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A pressure gauge measures the gas or liquid pressure inside a system. It monitors fluid pressure in a range of applications including the aerospace, transportation, healthcare, and production sectors.
- To ensure the quality of the product and as a safety precaution to be aware of leakage or increased pressure in a system, it is crucial to evaluate the pressure in a system.
- It is important to understand the fundamentals of pressure before selecting a pressure gauge.
| Table of Content |
Key Terms: Pressure, Gauge, Atmospheric, Absolute, Vacuum Gauges, Instrument, Pascals, Meter, Diaphragm, Mechanical Gauges, Application, Fluid.
What is a Pressure Gauge?
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Pressure gauges are instruments that are used to measure atmospheric pressure. The pressure as compared to atmospheric pressure is known as gauge pressure. Gauge pressure is positive for pressures higher than atmospheric pressure.
- For pressures lower than atmospheric pressure, gauge pressure is negative.
- Vacuum gauges and pressure metres are other names for the pressure gauge.
- A manometer is a device that measures and displays pressure by using the weight and surface area of a liquid column.
- Most gauges determine the zero point as the pressure in relation to atmospheric pressure.
- Therefore, gauge pressure is the name given to this type of reading. Analogue and digital pressure gauges are both available.
The following formula determines the pressure difference between the system and the atmosphere:
P = Pa + ρgh
Here,
P = pressure at any point
Pa = atmospheric pressure
This indicates that the pressure at any given location is always more than the atmospheric pressure by a factor of ρgh.
When P – Pa = ρgh
then,
P = pressure of the system
Pa = atmospheric pressure
(P - Pa) = difference in the pressure of the system and the atmosphere.
ρgh = Gauge pressure
Read More: Unit Of Pressure
Types of pressure gauge
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Most standard pressure gauges are designed by keeping in mind either a commercial or industrial function when considering usability and purpose. Some of the types of pressure gauges are:
- Bourdon Tube Pressure Gauge
- Diaphragm Pressure Gauge
- Capsule Pressure Gauge
- Absolute Pressure Gauge
- Differential Pressure Gauge
- Bellows Pressure Gauge
- Manometer Pressure Gauge
- Piezometer Pressure Gauge
Bourdon Tube Pressure Gauge
Pressure gauges are most frequently used with bourdon tubes. It is a mechanical device that gauges pressure devoid of an electrical source.
- Steel was used in its construction to prevent rust and wear.
- The range of pressure that a bourdon tube pressure gauge can measure is 0.6 to 7000 bar (8 to 10000 psi).
- It works with liquid or gaseous media for vacuum applications as well as low and high pressures.
- It is a small instrument that works well for dynamic pressure loads and strong vibration applications.
Bourdon tube pressure gauge
Diaphragm Pressure Gauge
It is the instrument used to gauge the fluid pressure present in a system. It is intended to measure the intensities of low pressure systems.
- The term "membrane pressure gauge" also applies to diaphragm pressure gauges.
- The diaphragm, a thin, flexible membrane, is used in this device to deflect the air.
- Using a needle that is moved by a pinion arrangement with the diaphragm, the pressure is represented.
Diaphragm pressure gauges
Capsule Pressure Gauge
In this kind of pressure gauge, a capsule is created by welding two corrugated diaphragms together at their edges. The key component for measuring the pressure is this capsule.
- One of the centre diaphragms has a hole that allows the medium to pass through.
- Depending on the amount of pressure, the diaphragms can expand or compress.
- The capsule pressure gauge is employed to determine the pressure of substances with up to 600 mbar and is used to determine the pressure of gases.
Capsule pressure gauge
Absolute Pressure Gauge
These are the instruments that work best for measuring pressure independently of atmospheric pressure variations. An unpressurized reference vacuum measurement is fastened to the side of the measuring element.
- As a result, it possesses constant pressure of zero.
- They are primarily used in laboratories for research.
- Absolute pressure gauges are employed in the fields of aviation, HVAC (heating, ventilation, and air conditioning) systems, and distillation since they are unaffected by altitude.
