# Gauge Pressure | Its Important Properties with 30 FAQs

## Definition of Gauge Pressure

Gauge Pressure can be defined as the pressure that is relative to the atmospheric pressure.  For pressures that are above atmospheric pressure, gauge pressure is taken to be positive while for pressures below atmospheric pressure, gauge pressure is noted to be negative. Gauge pressure is referenced to be zero at atmospheric pressure.

For example, while filling air in a flat tyre, the air is inside the tyre is filled in terms of gauge pressure and the atmospheric pressure is observed to be zero. This is because the tire gauges are designed to operate at 0 atmospheric pressure.

## What is the relationship between Gauge pressure and True pressure?

Gauge Pressure can be formulated as the difference between absolute pressure and atmospheric

Pabs = Pg + Patm

where Absolute Pressure is denoted as Pabs, Atmospheric pressure as Patm and Gauge pressure as Pg

If the tire gauge reading is observed to be 36 psi (pounds per square inch), then the Absolute Pressure will be sum of the atmospheric pressure (which is a constant, i.e., 14.7 psi) and gauge pressure reading

i.e.  Pabs = Pg + Patm

= 36 psi + 14.7 psi

= 50.7 psi

## What is the gauge pressure of the trapped air?

The gauge pressure of air trapped in a vessel or a tube can be measured using a manometer. A manometer is a U- shaped tube often filled with mercury as a fluid to measure pressure. The difference in the height of fluid (i.e., mercury) is used for measuring the gauge pressure.

For example, the gauge pressure can be measured using a U- tube with one end exposed to the atmosphere and a balloon connected to the other end. The absolute pressure is greater than the atmospheric pressure by an amount hρg which is taken to be the gauge pressure.

## What is the gauge pressure at the water mercury interface?

The water mercury interface is not affected by the atmospheric pressure. The equation for calculating gauge pressure at the water mercury interface is

Pg  = hρg

Where h is the height displaced, ρ is the density of mercury and g is acceleration due to gravity

## What is Pressure Gauge?

Pressure Gauge is a tool used for measuring the pressure exerted by a fluid which can be liquid or gas, per unit area which is expressed in terms of Newton per square meter or pounds per square inch.

## What is the liquid in pressure gauge?

Liquid Glycerine is often used in pressure gauges due to its excellent vibration dampening properties at room temperatures. They usually operate in the temperature range between -200 C and + 600C.  There are other fluids which are used as liquids in pressure gauges depending on the application, but the most promising liquid is glycerine.

### Working principle of Pressure Gauge

Pressure gauges work using principle of Hook’s law which states that the force required to compress or expand a spring depends on the distance i.e., F = kx, where k is the spring constant, x, the distance to which the spring is compressed or expanded, and F is the forced applied.

When pressure is applied on an object, there exists an inner pressure force and an external pressure force. Further, the pressure exerted in a Bourdon tube will be more in the inner surface due to the smaller surface area compared to the outer surface

## What is Pressure Gauge calibration?

Pressure Gauge calibration is the comparison of values of the unit that is being tested to the values that are measured from an accurately calibrated device. The pressure gauge is usually used for calibrating and tuning fluid flow machines. The fluid flow machines would be unreliable if not calibrated using a pressure gauge. Pressure gauges are calibrated according to the National Standards (NMISA).

## What is Bourdon Tube Pressure Gauge?

These types of pressure gauges are used for measuring relative pressure in the range of 0.6 to 7000 bar. They belong to the category of mechanically driven pressure measurement devices as they do not require electrical energy to power.

### Working Principle of Pressure Gauge

The Bourdon Tube Pressure Gauge has an oval-shaped cross-sectional area with tubes that radially packed. The pressure exerted by the measuring source creates a motion on the other end of the tube which is not clamped. This motion that is created on the other end of the tube is taken to be the pressure which is measured. A C- shaped Bourdon tube can be used for measuring pressures up to 60 bar. Bourdon tube packed with windings of exact angular diameter i.e., helical tubes are used for measuring high pressures that exceed 60 bar.

Bourdon tube pressure gauges are manufactured according to set standard of EN 837-1. There are Bourdon tube pressure gauges which are filled liquid, these types of gauges are used for critical applications where the readings to need to be accurate and precise.

## What is Oil Pressure Gauge?

Oil pressure Gauges can be categorised into mechanical gauges and electrical gauges

### Mechanical Pressure Gauge

This gauge measures the pressure of oil at the end of pipe connecting the pump and the filter. To measure the pressure, an oil take-off pipe taps on the engine block. The needle movement in the dial indicates the measured pressure.

