A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Noell Wilson
Date: 2020 Aug 8, 07:35 -0700
My first test with the Starrett and the Hall level were just to detect comparative movement. My three legged mirror platform has 1/4-20 adjusting screws and, even with a screwdriver, the smallest possible bit of screw rotation gave about 1 division mark on the vials.
I did the “feeler gauge” bit later and ended up going down a rabbit hole. I know that level accuracy is usually expressed in inches/foot (in the USA) so I leveled my three legged platform with the, no model number, Starrett. I started with a piece of paper (0.004”) under one end of the 8” level and the bubble jumped too much.
I then cut the cellophane window ( 0.001”) out of an envelope and the bubble moved about one division mark. That’s equivalent to 0.0015”/ft to move one division mark.
It’s hard to find the definition on a level but I find several references to the accuracy being based on the bubble moving one mark.
0.001”/8” is 0.0072 degrees or 0.432 minutes, or 23 seconds. Good enough. (Please don’t let me have messed that up!)
I’m still a bit in a rabbit hole about my unmarked Starrett level reading the equivalent of 0.0015”/ft. A new 98-6 Starrett claims 0.005”/ft. The next step up is a $1000 Starrett 199Z at 0.0005”/ft.
I’ll probably mount the Hall vial on three legs as suggested.