Pressure Gauge Measurement Techniques
Pressure is a scalar quantity that describes the ratio of force acting perpendicularly per unit area. This quantity finds applications in various industrial settings as well as the everyday physical processes. When dealing with liquid or gaseous media, pressure is defined as the amount of force applied perpendicularly to the wall of the container at any given point.
Before we look at the different pressure measurement techniques, let’s first understand the different forms of pressure and the basics of pressure measurement.
Pressure Measurements Basics
When determining the pressure of liquid and gases, stagnant fluids are often measured in terms of static pressure, which is considered uniform in all directions. However, a moving fluid exerts additional pressure called dynamic pressure. This means that the total pressure of a flowing fluid is the sum of its static and dynamic pressure.
That said, there are three main forms of pressure. These are:
- Absolute pressure – this is defined as the total amount of force acting on a unit area. The earth’s atmosphere at sea level has an absolute pressure of one atmosphere (1 atm). However, a perfect vacuum has an absolute pressure of zero.
- Gauge Pressure – this is the pressure displayed by most gauges and is defined as the pressure of a media relative to the atmospheric pressure. In other words, gauge pressure is calculated by subtracting atmospheric pressure from absolute pressure. The negative sign is often omitted for gauges designed to measure vacuum pressure, but the value is denoted with the letter “g.”
- Differential pressure – this is the difference in pressure between any two given points or media. This pressure plays a critical role when controlling fluid flow and regulating flow direction.
Pressure Gauge Measurement Techniques
Pressure measurement is a popular practice rooted deeply in the history of science and engineering. Nearly all systems that interact with liquids or gases have some form of pressure gauge measurement to ensure optimal operations and safety. A majority of the pressure gauge systems in the market use the widely accepted aneroid/mechanical gauge technology.
This technology uses a metallic pressure-responsive element, which deforms once the system has a pressure differential. As the element deforms, its size and shape change, and the displacement is measured and converted into rotary motion on a pointer scale. That said, all the modern pressure gauges in the market use this basic operational principle.
Here are the three main types of aneroid gauges:
This is a thin, closed-end tube that’s flat and flexible and often used to measure gauge pressure. When the fluid pressure is applied to the inside of the bourdon tube, the tube’s oval cross-section becomes more circular, strengthening the tube. This causes the tube to change its shape, creating a pattern at one end of the tube that’s then converted into pointer rotation with gears and links.
The bourdon tube is perhaps the most popular gauge pressure in the market. They have excellent linearity, sensitivity, and accuracy, making them ideal for a wide range of applications. Pressure gauges in this category have a measuring range of 0 to 0.6 bar and 0 to 1600 bar, while the accuracy class falls within 0.1 and 4.0.
The diaphragm pressure gauge features a metallic or ceramic diaphragm, a dial, a pressure entry point, and a pointer. One side of the diaphragm is either exposed to the atmosphere/environment where the gauge pressure is to be measured or is sealed against a vacuum in which the absolute pressure is to be measured.
As the pressure builds up, the diaphragm flexes, and through the linkages and gears, the displacement is converted into dial measurements for easy reading. Diaphragm pressure gauges are suitable for highly viscous fluids and corrosive gases and liquids. The measuring range for diaphragm gauges lies between 0 and 2.5 mbar and 0 to 25 bar, while the accuracy class falls between 0.6 and 2.5.
The capsule element is designed to measure air and dry gases at relatively low pressures. The gauge features two circular membranes that are joined on the outer edge. It also consists of a pressure chamber, a capsule element, stem with pressure connector, a dial, a window, and a pointer. One diaphragm has an orifice in the middle that allows the fluid media to enter the gauge. The chamber expands, and contracts as the pressure differential between the inner and outer media builds up. This causes a displacement, which is then translated into a dial measurement.
This pressure gauge is often used for precise pressure measurements, particularly gaseous media. Typical applications include low-pressure vacuum pumps, pneumatic systems, overpressure monitoring, and breather valves systems. The measuring range lies between 0 to 1 mbar and 0 to 600 mbar, while the accuracy class falls within the 0.1 and 2.5 range.
Choosing a Pressure Measuring Gauge
From the different types discussed above, it’s apparent that each pressure gauge has its specific applications, measuring range, and accuracy class. These are some of the critical factors you always want to consider before buying a pressure gauge in the market. You should look out for pressure ranges, chemical compatibility of the pressure gauge material, safety, and service life.