I’m working on making a torsion box type surface in my home shop right now. It is an interesting intellectual challenge. It’s not hard to build a surface that’s physically strong. What’s tricky is building a surface which is as close to flat as possible. Ideally I would use a reference surface to base mine from. But I don’t have anything flat enough. I have a slab floor but I don't trust it to be flat enough. What is flat?
Water. <---frozen? calm? very hard to practically use.
Plate steel or aluminum. <---expensive
Machined granite/stone. <---super expensive
Self-leveled epoxy. <---less expensive but still $100/gal + time + mold
Straight edge and level/multiple measurements. <---inexpensive but subject to human error
I'm going with the cheapest option, the level/straight edge approach, but I have to be meticulous if I want a good result. The plan is to temporarily fix one side of the table as I'm building it to the wall of the shop, then level it as perfectly as I can, then build and shim the other three sides of the table top relative to the first one using the level. But do I have a good enough level? There are new digital ones now, do I need it?
I started looking at the tolerance on the models available. Since I’m trying to stay in the sub $100 price point, the digital levels have an accuracy of 0.1 deg. Placed at the center of my 9’ span that’s 0.094" at each corner (about a tenth of an inch). That’s perfect usage, so my actual result will be worse. But guess what? Bubble levels (called “spirit” or “vial” levels) actually have better accuracy then the fancy digital ones. And you can pick one up for $10-15. Per Johnson Level their accuracy is .029 deg. I put that into Sketchup and I find my perfect usage would result in .027” (about 3 hundreths of an inch).
Isn’t that interesting? Careful use of a bubble vial level will result in a more accurate result then the fancy new digital level! And they are way cheaper/you already have one. Look, I know there’s merit to the idea that the digital one is easier to use, and the bubble one leaves one with a bit of uncertainty as to if the bubble is perfectly centered. So the higher precision of the bubble coupled with the lower ease of use, vs the lower precision of the digital level paired with its friendlier use means that the net practical result might be the same. But the bubble level costs ⅓ to 1/10 of the digital.
Don’t throw away that bubble level. It’s more precise than you thought. You can get a fancy digital one if you like but it’s not more precise unless you pay for a high end model ($100+).
Few users have a legitimate need for the accuracy obtained by higher end levels, but in our tech age it's hard to resist the siren call of the digital level.