Making Sure the Level is Level
Q: How can I make sure my level is accurate?
Is there any way to reset a spirit level that isn't accurate? I've been told that turning the level counterclockwise about four times will reset it.
—Brian Bolfing, Crawford, TEX.
Tom Silva replies: Not exactly. Here's how to go about it. Place the level on edge on any flat surface—it doesn't have to be level, as long as you can see both ends of the bubble in the vial. Now keep the level horizontal, rotate it 180 degrees, and put it down in the same place. If the bubble's position hasn't changed, the level is accurate. But if the bubble is in a different position, the vial is not accurate. Use the same technique to check the vials in the ends of the level, which show you whether a surface is vertical. When you find a vial that's gone bad and can't be adjusted, cover it with tape so that it won't mislead you. If none of the vials are reliable, you have a nice straightedge, not a level.
An adjustable vial has screws holding it in place. To reset it, hold the level flat against a wall surface, move it until the bubble reads level, then draw a line on the wall along one edge. Now flip the level around, position it on the line, and move it until the bubble reads level. Draw another line along the same edge. If the level isn't accurate, the two lines will diverge. Now position the level so that one end is where the two lines meet and the other is halfway between the diverging lines. This is true level, so loosen the screws and adjust the out-of-whack vial until it reads level. Retighten the screws and retest it.
Shown: Tom Silva depends on accurate levels for many different jobs, so he gives the vials a quick check every so often to make sure they're still on the money.