Pressure Gauge Types, Working Principle

Pressure gauges are devices that are used to measure the pressure of the system. To understand the concept of pressure gauge and how it works, first of all, we need to understand what is pressure.

Pressure is an essential component of our daily life. Generally, we speak about atmospheric pressure, vacuum pressure, blood pressure, and gauge pressure. So it becomes really very important to know what is pressure and how we can measure it.

Pressure is generally defined as the force exerted by a medium, particularly fluid, on a unit area. Mathematically, it can be defined as

Pressure (P) = Force (F) / Area (A)

Now, let us first understand what are pressure measurement scales

Measurement scales

• Gauge pressure
• Total absolute pressure
• Differential pressure
• Vacuum
• Absolute pressure

Let us understand each of them one by one:

Gauge Pressure

Gauge pressure is defined as the pressure that is measured above the local atmospheric pressure.

Total Absolute Pressure

Total absolute pressure is a total pressure measured from zero pressure as the reference point. The total pressure is a sum of atmospheric pressure plus gauge pressure.

Total absolute pressure = Atmospheric pressure + Gauge pressure

Differential Pressure

Differential pressure is generally the difference in the pressure that has been measured between the two points.

Vacuum Pressure

Vacuum pressure is defined as the pressure when the pressure is less than the local atmospheric pressure.

Vacuum = Atmospheric pressure − Absolute pressure

Absolute Pressure

Absolute pressure is the pressure that is measured above the total vacuum or zero absolute.

The relation among these are given as below:

Pressure Measuring Units

The following are the units and conversion that are normally used in Pressure terminology:

• 1 Pa = 1 N/m2
• 1 atm = 1.013 × 10⁵ Pa = 760 mmHg
• 1 mmHg = 1 Torr
• 1 Torr = 133.3 Pa = 1.316 × 10−3 atm
• 1 bar = 10⁵ Pa

Types of Pressure Gauges

The different instruments or devices used for the pressure measurement can be classified as
follows:

1. Gravitation-type manometers
2. Mechanical displacement-type manometers:
   (a) Ring balance
   (b) Bell type
3. Elastic pressure transducers:
   (a) Bourdon tube pressure gauges
   (b) Diaphragm-type gauges
   (c) Bellow gauges
4. Electrical pressure transducers:
   (a) Resistance-type pressure transducer
   (b) Potentiometer devices
   (c) Inductive-type transducer
   (d) Capacitive-type transducer
   (e) Piezoelectric pressure transducer
   (f) Bridgman gauges
5. Low-pressure measurement gauges:
   (a) McLeod gauges
   (b) Pirani or thermal conductivity gauges
   (c) Ionization gauges
6. Engine indicator (for varying pressure measurements)

Let us discuss a few of them:

Burdon Tube Pressure Gauge

Bourdon tube pressure gauge is one of the widely used pressure gauges. It was developed by E Bourden in 1849. This job is composed of C-shaped hollow metal which is having elliptical cross-sections. One end is fixed and pressure is applied through that part and the other is free and closed. When the pressure is applied the tubes get straightened and get a circular cross-section. This pressure causes the free end to move. The deflection is directly proportional to the difference in pressure between inside and outside.

Now to measure the pressure the moment of this free end is magnified and transmitted to a pointer that moves over the scale. That pointer moves with the scale through a linkage and Gear mechanism. The Bourden tube measures the gauge pressure because the reference pressure is atmospheric.

Diaphragm Pressure Gauge

The diaphragm pressure gauge works on the same principle as that of the bourdon pressure gauge but it measures the pressure using the diaphragm instead of a flexible tube. The Diagram pressure gauge is elastic and it gets displaced when the pressure is applied. In this case, the diaphragm is placed between two flanges and is used to determine the difference between the applied pressure and the reference pressure. As the pressure gets built, it flexes the diaphragm which through the mechanical linkage of gears can turn into dial measurements.

Capsule pressure gauge

A capsule pressure gauge has two thin and concentric diagrams that are sealed by the outer edges. Out of these two diaphragms, one is having a hole in the center which allow the medium to enter in. So when the pressure is applied, the diaphragm contract or expense. This changing shape creates rotary movement that appears on the gauge. Pressure gauges are used to measure air and dry gases at low pressure and this gauge work in a similar fashion to that of below pressure gauge.

Frequently Asked Questions

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