types of barometers

There are three basic types of barometers.

1. Water Barometers

A water barometer is the simplest form of a barometer. It often consists of a sealed vessel about half full of water. Connected to this vessel is a spout that extends above the water level and is open at the top. The water barometer is filled through a method that creates a vacuum in the vessel. This means the water level in the spout will show variations in atmospheric pressure. Often the water is coloured with food dyes to make reading the water level easier. Extreme low pressure can sometimes cause the spout to overflow.

The water barometer is also sometimes called a 'Weather Glass', 'Thunder Glass' or 'Goethe Barometer' (after German writer Johann Wolfgang von Goethe).

The TFA Weatherball is a contemporary example of a water barometer.

2. Mercury Barometers

Mercury barometers use a long tube and reservoir arrangement with the tube closed at one end. Atmospheric pressure moves mercury from the reservoir up and down the tube according to pressure variations. Tubes often need to be fairly long – around 75 cm.

Observing the mercury moving up and down the tube soon gave us the term 'inches of mercury' when measuring atmospheric pressure.

Readings in mercury barometers are not just affected by atmospheric pressure, they are also affected by temperature. For this reason, many mercury barometers include a thermometer so that a compensation for temperature can be calculated.

Unfortunately mercury is a dangerous substance – banned by many couriers and Australia Post – and glass tubes are very fragile. For this reason and the safety of our customers we do not sell mercury barometers.

3. Aneroid Barometers

The aneroid barometer measures the atmospheric pressure exerted on a small metal capsule which contains a vacuum. Changes in pressure will affect the height or thickness of the capsule. This type of barometer is purely mechanical and uses levers, linkages and a spindle to convey changes in the height of the capsule to a needle on a dial. Often a 'marker' needle is fitted to this dial so that changes in reading can be observed.

Aneroid barometers can often be 'sticky' so you need to lightly 'tap' the face to make sure the reading is accurate. The vibration caused by a gentle 'tapping' can overcome any resistance to movement in the capsule and linkages.

The word 'aneroid' first appeared in the French language and is taken from two Greek words meaning 'without water'.

Aneroid barometers are by far the most common barometers today and give a reasonably accurate pressure reading. The most accurate readings come from high-end meteorological barometers.

Aneroid barometers need to be calibrated to their specific location.

Barographs

Barographs often use several aneroid capsules stacked together and record pressure readings on paper fitted to a drum. The drum is slowly rotated by either a quartz or mechanical clock movement.

Next:  Units of measurement