If you are a driver you must have been to a mechanic to fix your vehicle. If not that, you’d definitely have visited a fuel pump to refill the fuel where you must have come across the air refilling machine. What do you see there? A machine that fills the air in the tires of your vehicle using a pressure gauging instrument. Don’t know what that is? Well, a pressure gauge refers to a method of measuring fluid, gas, water, or steam intensity in a pressure-powered machine to make sure that there are no leaks or pressure changes that would affect the performance of the system. These systems are designed to work within a definite pressure range which means they can’t handle the pressure higher than their capacity. That leads to an important point that we should always be attentive while going through the process or it can lead to a lot of unfavorable results. This isn’t anything new or confined to our generation only, people have been using it for centuries however there have been improvements as per the demand of the times.
There are a number of pressure gauging types and they are mostly differentiated on the basis of their application type. And based on this characteristic, they are manufactured in different sizes, shapes, styles, and with different materials. There’s a different type of pressure gauge for different types of use. What’s common and most important in all of these is the measurement of pressure. Now the question arises: how do we do that? If you were also thinking about the same question then you certainly landed on the correct page. Let me tell you how; Pressure is determined by the perpendicular force applied per unit area of a surface, a calculation that is determined through the use of different techniques and methods. The correct measurement of pressure is dependent on the reference the pressure reading is measured against.
Pressure gauges are mainly divided into two types; Hydrostatic and mechanical.
Hydrostatic Pressure: The pressure generated by the weight of a liquid above a measurement point when the liquid is at rest is called hydrostatic pressure. Important to mention that the height of the liquid influences the hydrostatic pressure. Also, the hydrostatic properties of a liquid are not constant and are influenced by liquid density and gravity. These two factors determine the hydrostatic pressure.
Mechanical Pressure: Mechanical pressure is measured by a bellows, diaphragm, or Bourdon tube that converts fluid pressure into force. This is the property that makes it different from hydrostatic pressure.
Now that you know the basics, let’s move to the types –
Bourdon Tube Pressure Gauge:
Bourdon tube pressure gauges are used for the measurement of relative pressures from 0.8 – 100,000 PSI. They are classified as mechanical pressure measuring instruments and operate without any electrical power. Bourdon tubes are radially formed tubes with an oval cross-section.
Diaphragm Pressure Gauge:
A diaphragm pressure gauge, also known as a membrane pressure gauge, is a device that utilizes the deflection of a flexible thin membrane called the ‘diaphragm’ to measure the pressure of the fluid in a system. Diaphragm pressure gauges are used for relative pressure as well as for vacuum, compound, and differential pressure applications. Due to the presence of a diaphragm, these gauges are extremely suitable for use on viscous media.
Capsule Pressure Gauge
To accurately measure pressure below 10 psi (690 mbar), mechanical gauges use a capsule element. Capsule gauges are found in applications that operate in a vacuum or pressures as low as 1″ water column. In most Bourdon tube pressure gauges, the range limitation on the lower end is 10 psi (690 mbar).
Absolute Pressure Gauge
Absolute pressure gauges are typically used in research and scientific laboratories where fluctuating atmospheric pressure can become an issue and in aeronautics where precise measurements are critical to determine altitude. Absolute pressure is measured relative to a full vacuum. In contrast, the pressure that is measured against atmospheric pressure (also known as barometric pressure) is called gauge pressure. A full vacuum has an absolute pressure reading of 0 PSIA and the average barometric pressure at sea level is ~14.7 PSIA.
Bellows Pressure Gauge
Bellows pressure gauges are devices used to measure absolute and differential pressure. The main element, the bellows, is a flexible membrane in the gauge whose expansion and contraction support the measurement of pressure difference in the system. The bellows are used in two forms. In one arrangement, pressure is applied to one side of the bellows and the resulting deflection is counterbalanced by a spring. This arrangement indicates the gauge pressure. In the second arrangement, the differential pressure is also indicated.