Absolute pressure gauges
Differential Pressure Gauge
The differential pressure gauge measures the pressure difference among two different chambers that are separated by an item that oscillates in response to pressure changes.
- An internal diaphragm of the differential pressure gauge keeps the media chamber and vacuum chamber apart.
- With increasing pressure, the diaphragm bends into the vacuum chamber.
- By converting the movement and change, a pressure value is generated.
Differential pressure gauge
Bellows Pressure Gauge
Devices for measuring low-pressure applications include bellows pressure gauges.
- The accordion-like bellows in the devices are made of thin-walled, linked tubes of springy metal,
- It is sealed in the gauge's open end.
Bellows pressure gauges
Manometer Pressure Gauge
It is a tool that measures the pressure on a fluid by balancing one fluid column against the other.
- The manometer pressure gauge has a tube that is attached to the location
- Where pressure will be measured and has an open end that faces the atmosphere on the other.
Manometer pressure gauge
Piezometer Pressure Gauge
An instrument known as a piezometer is used to determine the liquids' mean pressure range. A straightforward glass tube with openings at both ends serves as a piezometer pressure gauge.
- The glass tube has one end that is attached to a pipeline where the fluid pressure is being monitored
- The other end is exposed to the atmosphere.
- In order for the liquid in the pipe to rise into the tube, the tube is attached vertically.
- The fluid level in the tube's height provides a reading of the pressure.
Piezometer pressure gauge
Read More: Barometer
How does a Pressure Gauge Work?
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Aneroid gauges, commonly referred to as mechanical gauges, are the most widely used method for measuring pressure in a system out of all the methods that have been devised.
- A metallic pressure responsive element is used in aneroid gauges to measure pressure.
- The rotation of a pointer on an analogue scale display can then be calculated from the deformation of this part.
- Bourdon tubes, diaphragms, and capsule elements make up the three basic categories of aneroid pressure gauges.
Accessories
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There are numerous accessories that can be used with pressure gauges. Some of them are:
- Gaskets and seals: These guarantee a good seal.
- Flat: The seal is flat and prevents additional rotation of the pressure gauge readout.
- Design: The seal includes a design that allows for a 1 / 2 or full turn of the pressure gauge's reading to guarantee optimum installation orientation. Profile seals can either be internally or externally centred.
- Safety caps: The pressure gauge should have a rubber cap over it to increase durability and provide shock absorption.
- Reducers: If the pressure gauge's input and the connection size at the output are different, a reducer can be employed. If both have a different connection type for example, BSPP and NPT that can also be helpful.
- Push-on connectors: Push-on connectors make it simple and quick to connect or disconnect a hose from a pressure gauge.
- Snubbers: A pressure snubber reduces the impact of pressure pulses and spikes, preserving the readability of the pressure gauge and extending its useful life.
Read More: Relation between bar and atm
Selection criteria
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Pressure gauges come in a variety of styles, each of which is used in particular applications and sectors. The choice of these instruments is influenced by a number of variables, including accuracy, dial size, environment, media, and operating pressure range.
- Additionally, there are several uses for pressure gauges, such as in swimming pool filters, hoover cleaners, compressors, and water pressure gauges.
- To find out more about all the elements involved in choosing a pressure gauge, see our selection guidelines for pressure gauges.
Pressure units
There are numerous display devices available for pressure gauges. The commonly used pressure gauge units are listed in the table below, along with a conversion to their counterparts in Pascals.
| Pascal (Pa or N / m2) | ||
| 1 Bar | 105 | |
| 1 at (kg / cm2 or kgf / cm2 or Technical Atmosphere) | = 9.80 665 × 104 | |
| 1 atm (Standard Atmosphere) | = 1.01 325 × 105 | = 760 Torr |
| 1 Torr (mm Hg or Millimeter of mercury) | = 1.333 224 × 102 | |
| 1 cm H2O (cm Wc or Centimeter of water) | 98.0665 | = 10 mm H2O |
| 1 mm H2O (mm Wc or Millimeter of water) | = 9.80 665 | |
| 1 lbf / in2 (Psi) | = 6.8 948 × 103 | = 16 ozf / in2 |
| 1 oz / in2 (oz / in2) | = 4.30 922 × 102 | |
| 1 inHg (inch of mercury) | = 3.37 685 × 103 |
Pressure ranges
For commonly used pressure gauges, European Norm EN 837 specifies standardised practises, design criteria, testing, and installation instructions.
