I have noticed that when I fill the 36 inch glass tube of my mercury barometer, that there are thin bubbles of air stuck between the mercury and the inside wall of the tube; can't seem to get rid of them. Maybe a vacuum needs to be pulled before pouring in the mercury, but that seems too extreme for my "frugal" means. I'm thinking that the reason it steadily drops over some days to less than 30 inches, when the value is being reported as 30 inches, is because of these bubbles. I'm thinking that the vacuum has been compromised. So I wonder...is it OK to just move the inches scale to the lower point? Does it become temperature sensitive? Does the length of an inch on the scale need to change too?