Barometer

A barometer is a type of manometer that specifically measures a pressure relative to a prevailing atmospheric pressure (manometers more specifically measure pressure differences). Air in the atmosphere has a weight which presses against all surfaces on the earth as gravity pulls it inward, barometers measure this pressure. Different barometer designs exist (similarly with manometer designs), essentially there are those containing a fluid (mostly mercury) in a closed or open tube design, those without a fluid, using mechanical parts called an aneroid barometer and contemporary or digital barometers.

Barometers measure atmospheric pressure, the units were originally referred to as atmospheres, relating to a unit of air pressure at sea level at a temperature of 15 degrees C, later this was defined as 1.01325 x 105 pascals. Today this is often referred to as bar which is simply a pressure unit, and defined as 100 kilo-pascals. So one atmosphere (atm) is almost the same as one bar, but not quite, more specifically 1 atm is equal to 1.01325 bar. These devices often found outside windows in homes, used to measure the outside air pressure to assist in weather predictions, as rising air pressure indicates a good weather forecast whilst falling pressure might forecast rain or bad weather.

The first fluid barometer was invented by Italian physicist Evangelista Torricelli in 1643, it contained water and was over 10 metres in height. Water is relatively light in weight, the tall tube compensated for the heavier weight of atmospheric pressure, which was later redesigned with mercury as it is over ten times heavier and allowed for a much shorter tube. The design contains a single tube sitting in a pool of mercury, as atmospheric pressure applies a for to the pool the measure rises in the tube. A cistern barometer patented by Daniel Quare in 1695 was a sealed design so prevented spillage of mercury (a toxic substance now banned) and allowed easier transportation. The angle or diagonal barometer expanded the barometer scale allowing for better readings.

In 1844, Lucien Vidie a French lawyer and engineer (1805-1866) successfully produced a metallic barometer that used springs to measure atmospheric pressure known as an aneroid barometer. These devices have a sealed metal chamber which expands and contracts, depending on the atmospheric pressure surrounding it around it. A series of rods and cogs measure the mechanical movement of the chamber as it expands or contracts, to a scale aligned with atmospheres or bars. A barograph is a type of aneroid barometer linked to a pen arm that continually records the vertical displacement of the chamber over time creating a graphical record of atmospheric pressure changes.

Modern digital barometers use capacitive or piezo-resistive micro-electromechanical (MEMs) sensors to measure pressure, these combine electronic and mechanical elements in small integrated circuits (IC) to sense pressure differences and record them digitally.

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