Inverted spirit level not true, why?

[oldman2]

My Betts eurobox 36" level has not been mistreated and when tested appears to read level in normal use but for some reason I cant think of when used inverted to check ceiling runout its well out of calibration.

How can this be? its just a straight vial filled with liquid and a bubble, if it centers ok used normally I would expect the same when inverted.

 

[Jasonb]

The tube is not a straight cylinder, there is a very small crown to them internqally, in fact you can get fast & slow bubbles, a fast one has a more pronounced crown so the bubble will settle there quicker unlike a machinists level which is very accurate and has the smallest amount of crown.

Why can't you hold the level with the "upper" surface against the ceiling and look at the bubble from the side? thats what I do

Jason

EDIT its all out there on the net and you thought I was making it up

 

[oldman2]

Thanks Jason, I have several other Stanley levels and they all seem to be able to be used inverted, so I assumed I could do the same with the Betts one. Seems not.
I can use the betts non inverted but it just seemed easier on me to look from below rather than squint at the side of the level.
I have marked that one as not to be inverted now

You live and learn.

 

[TicklyT]

Some spirit level vials are curved cylinders, some have a barrel shaped 'straight' bore. A barrel shaped vial will work in any orientation.