# What Is Pressure?

Pressure is the physical magnitude that measures the instantaneous force on a surface unit, applied perpendicular to it. Its unit in the SI is the pascal, which is equivalent to a force of a Newton acting uniformly on a square meter, and other units used are, for example, the millimeter of mercury, the bar and the atmosphere. In everyday life we ​​find many examples of pressure that we use often: atmospheric pressure to talk about weather changes, blood pressure to talk about health, car wheel pressure in the motor field. We even talk about pressure on issues that have no link to physics: the pressure of a job interview.

Strength and pressure are very different concepts. The pressure (symbol P) is the physical magnitude that measures the instantaneous force on a surface unit, applied perpendicular to it. His unit in the SI is the Pascal. Pascal is equivalent to one Newton per square meter (N / m2). Other units can also be used to measure pressure such as the millimeter of mercury, the bar and the atmosphere.

Pressure can be expressed in terms of absolute pressure or relative pressure (also called normal pressure, gauge pressure or gauge pressure). The absolute pressure is the sum of the relative pressure and the atmospheric pressure. The standard atmospheric pressure is 101,325 Pa.

The pressure P indicates, for a given moment, the value of the normal force F at each point of a surface S.

## Press absolute and press relative

The pressure can be of two types:

• Absolute or real pressure (measured in the ata technical system, absolute technical atmosphere): it is the pressure measured by taking the vacuum as a reference;
• Relative pressure (measured in the technical system in relative technical atmosphere): is the pressure measured by taking another pressure as a reference (for example, the atmospheric ground pressure).

If there is a (absolute) pressure of 10 atmospheres inside a pressure vessel and the atmospheric (absolute) pressure is present outside it, the relative pressure inside the vessel (i.e. the pressure difference between the inside and outside of the container) are 9 atmospheres. It must be borne in mind that a pressure can assume values ​​below atmospheric pressure (in this case we speak of "depression" or "vacuum").

## Units for measuring pressure

The unit of the international system for force is the Newton (N), for the surface is the square meter (m2), so for the pressure it will be (N / m 2 ). This value, given that it is commonly used, has been given a special name, Pascal (Pa), in honor of Blaise Pascal.

Pascal (Pa) is the unit of measurement of pressure in the International System. But it is a small unit, and sometimes it is convenient to use other units of pressure measurement:

• Hectopascals (HPa). Used in meteorology. This unit is equivalent to 100 Pa.
• Atmosphere (atm). The atmosphere is a unit that indicates the pressure that the earth's atmosphere produces on average on the surface of the earth. At the pressure at this point we give the value of 1 atmosphere. The name of this unit will be atmosphere (atm) and is equivalent to 101,300 Pa.
• Kiloponds per square centimeter (kp / cm2). This pressure unit is used in engineering. In this field, it is usually treated with very large weights that makes it not useful to talk about Newtons. The kilopond is the one that weighs a mass of one kilogram, and therefore equals 9.8 N.

### Standard atmosphere

Within the different pressure units we want to highlight the atmosphere for its wide use.

The standard atmosphere or atmosphere, abbreviated as atm, is a unit of measurement, defined with precision of six digits in the International System, to approximate an amount that varies according to place and time. It is approximately equal to the air pressure at sea level and is defined as: 1 atm = 101 325 pascal.

Pressure is sometimes measured in relation to atmospheric pressure.

An example is the air pressure inside a car tire, for example, a pressure of '2.2 atmospheres' means 2.2 atmospheres above atmospheric pressure, that is, the absolute pressure is equal to 3.2 atmospheres.

In addition, the relative pressure cannot fall below a negative value, set at −101 325 Pa. This is justified by the fact that by adding the atmospheric pressure (101 325 Pa) an absolute pressure equal to zero is obtained, the called absolute void. It is physically impossible to obtain pressures below absolute vacuum.

With the spread of the use of the International System also in the meteorological field, atmospheric pressure is measured in hundreds of Pascals (or hectopascals, abbreviated as hPa).

## Pressure measuring devices

Among the different devices to measure the pressure we find:

• The barometer, to measure the atmospheric pressure.
• The pressure gauge to measure pressure differences between two fluids).

### Barometer

A barometer is an instrument that measures atmospheric pressure. Atmospheric pressure is the weight per unit area of ​​surface exerted by the atmosphere. One of the best known barometers is mercury.

There are different types of barometers depending on the physical principle of operation and the constructive solution. The most common barometers in meteorological practice are classified as follows:

• Mercury Barometer Liquid-based barometers indicate the atmospheric pressure measured by the height of the liquid column (usually mercury), attached to the tip and the lower end connected to a small liquid container (the atmospheric pressure value is proportional to that of the mass of the liquid column). Mercury barometers are the most accurate, as they are used in weather stations.
• Water barometers: Goethe barometer and FitzRoy barometer.
• Gas barometer
• Mechanical barometer (aneroid). In practice, mechanical barometers (aneroid) are used more frequently. They contain no liquid (from Greek: aneroid = no water). They indicate the atmospheric pressure acting on an elastic metal box with thin walls, with a small hole. When the atmospheric pressure decreases, the box widens and, when enlarged, contracts, acting on a spring. Often, in mechanical barometers there are up to ten metal boxes, connected to each other, which, when changing the value of atmospheric pressure, moves an indicator on the graduated circumference according to the mercury barometer model.
• Barograph. It is a barometer that automatically records the curve of the heights reached by an airplane.

### Manometer

The manometer is a measuring instrument for the pressure of fluids contained in closed containers. There are two types of pressure gauges, as used to measure the pressure of liquids or gases.

The pressure gauge is based on balancing the measured pressure with the force of the elastic deformation of a tubular spring or a more sensitive two-plate membrane, one of which is sealed in a support and the other through a rod connected to a mechanism. tribo-sector that converts the movement of the elastic sensor into a circular movement of the arrow.

## Examples of orders of magnitude of pressure

0.5 Pa Atmospheric pressure on Pluto.

9.81 Pa The pressure caused by a depth of 1 mm of water.

1 kPa Atmospheric pressure on Mars.

10 kPa The pressure caused by a depth of 1 m of water, or the difference in pressure between sea level and 1000 m high.

101.325 kPa Atmospheric pressure at sea level.

10 GPa Pressure necessary for diamond formation, and some other minerals.

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Published: November 15, 2016
Last review: February 25, 2020