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Atmospheric pressure decreases as altitude increases. The atmospheric pressure at the top of Mount Everest is about one third of that at sea level. Official pressure readings are corrected to sea level. So to be accurate an aneroid barometer needs to be calibrated to your specific location. In doing this, you compensate for any natural variation caused by altitude.
To calibrate your barometer you will need:
1. A small flat-head screwdriver.
Aneroid barometers feature a small adjustment screw on the back of the unit. The barometer needle is moved by turning this screw.
Some barometers ship from the factory with several turns of 'slack' in the screw. If you turn the screw and the needle does not move, you may need to keep turning until it responds.
A word of caution: never force the screw if it doesn't want to turn and don't back the screw off too far so that it falls out! Don't turn it too far in either direction.
2. An accurate reading of the current atmospheric/barometric pressure for location.
If you don't know how to find this, the best place to start is the Bureau of Meteorology's website.
Try and find the latest reading nearest your location. Hopefully you?ll find a page which gives half-hourly or hourly pressure reading (click here for an example).
Using the most recent reading, adjust the screw until the needle gives the correct reading on the dial. Lightly tapping the face of the barometer can help the needle move if the linkage seems 'sticky'.
If you can only find observations taken at three hourly (or greater) intervals for your area, you'll need to do your calibration just after one of those readings has been taken.
Alternatively you can record the reading on your barometer at the time the official readings are taken, and then compare that to the official reading when it is published, You can then adjust for the difference. For example, at 9.00 am you record 1020 hPa. When the official reading is published for your area, it shows 1010 hPa at 9.00 am. Therefore you should adjust your barometer to read 10 hPa less than it currently does.
If no official readings are available within about sixty kilometers from where you live, the Bureau of Meteorology advises that you can:
"read your barometer, preferably at 9 am or 3 pm, and forward the reading, together with the time and date that you made it, to the Regional Office of the Bureau of Meteorology in your State or Territory. An officer will compare your reading with the isobaric chart and tell you what adjustment (if any) is needed."
