Friday, April 26, 2013

Since mercury expands and contracts with temperature changes, how can it give consistent barometric measurements?

You’re correct that mercury exhibits thermal expansion, but it’s not enough to affect a mercury barometer reading for most purposes. Sometimes the degree of precision of pressure measurements is such that temperature variations will make a difference. In this case a mathematical correction is made to the measured height of the mercury column to account for the change in volume. A table of corrections is shown below for temperatures above the calibration temperature of 0 degrees C. It shows the correction...

You’re correct that mercury exhibits thermal expansion, but it’s not enough to affect a mercury barometer reading for most purposes. Sometimes the degree of precision of pressure measurements is such that temperature variations will make a difference. In this case a mathematical correction is made to the measured height of the mercury column to account for the change in volume. A table of corrections is shown below for temperatures above the calibration temperature of 0 degrees C. It shows the correction for 760 mm Hg at 20 degrees C to be 2.47 mm. This is a difference of only about 0.3%.


Mercury barometer readings are also affected by altitude, because the height to which air pressure will push a column of mercury depends on the local value gravity. This can also be mathematically corrected when required for a desired degree of precision.


You might be wondering why the expansion of mercury seems more significant in a thermometer than in a barometer. This is because the mercury in a thermometer is contained in a very thin tube. The small diameter of the tube causes a more noticeable change in height when the volume of the mercury changes.

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