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.
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
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 of water.
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
is about 10 N/cm2 at sea level but varies with the
weather, and of course altitude.
We sometimes measure high pressures in
atmospheres (atm) or bars.
Pressures inside cylinders of compressed air and other gases are often given in atmospheres or bars. These are of course gauge pressures.
Meteorologists measure the pressure of the atmosphere in bars or more usually millibars (mb).
The pressure at sea level in England varies from about 900 - 1100 mb, and the global average is 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 is about 10 m of water or 76 cm (30 inches) of mercury.
Mercury (Fortin) barometers are still used today, so
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). They are often calibrated in millibars, but the numbers 28 to 32 on 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.
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.
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
© Barry Gray March 2008