A topic that comes up frequently concerns scope and/or anti-cant device leveling. Many members consider a precision level is the best way to confirm/compare their devices accuracy. But, how do you really know which one is reading right? Precision machining is my background. But, metrology, the science of measurement is a passion of mine. It doesn't matter if the level is a $.99 one from Home Depot or a high-precision level from Starrett, they are all checked/calibrated the same way. The principle is known as reversal.
To confirm a level IS level is very easy. You don't even need a completely level surface. As long as the bubble doesn't "peg" at one end or the other you're good to go. Use one edge and one end of the level like a straight edge and draw 2 lines so that when you re-position the level it is placed back in the same spot. Set the level down and note where the edge of the bubble is lined up. Pick up the level and turn it 180° and set it down. Again, note where the bubble lines up. If they are not at the same point the error is 1/2 the difference. If you're lucky and the vial has multiple graduations and is able to be adjusted you can center up the bubble easily. Most decent carpenter's levels have an adustment, either a jackscrew or the entire vial can be rotated. For example, if the bubble stops at the 4th line and after spinning it around it is at the 2nd line, 4-2=2, 1/2 of 2 is 1. Use the adjustment screw and bring the bubble to the 3rd line. The bubble should line up with the 3rd line, no matter which end the level is flipped to. If your level is not adjustable, you can either shim the base or use a Sharpie to mark the true level point.
This can also be helpful if you or someone you let borrow the level drops, dings or otherwise abuses it. Easy check, easy reset.
To confirm a level IS level is very easy. You don't even need a completely level surface. As long as the bubble doesn't "peg" at one end or the other you're good to go. Use one edge and one end of the level like a straight edge and draw 2 lines so that when you re-position the level it is placed back in the same spot. Set the level down and note where the edge of the bubble is lined up. Pick up the level and turn it 180° and set it down. Again, note where the bubble lines up. If they are not at the same point the error is 1/2 the difference. If you're lucky and the vial has multiple graduations and is able to be adjusted you can center up the bubble easily. Most decent carpenter's levels have an adustment, either a jackscrew or the entire vial can be rotated. For example, if the bubble stops at the 4th line and after spinning it around it is at the 2nd line, 4-2=2, 1/2 of 2 is 1. Use the adjustment screw and bring the bubble to the 3rd line. The bubble should line up with the 3rd line, no matter which end the level is flipped to. If your level is not adjustable, you can either shim the base or use a Sharpie to mark the true level point.
This can also be helpful if you or someone you let borrow the level drops, dings or otherwise abuses it. Easy check, easy reset.
