of the pressure gauge material to ensure compatibility.
Closing Comments
Pressure gauges are the everyday instruments used in various industrial settings where gas or liquid media at varying pressures are to be controlled. Most pressure gauges in the market use the mechanical principle of element displacement, which is converted to a rotational force with a resultant deflection on a calibrated scale.
Choosing the right pressure gauge often narrows down to your unique use cases. Some of the factors you should prioritize include material compatibility, accuracy class, pressure ranges, and pressure units. If you have any challenge differentiating these technical parameters, it’s best to seek professional guidance.
That said, there are also several accessories that you may need to pair up with your pressure gauge for optimal results. Some of these accessories include snubbers that dampen the effects of pressure spikes & pulses and safety gaps that help absorb shock and add durability to your pressure gauge. Others are reducers, seals, and push-on connectors.