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The accuracy of water levels

Q: What accuracy can I expect using a water level? When I fill a tube with water (all air removed) and hold the ends next to each other, the water levels are never the same height, and the difference varies every time I move the tubes. Can you tell me the reason for this?


Rick Strub, Amherst, NY


A: Tom Law, former consulting editor of Fine Homebuilding, replies: In its natural state, water infallibly seeks its own level. The problem is that when we put water into hoses or tubes, we’re doing something unnatural with it. In order for a water level to work properly, the water has to be able to move freely. The tube cannot be crimped or have air bubbles in it, and the tube has to be open to the atmosphere on both ends (no plugs or fingers blocking either end of the tube). Under those conditions, a water level simply has to work with 100% accuracy.

There are some things that could prevent the level from working properly that you may not think of. First of all, use a tube with l/2-in. I. D. or larger. In a small-diameter tube, surface adhesion makes the water’s surface concave—the water wants to creep up the inner walls. In a larger tube, there is still the same adhesion, but the surface appears flatter.

If you’re using a water level outside in the winter, ice can form and obstruct the flow of water, but you won’t be able to see it. The water must be the same throughout. Antifreeze is often used in a water level to avoid freezing (the color of antifreeze also makes the water level easier to read). The antifreeze should be mixed thoroughly with water before the tube is filled. If you pour the antifreeze directly into the tube, it may not mix equally with the water. And the specific gravity of the solution could be greater in one end of the tube than in the other.


From Fine Homebuilding 77, pp. 22 November 1, 1992