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Recently I have come across a mercury-filled manometer for vacuum distillation that somewhat resembles a U-tube shaped manometer, but with one arm sealed and completely filled with mercury. Searching through the Internet, I have found that such manometer is being called Anschütz/Bennert manometer. Here is a drawing showing what I am talking about:

Anschütz/Bennert manometer

Also, the page "Vacuum or reduced-pressure distillation" leads to a website showing the whole vacuum distillation set up with such a manometer (Fig. 15.5):

Since one arm of this manometer is sealed and completely filled with mercury, it doesn't seem like the difference in height of mercury in both arms could be related to pressure difference between vacuum in the system that the manometer is connected to and air—as it is being described most commonly for U-tube shaped manometers that have one arm open to air. Thus it makes me a little confused what is the working principle here. I have been searching for an explanation, but I didn't find anything for this particular case.

How does such an Anschütz/Bennert manometer work and how can I read the vacuum pressure from it?

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All operations should be gentle:

  • open the valve to the system evacuated
  • if the vacuum applied is good enough, the level of mercury in the right hand (open) leg rises, the level in the left hand (closed) leg decreases
  • after stabilization of the two levels, move the sliding rule behind the U shaped tube to read the total of the difference of the two levels in mm, which by filling with mercury equates to pressure in expressed in mmHg (link to a pressure converter, but no particular endorsement for this one). Because the left leg is closed, the recording is about absolute pressure.
  • if no further monitoring is needed, close the valve to the system evacuated.

To return the «ground state»

  • check the valve, which initially should be closed
  • detach the manometer from the system to monitor
  • gently open the valve to return to atmospheric pressure; the level of mercury in the left hand leg should rise so that there is no empty head space left
  • close the valve again, and stash it in the shelf

If you return too rapidly to atmospheric pressure, the momentum of mercury can be enough to shatter the glass tube.


Depending on local law applicable, finding mercury manometers / thermometers becomes more difficult/impossible, though this short leg manometer as well as the swivel Mc-Leod type (the later more suitable for oil pump vacuum) still have their point beside the electronic ones. But they can break (then there can be some trouble [OSHA]), are not so handy in an already crammed fume hood, have no data link etc.

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