Figure 4. Mechanical Oil Gauge (Credits Lumen Learning).

### Working Principle

By tapping of the engine block, oil is sent to the gauge by using a copper or plastic bore. The pipe is arranged in such a way that it will be exposed to minimum damage to prevent leakage of the engine oil. The gauge is composed of a coiled tube which is termed as bulb. The open end of the bulb is connected to outer casing of the gauge.

The oil fed that is fed into the supply pipe is at almost the same pressure as when the oil leaves the engine. Under this pressure, the bulb tries to maintain its position, in doing so the needle in the dial moves and indicates the pressure. The higher the pressure, the larger will be the degree of movement of the needle.

### Electrical Pressure Gauge

This gauge measures the pressure of oil at the end of pipe connecting the pump and the filter. To measure the pressure, a screwed sensor taps on the engine block. The needle movement in the dial indicates the measured pressure.

### Working Principle

This type of pressure gauge is powered by electric current which is supplied from one of the wires that is present in the dashboard. The current that is supplied through the wire passes through a coil which is wound with a wire in the needle’s pivot. A magnetic field is produced which causes the needle to move within the dial depending on the measured pressure.

The extend to which the needle moves or the reading it shows depends on the current that flows through the gauge. The contributing factor is the resistance offered by the gauge wire that is earthed in the engine block using the sensor. All gauges are illuminated for the ease of reading the measurement at night.

## What is a Magnehelic differential pressure gauge?

It is an instrument used for measuring pressure as well as pressure differences. This pressure gauge was modelled and developed by Dwyer which has currently set standards for the pressure gauges used in industries. It is primarily used for measuring positive and negative pressure i.e., vacuum.

### Working Principle

A magnehelic pressure gauge is composed of a diaphragm that is sensitive to pressure changes. The dial of the pressure gauge responds based on the pressure applied. The appropriate positioning of the instrument is required for proper functioning of this pressure gauge. It should be placed at the right level and in a vertical position or else the diaphragm will give inaccurate readings as it will sag.

## What is the most accurate tire pressure gauge?

Industries and labs had been using conventional analogy tire gauges for measuring pressure since ages. But ever since the discovery of digital instruments has led to the use of digital pressure gauges which provide the most accurate reading. It is easy to operate a digital tire gauge, that is to switch on the gauge and position it on the valve stem to get the corresponding reading.

## What is Compound pressure gauge?

This type of pressure gauge is used for measuring both positive and negative pressures in a vacuum. Few examples where compound pressure gauge is employed are

• for leaking testing in pressure lines,
• for measurement of low pressures, and
• for pressure measurements in test chamber

Its capable to measure positive and vacuum pressure only for pressures below 200 psi.

### Working Principle

The compound pressure gauge consists of a sensor that is capable of measuring both positive as well as negative vacuum pressures. The zero pointer of the instrument is referenced at ambient pressure. The gauge consists of a vent hole which allows to compensate for the changes in atmospheric pressure.

## What is Digital pressure gauge?

These are pressure gauges that can measure the pressure from fluid and provide direct reading of the pressure measurement unlike Analog pressure gauges which require an operator to manually read the positioning of the needle in the dial for the respective pressure reading.

## What is kPa on a pressure gauge?

kPa (kilo Pascal) is a unit of pressure measurement and is thousand times Pascal which is the SI unit of pressure.

## What is psi in pressure gauge?

Psi in a pressure gauge is pounds per square inch which is the unit for the measured pressure. It is the pressure exerted by one pound force over an area of one square inch

## What is bar on a pressure gauge?

Bar is the metric unit for pressure and not the international unit. Bar is equivalent to 100,000 Pascal.

## What is Supply pressure gauge used for?

A supply pressure gauge helps in determining the amount of air or water or fuel in a tank. Air brake vehicles are usually provided with a supply pressure gauge to measure the amount of air in the tank. For vehicles with dual air brake system, there is a pressure gauge for every half section of the system

## What is Water gauge pressure?

Column of water is sometimes used for measuring pressure. A non- SI unit for measuring pressure is inch of water and can be defined as the pressure that a column of water that is 1 inch height exerts under standard conditions.

## What is a Dial gauge pressure canner?

A pressure canner is a vessel that is fitted with a lid that has a dial or weighted gauge that regulated the steam that builds up inside. The steam that is build up inside the vessel is released when the pressure exceeds the limit the vessel can handle.  Further, the steam that is build up inside is hotter than boiling water. Dial gauge regulators are found in older types of pressure canners. The dial displays the exact pressure build up inside the canner.

