National Physical Laboratory

How do I transport my mercury barometer? (FAQ - Pressure)

Fortin Barometer
Fortin Barometer

Mercury barometers must be transported with extreme care - the prime concerns are to prevent air from entering the vacuum space or the tube from being broken and leaking mercury.

Fortin Barometers

These have tubes which can easily be broken if the mercury is allowed to oscillate up and down wildly and hit the top of the tube. This can occur, for example, simply by walking whilst holding the barometer in an approximately upright position (cistern at the bottom).

Air can enter the tube if the barometer is inverted whilst there is still air in the cistern (see picture to left).

To transport a Fortin barometer the cistern mercury level should first be raised until the cistern is full by turning the axial screw (see S in diagram) and then continuing to turn it until the mercury in the tube has risen to within about 25 mm of the tube's top. (A small gap is left to ensure that expansion of the mercury does not rupture the cistern's leather bag.) The barometer shall then be inclined slowly until the mercury just touches the top of the tube, and then on until the instrument is horizontal or, preferably, until the cistern end is higher than the body - until completely upside down if preferred. The barometer can then be safely transported with care, but sealing it in a rupture- and leak-proof plastic bag is recommended to contain any mercury spilt by accident.

Kew Barometer
Kew Barometer

Kew Station Barometers

These are more difficult to break than Fortin barometers by transporting them in an upright position because the mercury cannot oscillate so wildly. It is nevertheless safer and better practice to turn them slowly until they are horizontal or, as with the Fortin, with the cistern end higher. There is, of course, no axial screw to adjust before inversion but the geometry of the cistern is such that air will not enter the tube when the barometer is carried in this way.

The barometer can then be safely transported with care, but sealing it in a rupture- and leak-proof plastic bag is recommended to contain any mercury spilt by accident.

Bench Barometers

Bench barometers have tubes which are susceptible to damage caused by mercury oscillating wildly during transportation. There are essentially two types of bench barometer in regular service and the pre-transportation procedures are different for each:

  1. Those with transportation sealing screws: With the transportation screws only loosely fitted, the barometer should be tilted slowly forward until the mercury rises to within approximately 25 mm of the top of the tube (there is usually a small sight-hole in the barometer frame to allow this) and the screws tightened. The mercury should then remain near the top of the tube when the barometer is returned to the vertical position. Two pairs of rubber tube-clamps should be inserted symmetrically along the barometer's length, between the frame and the tube, to provide extra support for the now heavy tube. They may also be transported in the same way as those without transportation screws.

  2. Those without transportation sealing screws should be evacuated to a pressure of just a few millibar and have their external pipework sealed. Some instruments have a valve which permits this but those which do not (the majority) may be sealed by clamping a short piece of rubber tube fitted to the barometer's pressure port. Barometers should not be transported with the mercury raised to near the top of the tube when employing the external valve or pipe-clamp method of sealing because the mercury could still oscillate and break the top of the barometer's tube.

After completing the pre-transportation procedure, the bench barometer should be transported, with care, in its normal upright position, but sealing it in a rupture- and leak-proof plastic bag is recommended to contain any mercury spilt by accident.

Last Updated: 25 Mar 2010
Created: 9 Aug 2007

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