Study Material

# Pressure gauges

## What are pressure gauges?

Pressure gauges are tools used to measure a physical quantity called pressure. Pressure is defined as the quotient between the perpendicular component of the force and the area on which it acts, being defined through the following equation: types of pressure gauges

P = F / A

Where P is the pressure and F is the force perpendicular to the area A. The unit of pressure in the International System of Units is the pascal, abbreviated Pa. Since it is a small unit for frequently handled pressures, multiples of the pascal , like the kilo, the mega and the giga are in widespread use. types of pressure gauges

To measure pressure, a multitude of devices have been created, which can be grouped into one of these categories, according to the physical principle they make use of:

• Based on mechanical principles, such as deformation caused by a force.
• Those that make use of electrical principles, which convert a force into an electric current.

## Where are pressure gauges used?

Pressure is, along with temperature, one of the most widely measured quantities in the world. This is due to the fact that fluids, be they gases or liquids, exert pressure on the walls of the containers that contain them and on the objects inside them.

Pressure gauges are essential instruments in various areas, the main ones being: types of pressure gauges

• Natural sciences, such as geosciences and in medical, climatological and environmental studies.
• Various industrial processes: pharmaceutical, food, metallurgical and automotive, among others.

In climate studies, the measurement of pressure is very important, since it is a main indicator of the climate: low pressures are associated with rains and storms, while high pressures indicate clear skies.

On the other hand, in all industrial and metallurgical processes there are numerous fluids continually exerting pressure on pipes and vessels, whether they are at rest or in motion. Hence the need to know its value at all times, in order to properly regulate these processes. types of pressure gauges

Another very familiar example is the check of automotive power , for which the level of compression in the engine cylinders is measured, which is the pressure measured when the piston is at the minimum volume point . Also, the tires must be filled to an adequate pressure, which is checked with a portable gauge designed for this purpose.

Blood pressure or blood pressure is an indication of the general state of health of people and is a routine examination, which is easily performed with a device called a sphygmomanometer or blood pressure monitor .

### Pressure classes

The pressure gauge to use depends on the pressure class to be measured: types of pressure gauges

• Absolute pressure , is the one that is measured with respect to absolute vacuum and is always positive. Atmospheric pressure is absolute and its standard value, at sea level, is 101325 Pa. It is measured with a barometer, which is why it is also called barometric pressure .
• Relative pressure , is measured with respect to a reference pressure, which is atmospheric pressure. Its value can be positive or negative. The instrument to measure it is the manometer.

## Types of pressure gauges

Depending on the type of pressure to be measured, various instruments have been designed. These are the main ones:

### Barometers

Barometers measure atmospheric pressure. There are several types, depending on the sensor mechanism.

• #### Mercury barometer

This instrument was invented by the Italian physicist Evangelista Torricelli (1608-1647). It consists of a tube filled with mercury inverted over a container that is also filled with mercury and open to the atmosphere. types of pressure gauges

By inverting the tube, the mercury descends, leaving a space at the top where it becomes almost empty, except for a small amount of mercury vapor. There the absolute pressure is quite close to 0. types of pressure gauges

The force exerted by the air on the exposed part of mercury and the weight of the mercury column in the tube are balanced, since the whole assembly is at rest.

Now, the force of the air on the surface of the mercury is exerted through pressure, and this in turn is related to the height of the column of mercury in the tube, a value close to 760 mm and equivalent to 101 325 Pa .

• #### Water barometer

This barometer uses water instead of mercury, but the liquid column is longer, since the water is lighter: 10.3 m high.

• #### Aneroid barometer

This device does not use liquid, but a metal chamber partially under vacuum that contracts or expands according to the pressure to which it is subjected. The pressure difference is transmitted by a spring, the elongation of which moves a rod attached to the gauge indicator. The pressure is read on a display fitted with a scale. types of pressure gauges

When a recording system of some kind, such as paper and ink, or a digital screen is added to the instrument, then the instrument is a barograph and they allow the atmospheric pressure variations to be displayed in graph form, useful for predicting the weather.

### Pressure gauges

Manometers usually measure relative pressure, although they can sometimes give absolute readings. types of pressure gauges

• #### U-tube manometer

It is used to measure small and moderate relative pressures. The simplest consists of a U-shaped tube made of glass or plastic, filled with fluid, such as mercury, with one of the branches sealed or open to the atmosphere and the other connected to the pressure to be measured. types of pressure gauges

When pressure is applied at one end, the mercury rises up the other branch until the weight of the column balances the pressure at the end. The difference between the original level and the new level is proportional to the pressure difference between the two points.

A variant of the U-tube manometer is the inclined tube manometer, which is used to measure small pressure differences.

• #### Dial gauge

This type of manometer transfers the pressure variations to a graduated scale or dial, through some mechanism. This can be, for example, the deformation of a spring or a C-shaped or spirally wound tube, which tends to straighten when pressure is applied at one end or to bend more if it decreases.

The tube is called the Bourdon tube , after Eugene Bourdon, the French engineer who invented it in 1849.

The deformation of the tube moves a stop provided with a mechanical system with gears that converts the linear movement of the tube into one of rotation at the other end, there an indicator needle is connected that can be moved on a dial. The position of the needle on the graduated scale on the dial indicates the pressure value. types of pressure gauges

• #### Digital pressure gauge

They cause a pressure difference to cause a variation in some electrical quantity such as resistance, for example. Said variation can be converted into an electric current that is proportional to the pressure difference and the reading is made on a digital display.

In a pressure gauge of this type, the elastic deformation of the sensor made of a conductive material produces a change in its electrical resistance. The change in resistance is related to the change in pressure. In the same way, you can vary the capacitance in a capacitor or the inductance in an inductor.

Another kind of sensor is of the piezoelectric type, such as quartz or tourmaline, in which a change in pressure on a crystal of one of these minerals is translated into a voltage. types of pressure gauges

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