- Information on the design of dials with concentric scales is available in EN 837-1 and EN 837-3.
- From low pressure water pressure gauges to high pressure hydraulic pressure gauges that are frequently equipped with snubber
- Pressure gauges can be used with a wide range of ranges.
- The maximum working gauge pressure shouldn't be higher than 50 % of the full-scale range if the process involves pulsation.
Nominal size
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A gauge's diameter is known as its nominal size (NS). 40, 50, 63, 80, 100, 160, and 250 mm are the nominal diameters for gauges according to EN 837.
Pressure gauge accuracy
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The maximum margin of error that each pressure gauge is permitted to have in terms of the % of the maximum scale reading is determined by accuracy classes (KL).
- For instance, a pressure gauge with an accuracy class of 4 and a maximum reading of 10 bar may vary from the real pressure by 4 % (0.4 bar).
- Another illustration is a gauge with a 0 - 100 bar scale and a 2 % accuracy.
- This shows that in its overall whole range, the gauge is correct to within 2 bars.
- Installing a gauge with poor precision might result in inaccurate readings, and utilising a gauge with excessive accuracy raises the expense of buying, calibrating, and maintaining that gauge.
| Accuracy class | Limits of permissible error (Percentage of Span) |
|---|---|
| 0.1 | 0.10 % |
| 0.25 | 0.25 % |
| 0.6 | 0.60 % |
| 1 | 1 % |
| 1.6 | 1.60 % |
| 2.5 | 2.50 % |
| 4 | 4 % |
Types of mounting and connection
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The types are mentioned below:
- Standard threaded connection: This gauge just has to be screwed into a thread that is already present. Compression seals for tapered threads and o-rings for parallel threads are used to seal the threads.
- Integrated pressure gauge: A female thread is used to mount this pressure gauge.
- Flange pressure gauge: For individuals who want to attach the pressure gauge on a control cabinet, a flange mounting option is available.
Read More: Phase changes
Safety and service life
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En 837 - 2 states that a pressure gauge must be chosen with a range that prevents the maximum working pressure from going above 75 % of the maximum scale value for constant pressure or 65 % of the maximum scale value for cyclic pressure.
- A pressure gauge with additional safety features, such as a blow-out device on the back
- It is required when employing dangerous pressure media, such as oxygen, acetylene, flammable compounds, and toxic substances.
- These safety precautions guarantee that anyone at the front of the scale won't be hurt by any leaks or component bursts under pressure.
Application of Pressure Gauge
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Pressure gauges are used in a variety of industries for a variety of purposes.
- The pressure of liquids, vapours, solids, and gases can all be measured with pressure gauges.
- It is used to examine trucks' air brakes.
- In the process, sanitary, pharmaceutical, chemical, and petrochemical sectors, pressure gauges are utilised.
- Pressure gauges are used in the food, beverage, HVAC, refrigeration, and ventilation industries.
- They are also employed in the removal of probable leak routes.
Also Read:
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Things to Remember
- A pressure gauge is a measuring tool that measures the amount of pressure within a vessel
- When comparing two pressures, a differential pressure gauge is employed. The pressure difference is indicated by it.
- A diaphragm is used in the capsule pressure gauge as well, but two of them are sealed together to resemble a capsule.
- The pressure above vacuum is known as absolute pressure. Gauge pressure + one atmospheric pressure = absolute pressure.
- Barometric pressure, often known as atmospheric pressure, is the amount of pressure that the atmosphere is exerting at a particular location.
- A natural gas pressure gauge measures natural gas pressure and helps find leaks. Typically, it is made up of brass or stainless steel.
