Units of pressure

Units of pressure

We measure forces in newtons (N) and areas in square metres (m2) so we measure pressure in newtons per square metre (N/m2). The SI unit of pressure is the pascal (Pa), after Blaise Pascal (1623 - 1662). He was a French mathematician and physicist.

1 N/m2 = 1 Pa

A pascal is a very small pressure indeed (less than the pressure exerted by a teaspoonful of caster sugar spread out over a 30 cm square kitchen floor tile) so we normally use N/cm2 or kPa. There are 10 000 square centimetres in a square metre and 1000 pascals in a kilopascal so

1 N/cm2 = 10 kPa

A pressure gauge connected to a gas cylinder is usually designed to read zero when the cylinder is empty, that is, when the pressure inside it is atmospheric pressure so that when you open the valve no gas will come out. We often refer to the pressure measured by a pressure gauge or manometer as a gauge pressure - to obtain the absolute pressure we must add atmospheric pressure to a gauge pressure.

The density of water is about 1 g/cm3 so we can show that the (gauge) pressure of the water at a depth of one metre is about 1 N/cm2. We often measure gauge pressures in centimetres or metres of water.

100 cm of water gauge = 1 N/cm2 (about)

Mercury is about 13.6 times heavier than water so we often measure higher pressures using a manometer containing mercury rather than water. We could use any liquid in a manometer (or barometer - see below) but we seldom use any liquid except water or mercury - we usually colour the water to make it easier to see.

Blood pressure, for example “120 over 80”, is almost invariably measured in millimetres of mercury. You can read more about blood pressure on the Heart Page.

Atmospheric pressure

Atmospheric pressure at sea level is about 10 N/cm2 or 100 kPa or about 10 m of water or about 760 mm of mercury, but varies with the weather, and of course altitude.

Meteorologists measure the pressure of the atmosphere in bars (from barometric unit) or more usually millibars (mb).

1 bar = 1000 mb = 10 N/cm2 = 100 kPa

The pressure at sea level varies from about 900 - 1100 mb, with a global average of about 1013 mb. Lower pressures are usually associated with rain and strong winds, and higher pressures with fine, dry and calm weather. Changes in pressure bring changes in the weather.

We measure atmospheric pressure with a barometer. The earliest barometers contained water or mercury, but water barometers are no longer used, although students sometimes make one in a science lesson. Atmospheric pressure at sea level is about 10 m of water or 760 mm (30 inches) of mercury.

Mercury (often called Fortin) barometers are still used today, although in some countries because of concerns about the safety of mercury they may no longer be made.

An anaeroid barometer contains a sealed metal box with flexible sides which has had some of the air drawn out of it. As the pressure on the outside of the box changes the box changes its shape and this makes a pointer move. Today anaeroid barometers are widely used, particularly in the form of barographs (which draw changes in atmospheric pressure on a graph against time). Today they are usually calibrated in millibars, but the numbers 28 to 32 on older anaeroid barometers in British homes refer to the atmospheric pressure in inches of mercury.

It is often said that an anaeroid barometer is so called because it contains a box with very little air in it (thinking the word comes from the Greek for without air). In fact anaeroid comes from the Greek for without fluid, that is, without water or mercury.

In common speech we sometimes talk about very high pressures as being so many atmospheres, so a thousand atmospheres (1000 atm) would be a thousand times atmospheric pressure. There is a slight problem: one atmosphere is sometimes taken to be the mean atmospheric pressure of about 1013 mb, and sometimes as 1 bar (1000 mb), so in scientific work it is best to use only bars.

Standard temperature and pressure

The density of a gas depends upon its temperature and pressure. We often give the density of a gas at a “standard temperature and pressure” (s.t.p.). The original IUCAP (International Union of Pure and Applied Chemistry) definition was 760 mm of mercury and 0oC, in SI units 101.325 kPa and 273.15 K, and this is still the definition most commonly used, although there are now also other definitions based upon 100 kPa and 288.15 K (15oC).

Vacuum Pressures

Very high vacuums, that is, very low pressures, are sometimes measured in torrs, after Evangelista Torricelli (1609 - 1647) who was the first person to make a mercury barometer, and therefore the first person to create a vacuum.

1 torr = 1 mm mercury or about 133 Pa

Imperial Units of Pressure

In Imperial (old British) units pressure is measured in pounds per square inch (psi). Car tyre pressures in the UK are still often given in psi. Atmospheric pressure is about 14.7 psi.

There are more units of pressure and the conversion factors between them on the Easy Unit Converter Web Site