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Barometric pressure, a measure of the force of air, is frequently used in forecasting the weather. It is usually measured with a Fortin barometer, in which mercury is enclosed in glass, and travels up and down a metal tube in response to air pressure changes. A scale helps to match the mercury levels with the barometric pressure at the time a reading is taken. The Fortin barometer is generally used in meteorological stations as well as in laboratories and schools. In 2007, the primary manufacturer of the barometer discontinued production as mercury can be harmful to the skin and generally toxic to humans.
A Fortin barometer typically has a screw that allows the mercury level to be adjusted. An accurate Fortin barometer measurement is often possible only when it has been calibrated to known pressure levels. The measurements should generally be taken at the same temperature. Hotter or colder conditions can skew readings, as mercury responds to these changes as well as the pressure.
The location of the barometer should be out of direct sunlight, and should not vary much in temperature. Corrosive substances stored nearby could damage the device, so they should be kept away from the barometer. A highly traveled room or area where there is vibration is generally unsuitable for placing the instrument as well. It is often best hung at a height where the scale can be read.
Setting up the barometer generally requires some care. It is usually advised to move the device into a vertical position slowly when it is being unpacked and hung on a wall. Air in the tube can cause damage and affect the accuracy of the mercury. Adjustments made with the screw should usually be done slowly also because of air possibly trapped in the tube. When it is correctly set up, a Fortin barometer is typically very accurate.
The vacuum of an old Fortin barometer is often compromised, and antique instruments often need to be disassembled for restoration. Sometimes there is a removable cistern, which contains the mercury. If need be, the liquid metal can be removed with a syringe, and stored in a jar while the interior of the barometer is properly cleaned. A pump can be used to seal off the tube once the mercury has been poured back in. Similar procedures might be done when servicing the barometer for maintenance or in cases when it needs to be returned to the manufacturer.
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