## What is a Weighted gauge pressure canner?

A pressure canner is a vessel that is fitted with a lid that has a dial or weighted gauge that regulated the steam that builds up inside. The steam that is build up inside the vessel is released when the pressure exceeds the limit the vessel can handle.  Further, the steam that is build up inside is hotter than boiling water. Weighted gauge regulators are made up of disc like pieces that must be placed on the vent pipe with preferred choice of weight and like the one-piece regulator, this regulator makes a rocking sound.

## What is a Fuel pressure gauge?

This type of pressure gauge is used as a diagnostic tool to ensure that the fuel pressure in the engine is maintained and is running at good performance levels. They also help to prevent any kind of damage that might occur due to pressure build up on the fuel pump or on the injector

## What is a Manifold Pressure Gauge?

This pressure gauge is used for measuring absolute pressure of the fuel-air mixture contained in the intake manifold. The diaphragm in the manifold pressure gauge is used for measuring the absolute pressure. The accurate power configuration and settings for an aircraft engine is obtained using the manifold pressure.

## What is a normal oil pressure gauge reading?

The normal oil pressure gauge reading when an engine is running should be between 25 and 65 psi. When the pressure gauge reading is higher than 80 psi, then there is problem of high oil pressure which needs to be dealt with.

## What is a Photohelic pressure gauge?

Photohelic pressure gauge is a Magnehelic pressure gauge equipped with a switch to adjust between high and low gas pressures. It is an advanced version of Magnehelic pressure gauge that helps in saving money with reduced usage of compressed air and provides a longer life for the pressure gauge.

## What is a Pressure gauge snubber?

High pressure required for waterjet cutting is smoothened out by using pressure gauge snubber. These high-pressure fluctuations are created by reciprocating pumps and controlling these fluctuations help in extending the life of the pressure gauge and reducing the calibration time. These gauges are preferred over a valve due to their small orifice which reduces the cases of clogging.

A pressure gauge snubber consists of a pressure vessel with a capillary that has a small bore. The pressure is accumulated in the gauge and the built-up pressure is smoothened out thereby reducing the fluctuations. The gauge is equipped with a steel filter at the inlet to the capillary to avoid dirt from entering or clogging the bore.

## What is a Strain gauge pressure transducer?

This transducer converts pressure into an electrical signal. The principle behind the working of a strain gauge pressure transducer is piezo resistance i.e., the change in resistance value with respect to the physical deformation or changes caused to the material when exerted by pressure. This transducer is when wired to a Wheatstone bridge can convert small changes in resistance to electrical signals corresponding to the pressure exerted.

## What is accuracy class in pressure gauge?

The accuracy class in pressure gauges help in determining the permissible percentage of error. The accuracy classes for pressure gauges are 0.1, 0.25. 0.6, 1, 1.6, 2.5 and 4. The gauges with pointer stop are the range of 10 to 100%.

## What is Bayonet ring in pressure gauge?

Pressure gauges have removable rings which are termed as bayonet ring.  A bayonet is an indentation on the outer surface of the ring. Usually, a bayonet ring has up to five indentations. The rings help in holding a gasket and window. The dial can be found on removing the gasket and window. This ring is mostly seen in pressure gauges where the operator must access the adjustable pointer.

## What is Bellows pressure gauge?

Thin walled cylinder with convolutions and metal as material of construction, are Bellow pressure gauge they are closed at one end while the other end is open and can move about. On applying pressure to the sealed end, the bellows will compress and move upwards. The rod in between the bellows and the transmission system will also move up and initiate the movement of the pointer. They can provide longer stroke length and exert greater forces. These bellows are fabricated using different materials depending on the application.

The deflection that is produced can be expressed as below

## What is Blowout protection in pressure gauge?

In case of high pressure, the entire disc will be blown and break into piece to release the build-up pressure. To protect the gauge from breaking or from being blown up, pressure gauge blows out protection is provided. An advisable design for protecting the pressure gauge from the over pressure is by separating the front and back part of the pressure gauge using a solid wall. Using such a design, the front part will not be affected though the back part will blow out thereby providing protection to the pressure gauge.

## What is Differential pressure gauge?

A differential pressure gauge helps in measuring the differences in two measured pressure. They are usually used for measuring pressure levels in closed tanks, over pressure in room and for controlling pump stations.

A pressure element divides the two chambers in a differential pressure gauge. If the pressures in the two chambers are the same, then there occurs no difference in the pressure element. On the other hand, if there exists difference in pressure between the two chambers, then the pressure element displaces, and mechanical movement indicates the pressure difference value.

## What is FSD in pressure gauge?

FSD in a pressure gauge means full scale deflection and the accuracy of the pressure gauge is in full range.