http://www.ebay.com/itm/26111795164...l?_from=R40&_sacat=0&_nkw=261117951643&_rdc=1
This is an eBay item LOOK at it. look at this and the 10 second bubble radius. I do not see a means to adjust. If it does, then.....
Then look at the starrett. The Starrett:
http://www.starrett.com/metrology/m...urrentPage=1&displayMode=grid&sortBy=none/asc
Basically they used the SAME BUBBLE until they get to the "precision". All their levels on this page are adjustable. This is needed because at some time it will be dropped. I looked a each item on this page.
"
80-90 seconds graduations or .005" per foot (0.42mm per meter)" meaning that it is capable of resolving about 1/2 mm in 1 meter or 0.005" per foot.
As the price goes up the vial does not change BUT the GROUND lenght does. That is the big ticket. If the surface is long and flat, short is OK. If surface is rough, need long base.
That is UNTIL you get to the PRECISION grade. "
Ground and graduated main vial of 10-second accuracy; one division equals 1/2 thousandth (0.0005) of an inch per foot, or 0.04mm per meter " Big difference. 3
So, in conclusion, if you want a longer lenght, if will cost. The bubble is the same. MEANING the graduations subtend the same angle of arc.
(Now adjustment is easy. Let me know if you want to know how AND you do not need a perfectly level surface.)
One minute of arc is 1" in 300 feet (about). This is 60 seconds. (really 57") OK?
So assume you need (want, require) 0.010 max error 4 feet away. This is your decision.
1 inch/300 ft = 0.003
.010/4 = 0.0025 <= this is 0.025" per foot.
(0.0025/.003)X60 = 0.8333 X 60 = 50 seconds. SO each readable graduation on the bubble needs to be less than 50" of arc for you to acheive your goal.
for what it's worth and respectfully submitted,
Mike
The guys below have told you what they used. This is the precision of your level.
I paid >$150 for my Bridge City 15 yrs ago. A thin paper strip, 0.003, causes it to move about 1/2 grad. It is only 8" long, but sensitive bubble. I have adjusted mine numerous times over the years.