- Bourdon tube gauges have a pressure range of 0 – 1.6 bar (0 – 23206 psi) to 0 – 0.6 bar (0 – 8.7 psi)
- The accuracy of their category is normally between 0.1 and 4.0.
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Sample Questions
Ques. What do you mean by gauge pressure? (2 marks)
Ans. The gauge pressure formula (P - Pa) is used to determine the actual pressure within the vessel to which the pressure gauge is attached. 'P' stands for the pressure measurement from the pressure gauge, and 'Pa' stands for ambient pressure in this formula.
Ques. How can a fire extinguisher pressure gauge be checked? (2 marks)
Ans. The extinguishing medium's pressure inside the extinguisher's canister is displayed by a pressure gauge. The needle's position on the scale's green zone is used to determine if it is. The extinguisher is in good condition if the needle is in the green zone; however, if it is in the red zone, it indicates that the extinguisher's internal pressure is below what is necessary.
Ques. What are the uses of pressure gauge in day to day life? (3 marks)
Ans. Pressure gauge can be used in our day to day life in various forms:
- In determining a vehicle's tyre pressure. When gauging the air brake pressure on trucks and locomotives.
- Scientific investigations into pressure.
- To ensure effective chemical production, pressure within every vessel must be monitored in sectors like the chemical ones.
- For creating the necessary pressure within the vessel in autoclaves.
Ques. Water is poured into a tank that has a vertical sidewall, an area A flat bottom, and a depth h. At the top surface, there is no pressure (P0). What is the tank's absolute pressure at the bottom? (2 marks)
Ans. If the tank's bottom area is equal to A,
h = water depth
Top surface pressure = P0
Absolute pressure, or P, is:
P = the top surface pressure + the liquid column pressure.
P = P0 + pgh
Ques. In a fluid with density, a solid cone with height H and base radius H/2 floats. With the use of a string, it is suspended from the ceiling. What is the fluid's force on the cone's curved surface? (3 marks)
The air pressure is (P0).
Ans. Assume that P0 is the atmospheric pressure.
If we suppose a tiny ring with radius r and thickness dr at height h from the cone's top,
Logically, h = 2r.
- This ring would be under pressure from: P = P0 + ρgh
- For this ring, force = PA
- dF = (P0 + ρgh)2πrdr
By incorporating the force from the radius r = 0 to r = H / 2
F = ∫0H / 2 (P0 + pg2r)2πrdrF = πH2 (P0 / 4 + ρgh / 2)
Ques. What do you mean by vacuum pressure? (2 marks)
Ans. Vacuum pressure is calculated in relation to outside atmospheric pressure. PSIV stands for pounds per square inch (vacuum). A vacuum pressure transducer's electrical output is 0 VDC at 0 PSIV (14.7 PSIA) and full scale output (usually 5 VDC) at full scale vacuum, 14.7 (0 PSIA).
Ques. What distinguishes atmospheric pressure from gauge pressure? (2 marks)
Ans. The difference between atmospheric pressure and gauge pressure is called gauge pressure. The gauge pressure is positive if it is higher than the atmospheric pressure. It is negative if the gauge pressure is lower than the atmospheric pressure.
Ques. What are the different types of pressure gauge? (2 marks)
Ans. The different types of pressure gauge are:
- Bourdon Tube Pressure Gauge
- Diaphragm Pressure Gauge
- Capsule Pressure Gauge
- Absolute Pressure Gauge
- Differential Pressure Gauge
- Bellows Pressure Gauge
- Manometer Pressure Gauge
- Piezometer Pressure Gauge
Ques. How do pressure gauges function? (2 marks)
Ans. A pressure-sensitive element is coupled to a measurement device and display, such as a dial or digital readout, to form a pressure gauge. Under the pressure of the gas or liquid, the pressure-sensitive element deforms, and the measuring mechanism converts this deformation into a readable measurement.
Ques. What is the purpose of an industrial pressure gauge? (2 marks)
Ans. In an industrial context, it measures a gas or liquid's pressure. The tool monitors and regulates fluid pressure in a variety of applications in the manufacturing, aerospace, automotive, and medical sectors.
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