I thought that I would pass the following along...
Some bubble levels use a fairly viscous fluid within the bubble levels -- viscous enough that it can take up to 20 seconds for the bubble to settle down to indicate whether the bubble level is level or not. We use bubble levels on our large telescopes to indicate when the telescope is pointed exactly at the zenith (straight up) or if the telescope is pointed exactly horizontal and parallel to the distant horizon.
The cheapest and very accurate bubble level which I have found which also does not use a viscous fluid within the level is a fairly small bubble level sold by ACE Hardware. The level has a white plastic case and is the size of a pen. It also has a metal pocket clip on one end and has a small magnet on the opposite end. The bubble level within the white plastic case is dead on accurate, and the bubble level installed in the white plastic case is accurate to within 10 arc minutes or 1/6 of a degree.
I don't know the ACE part number, but I thought that I would pass this info along since this described ACE level is by far the cheapest yet extremely accurate level which I have found to date. The levels are so accurate that, when I point one of our telescopes at the zenith based on the levels and then tell the control software that the telescope is pointed exactly at the zenith, I can shoot a shot with the telescope's CCD camera and find that the the pointing error either left-right or up-down generally is less than 10 arc minutes or within 1/6 of a degree. That is pretty darned accurate for any bubble level -- no matter how cheap or how expensive!