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:
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?