# Basics of Pressure and Pressure Gauges

Nearly all industries have pressure measuring devices or pressure gauges incorporated into their systems. These devices measure the pressure of working fluids helping technicians ensure the pressure readings remain within the set limits. Depending on the type/design of the system and the fluid being measured, the pressure gauges used may vary.

We have discussed more on pressure gauges and the basics of pressure in the sections below. You’ll also learn the different types of pressure gauges, their working principle, selection criteria, and everything in between.

## Fundamentals of Pressure

Pressure is the amount of force applied perpendicularly to a surface per unit area. A pressure gauge is used to measure pressure, but you first have to know what kind of pressure you are measuring.

Here are the three types of pressure:

• Gauge Pressure. Gauge pressure, also known as relative pressure, can be defined as the pressure of the medium relative to the surrounding pressure. In other words, this pressure is the difference between atmospheric pressure and the medium’s pressure.
• Absolute Pressure. This is the pressure of a medium measured relative to a perfect vacuum, i.e., at zero pressure.
• Differential Pressure. This is the pressure difference between the two points in a system. To measure differential pressure, you must use one point as a reference to help determine the pressure of the other point.

## Pressure Measurement

Several methods have been developed for measuring pressure, with one of the most used being the aneroid or mechanical gauges. Aneroid gauges use a metal that is responsive to pressure. The metallic element flexes and deforms when differential pressure is applied to it. Here, the amount of deformation is translated to a rotation of a pointer on a scale.

The bourdon tube, capsule element, and diaphragm are the three types of aneroid gauges.

### The Bourdon Tube

The bourdon tube is a C-shaped, thin-walled, closed-end tube that straightens when pressure is applied and regains its initial shape when the pressure is withdrawn. When pressure is applied, the change in shape creates movement at the open end of the tube. This is reflected on a pointer rotation on the scale.

Due to its high sensitivity and accuracy, the bourdon tube is commonly used in industries that utilize medium and high-pressure applications. Some of these industries include; chemical processing, automotive, marine, and aerospace industries.

### Diaphragm or Membrane Pressure Gauge

With this type, the gauge has a flexible thin membrane (diaphragm) that deflects when measuring pressure. The diaphragm deflection is translated through a link and gear to the pointer movement. These gauges are usually preferred for corrosive and extremely viscous mediums, e.g., those used in chemical, mining, and power stations.

### Capsule Element

This pressure gauge measures dry air and gases at relatively low pressures. It has two membranes where one has an opening that allows media to enter. Its expansion or contraction due to the differential pressure between the fluid media allows for pressure measurement. The capsule element pressure gauge is mainly used in vacuum pumps , breather valves, pneumatic systems, and filter monitoring, all at low pressure.

## Selection Criterion

Pressure gauges vary significantly in their designs and applications. It is equally crucial for you to understand the parameters that will help you select the correct pressure gauge that is most suitable for your needs. Here are some factors to consider during the selection process.

### Gauge Type

Pressure gauges are of different types, and each has its best application. The capsule element will be the best choice if you want to measure dry gaseous medium at low pressure. On the other hand, a diaphragm pressure gauge will be the most suitable for corrosive substances that crystallize easily.

You can find both digital and analog pressure gauges in the market. A Digital pressure gauge provides a better analysis of pressure in the system and gives a steady and precise reading of the output. Analog systems, on the other hand, are relatively cheaper but less accurate.

### Pressure Gauge Material

The cabinet or pressure gauge housing material should be compatible with the fluid media you measure. Common materials include Inconel, stainless steel, aluminum, and copper alloys. Each of these materials is designed for different conditions; for instance, Inconel is best for corrosive substances and high-temperature environments, while stainless steel is often used in the food industry.

### Pressure Range

This is the range between the minimum and the maximum pressure a gauge can read. Always choose a pressure gauge that is twice the system’s pressure to ensure safety and accuracy. Note that the operating pressure of your system should not exceed 75% of the gauge range.

### Fluid Temperature

All the pressure gauge components should withstand the temperature of the fluid media and the adjacent environment. Pressure gauges made of stainless steel should not exceed 100°C (212°F), while those of Inconel should not exceed 315°C (600°F).

## Summary

Pressure gauges find applications in several industries where liquids and gases must be measured to ensure optimal operation. As noted above, there are different pressure gauges designed for use with different fluid media and in varying environments. If you need help during the selection process, always seek professional help.