text: It is often necessary to maintain the Büchner flask and, incidentally, the vacuum flask. The rigidity of the vacuum pipes and the difference in height between the different parts of the apparatus (as visible in the diagram) make such an apparatus relatively unstable.
  Therefore, a three-pronged clamp should be used to maintain the Büchner flask. This clamp should be placed such that the two prongs surround the part of the flask connected to the vacuum tube, the lasting prong resting on the other side.
  If it is also necessary to maintain the vacuum flask we use either a mandible clamp, or a three-pronged clamp, depending on the apparatus and its stability. The clamp to use is left to the judgement of the operator.
  Before closing the tap, it is necessary to "break the vacuum" (letting in the air in through any area in the apparatus, by removing the funnel for example), otherwise water goes up the apparatus from the aspirator. The vacuum flask prevents the water from going up the Büchner flask.