There are three main types of barometers: mercury column, aneroid or barograph. As the mercury column involves a glass tube at least 84cm long it is not the most practical thing to have aboard. Both the aneroid and barograph use the same mechanism but the barograph records the air pressure using a pen on a paper attached to a drum moved by clockwork.
An aneroid barometer uses a small, flexible metal box called an aneroid cell (capsule), which is made from an alloy of beryllium and copper. The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen. In addition, the mechanism is made deliberately "stiff" so that tapping the barometer reveals whether the pressure is rising or falling as the pointer moves. It is not the actual pressure but this change in pressure that guides you on what the weather may be doing next. Most complaints from users that their barometer does not work properly are because they have failed to adjust it to the proper barometric pressure for the area in which it is being used. Aneriod barometers are fully adjustable. Adjustment can be made by turning a small brass screw on the back of the movement until the barometer needle is set on the current pressure for that geographic area on that day. After setting, move the gold needle on the glass over the registering needle so you can easily observe any rise or fall.
To enure that your barometer is working properly, you can do the following tests:
- Using the small brass adjusting screw, turn it so that it makes an arc from the 8 o'clock position to the 4 o'clock position and back again. It should do this smoothly and without sticking. Be careful to not unscrew so far that the screw falls out.
- Place the barometer in a clear plastic bag, blow air into it and seal tightly so the air will not escape. Gently push down on the bag. If your registering needle moves several millibars, it is most likely working as expected.