# Isn’t there suppose to be vacuum above the closed end of Mercury manometer?

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.

• That seems to be a very dangerous amount of mercury... Just saying Mar 9, 2021 at 22:08
• 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? Mar 10, 2021 at 0:11
• @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. Mar 10, 2021 at 5:37
• @DavidWhite from the bottom of the tubes U to the top mercury it’s 19mm height. Mar 10, 2021 at 5:39

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).