Pressure measurement is a common day-to-day activity in most industrial environments where safety, quality, and production processes are prioritized. Outside of the industrial setting, pressure measurements still manifest in various forms, whether in business, irrigation farms, water pump stations, sewer systems, etc.
Below, we’ve highlighted the basics of pressure and pressure measurements, including the working principle of pressure gauges and selection criteria. Read on to learn more.
What is Pressure, and Why is it Important?
Pressure is defined as the force exerted perpendicularly to a surface of a given object per unit area. Pascal (Pa) bars are the most common ways to express pressure, according to LumenLearning. As a physical quantity, pressure plays a critical role in various topics and concepts ranging from fluid and solid mechanics to thermodynamics. Pressure also constitutes nearly all the aspects of everyday life. For instance, chopping a piece of onion or cutting an orange will be a lot easier if you use a sharp knife rather than a blunt one.
The simple science behind this observation is that the blunt knife has a larger surface area in contact with the onion or orange. So, the force applied by the knife will exert less pressure. The area in contact with the orange or onion is smaller for the sharp knife, and the same force applied will exert more pressure.
Types of Pressure
There are three main types or forms of pressure. These are gauge pressure, absolute pressure, and differential pressure. Below is a quick overview.
Gauge Pressure
Gauge pressure is the pressure of a medium relative to the atmospheric pressure. In other words, it’s the amount by which the medium’s pressure exceeds the atmospheric pressure. A pressure gauge is used to measure this type of pressure, and the obtained value is often denoted by a letter “g”, i.e., 100 psi g. A pressure gauge can also measure vacuum pressure (pressure less than atmospheric pressure). Here, the measured pressure will be negative, and this can be labeled a vacuum gauge, omitting the negative sign.
Absolute Pressure
Absolute pressure is defined as the total pressure exerted on a body or object and is obtained by summing up the gauge pressure and atmospheric pressure, according to Avnet. A perfect vacuum, i.e., a space with no matter inside, has an absolute pressure (Pabs) of zero. The earth’s atmospheric pressure, on the other hand, has an absolute pressure of one atmospheric pressure (1 atm) at sea level.
Differential Pressure
Differential pressure is referred to as the pressure difference between any two points or mediums. Here, the pressure at one point is used as a reference to find the pressure of the second point. Often, differential pressure is calculated by subtracting one pressure reading from another. However, some pressure gauges measure the differential pressure between two mediums/points without manually doing the calculation.
How to Measure Pressure
There are different mechanisms or techniques for measuring pressure. One of the common and widely adopted technologies are mechanical gauges, also known as analog gauges. These gauges use a metallic pressure-responsive element that flexes elastically once there’s some pressure differential. This element deforms with respect to the pressure difference and causes the pointer on a scale to rotate, displaying the pressure reading.
Today, there are three main mechanical gauges in the market that employ the mechanical gauge’s working principle. They include the bourdon tube, diaphragm, and capsule element.
The Bourdon Tube
The Bourdon tube is a C shaped thin-wall tube that straightens once the fluid pressure is applied on one end to the inside of the tube. Once the fluid pressure disappears, the tube regains its shape. The changing shape creates a motion pattern, which is then converted into some pointer rotation. This mechanical pressure gauge can measure pressures in the range of 0 to 0.6 bar and from 0 to 1600 bar. The accuracy class of this pressure gauge is somewhere between 0.1 to 4.0.
The Diaphragm Gauge
The diaphragm uses deflection of its flexible membrane to measure pressure changes and turn this into pressure measurements. One side of the diaphragm can be sealed against a vacuum to measure absolute pressure or exposed to the atmosphere to measure gauge pressure. The flexible membrane is displaced as pressure builds, and a pressure reading can be taken simultaneously. This type of pressure gauge is best used with corrosive gases and highly viscous media. The measurement range is between 0 to 2.5 bar and from 0 to 25 bar, while the accuracy class is somewhere between 0.6 and 2.5.
The Capsule Element
This pressure gauge is designed for low-pressure measurements of air and dry gases. It consists of two circular parts joint along the outer side and one diaphragm with an inlet opening in the middle. The contraction and expansion of the chamber due to the pressure difference between the inner and outer media allow for instant pressure measurement. The capsule element is commonly used in low-pressure pneumatic systems and vacuum pumps for precise pressure measurements of gases.
Pressure Gauge Selection Criteria
When choosing a pressure gauge in the market, there are certain things you need to consider. Here are the key factors to keep in mind:
- Accuracy class – this determines the maximum error margin that the pressure gauge can have. A pressure gauge with an accuracy class of 4% or 0.4 means it can deviate from the actual reading by 0.4 bar.
- Material – the chemical compatibility of material to different fluid mediums vary. So you should keenly choose the gauge material that won’t rust or react with the fluid.
- Pressure range – pressure gauges measure pressure in a wide range, and you should choose one that suits your unique applications.
Closing Comments
Pressure gauges come in different types and designs, and choosing the right one for your pressure measurement applications should not be a challenge. Before picking a pressure gauge, consider the selection criteria above and prioritize buying from a reputed manufacturer.