Sonic checkers

It's not the shape, but the material that makes issues. Think of it like taking a step into a pond. If the waters clear, you can see what you're doing. If the waters muddy, it could be 3", or 1' deep. The better testers can adjust for the clarity, and give you a good reading. The poorer testers can tell you its wet :) . If you're happy with the results, that's all that matters. I looked at a couple, talked to a guy or two, and i wasn't comfortable enough.