# Basics of Pressure and Pressure Gauges

Pressure is a force per unit area measured in units of Pascal. It’s a measure of how much something resists the force being applied to it, and it’s measured by how many Newtons of force is needed to push down on an area of 1 square meter.

Pressure gauges help us monitor the pressure inside different objects or places in our machines, such as boilers or reactors. They can be installed in pipes, tanks, and other vessels where they measure the internal pressure within this equipment or environment. Pressure gauges can also tell how much pressure there is in an engine or gas tank.

## Different Types or Forms of Pressure

There are three main ways you can measure the pressure of a fluid media. This gives us the three main forms or types of pressure. The first thing to understand about these three forms of pressure is that they are all related. Absolute pressure, gauge pressure, and differential pressure are all measures of fluid force per unit area. The main difference between them is where you measure it.

• Absolute Pressure: This is the sum of atmospheric pressure and total system pressure. You can calculate it by summing up all the positive pressures (if you have any) and subtracting any negative pressures.
• Gauge Pressure: This is the difference between absolute or total system pressure and static or external atmospheric pressure (the air outside your system). If you know how much static air weighs, you can use this formula: gauge pressure = absolute pressure – static air weight in pounds per square inch (psi).
• Differential Pressure: This is the difference between two points in a system; for example, it could be the difference between your high-side port and your low-side port on an oil pump assembly.

## Techniques for Measuring Pressure

There are many different methods you can use to measure pressure. Some are more precise/accurate than others, and some require specialized equipment. The method used will depend on the application, the environment where it is being measured, and other factors.

One widely adopted technique for measuring pressure is using aneroid pressure gauges. These gauges are accurate, reliable, and versatile. Popular aneroid gauges in the market are the bourdon tube, capsule element, and diaphragm.

### The Bourdon Tube

The Bourdon Tube is a pressure-measuring device that relies on the principle of a coil spring. The device consists of a mercury-filled tube and a piston that acts as the movable element. When pressure is applied to the tube, the mercury moves up the tube and presses against the piston. This causes the tube to elongate in proportion to the force exerted on it. As this happens, an indicator moves along an attached scale to indicate how much pressure has been applied.

This gauge type can be used in many applications, including measuring pressure in pneumatic equipment or hydraulic systems. It’s also used in automotive systems where precision is required for accurate readings.

### Diaphragm

A diaphragm pressure gauge is a type of pressure-measuring gauge that uses a thin membrane to measure the pressure of a fluid. The measurement is taken by measuring the deflection of the diaphragm in response to this fluid force. The deflection of the diaphragm moves an indicator needle on a scale that indicates the measured pressure level.

The internal components of a diaphragm-type pressure gauge include:

• A casing that holds all of these parts together and provides support for them while they are being used;
• A capillary tube or other means for directing fluid into the casing;
• A flexible membrane or diaphragm separates two chambers inside the casing, and
• An indicator needle or pointer is attached to one side of this membrane. It moves up and down as pressure changes inside the chamber.

A Diaphragm pressure gauge is ideal for corrosive gases and highly viscous media. The measuring range lies between 0-2.5 mbar and 0-25 bar with a 0.6 to 2.5 accuracy class.

### Capsule Element

This pressure gauge is designed to measure air/dry gases at relatively low pressure. The capsule is a small glass tube with two circular membranes inside it. One end of the capsule is sealed and connected to a tank, while the other has an opening that allows molecules to pass through it when there is pressure inside the tank. A pressure difference between the inner and outer media leads to the contraction or expansion of the chamber, which allows for pressure measurement. The capsule element is used for accurate pressure measurement in gaseous media.

## Choosing a Pressure Gauge for Your Application

Pressure gauges come in different designs, and it’s essential to understand the various selection criteria. Ideally, you want to understand your system requirements and applications to ensure you choose a pressure gauge with the desired pressure units, pressure ranges, and accuracy class.

Similarly, it would help if you understood the chemical compatibility of the materials used to manufacture the gauge. The nominal size of the gauge, mounting/connection design, and the need for additional accessories such as seals, reducers, and snubbers are also critical during the selection process.