I found this very old Mercury closed end manometer(I think so). Looking at the graduations on the left side it shows very low vacuum values and the right side values from 110-0-80. The reading is in mBars.

So, when I looked into some theory on manometers the closed one has some column of vacuum at the closed end. Which I guess is created due to the weight of Mercury going down? Is this setup correct or should the Mercury be drained and filled again to get that gap at the section with red arrow.

Sorry since this is the first time I’m using a Mercury type gauge. Most of the time I’ve used only analog needle gauges or digital ones.

mercury closed manometer / vacuum gauge

  • 2
    $\begingroup$ That seems to be a very dangerous amount of mercury... Just saying $\endgroup$
    – DJohnM
    Mar 9, 2021 at 22:08
  • $\begingroup$ Measure from the bottom of the tube to the top of the left hand side of the tube (the one filled with mercury). How many mm is that? $\endgroup$ Mar 10, 2021 at 0:11
  • $\begingroup$ @DJohnM Though from the close up picture it may appear like one of those old large manometers, it isn’t that big. The height is just 4.2” and base is 2.67”. The amount of mercury isn’t that much but still quite enough to be dangerous too. $\endgroup$ Mar 10, 2021 at 5:37
  • $\begingroup$ @DavidWhite from the bottom of the tubes U to the top mercury it’s 19mm height. $\endgroup$ Mar 10, 2021 at 5:39

1 Answer 1


Yes at the top of the left tube you need vacuum because you want to measure pressure by only watching the height of the mercury. You don't need vacuum to make this manometer work but it's super not pratical, infact if you would have air or some sort of gas its pressure would change with the change in height of the column of mercury (also the graduation would be non-linear), so that's why we use vacuum (notice how in your foto the left column has no gap at the top and its completely filled with mercury). But how it works? basically the pressure of the left column of mercury ($P=\delta gh$, where $\delta$ is the mercury density and h the height of the column) must be equal to the air pressure "entering" from the right tube. Infact the value 0 (aka vacuum) is positioned in the middle of the tube, where the pressure of the two column of mercury (height = h/2) is balanced (the mercury goign down in the left column "doesn't leave anything behind", so the vacuum remains).

  • $\begingroup$ Thank you for the detailed explanation. I got confused about the vacuum from the Torcelli barometer. My assumption was that the mercury at the left side would drop. But then I realized that actually that height should be 76cm and only a tube bigger would leave that vacuum. Since mine is smaller I guess that’s why the mercury is at the top. Also I saw a video by Cody from YouTube channel Cody’slab using a similar type of vacuum gauge(link below). I see that when the vacuum pump max capacity is reached he just measures the difference in height as millimeter of Hg with a normal scale. $\endgroup$ Mar 10, 2021 at 20:16
  • $\begingroup$ Link of video from Codyslab: youtu.be/hrgRHD1STFw $\endgroup$ Mar 10, 2021 at 20:17
  • $\begingroup$ So, basically this cannot be used to measure atmospheric pressure right? $\endgroup$ Mar 10, 2021 at 20:18
  • $\begingroup$ your device CAN be used to measure atmosferic pressure, but obviosuly has its limitation, you cannot measure a bigger pressure when the left column is completely filled (because the mercury hit the top of the column), infact see how the last value on the graduation (110) correspond with the left column being completely full. But you can meausure pressure < 110 because the mercury has room to move (in the vacuum both column would be half-filled, infact the value 0 is in the middle). $\endgroup$ Mar 11, 2021 at 21:09
  • $\begingroup$ the experiment in the link you sent create vacuum OUTSIDE the barometer and not in the column not exposed to the atmosphere (left column in the photo). the vacuum on the left column is always present, while an "atmospherc vacuum" can only be created with a pump (that's the backgroud noise in the video), infact notice how the two column becomes both half-filled(almost) when a vacuum/low pressure condition is formed by the pump. $\endgroup$ Mar 11, 2021 at 21:13

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