spirit level bubbles - calibration marks/meanings?

[Duncan Di Saudelli]

Hello

Every spirit level I have owned (you know, the standard Stanley thigs a yard long, with a vial filled with green liqwuid) has got two marks at each end of the vial - two thin lines close together at one end, and the same at the other.

Clearly when the bubble is centred the level is flat, but what's the graduation of these two lines at eaither end? If the bubble touches the inner line and then moves to touch the outer line, is it standard to be able to say that the inner line is so many degrees away from horizontal, and the second line is thus so many extra degrees?

DDS

[Dave]

It's down to drainage angles for gutters and other things.

Dave

[John Whitworth]

Please expand - I'll be putting some guttering onto the summerhouse fairly soon, and this sounds like useful knowledge.

Cheers

JW

[Chris J Dixon]

I have always believed that the idea was that, when looking at the bubble, you place your eye so that the four lines appear to be symmetrical. This ensures that you are looking perpendicular to the phial centreline, and thus minimise parallax error.

Chris

[The Other Mike]

The angle to move from the centre to the end of a vial is dependent on the curvature of the vial, think of a vial as a segment chopped out of a large clear tubular ring.

The bubble in a small diameter ring say 1ft diameter will be much more sensitive and move more with a small angle change than a bubble in a large diameter ring of 10ft diameter

[dom]

Modern ones seems to be straight bi-conical tube, rather than a parallel-sided curved tube.

Mine will show a true level regardless of whether the level is sat on its narrow face or wide face - now and again quite a useful thing.

[Dave]

Will do when I get it confirmed tonight.

Dave

[pcb1962]

Are you sure you have that the right way around? For a given angular displacement the bubble in a large diameter tube will move further than the bubble in the small diameter tube, giving finer granularity when observing a small change. At the extreme, a straight tube would be most accurate as any deviation from level would send the bubble straight to the end of the vial.

[Harry Bloomfield]

The Other Mike brought next idea :

T'other way round I think 8-o

[Dave]

Phoned my b.i.l. up an hour ago, he has been in the building trade since he left school and has only ever seen levels with 2 lines and not 4, So my original answer could be wrong. I'll keep asking and if I find anything out I will post again.

Dave

[The Other Mike]

I knew what I meant, even though my fingers didn't :)

[John Whitworth]

OK - cheers Dave.

JW

[marcheritier4]

yes I think you are right.

[Clive Arthur]

Christ, that took as long as the Chilcot report!

Cheers

[newshound]

Not sure about the parallel with Chilcot but the description of vial geometry is completely wrong, (proper) vial interiors are barrel shaped. There's a fascinating article on the web somewhere about how you can make your own *very* sensitive vials by controlled lapping of the bore.

[Harry Bloomfield]

newshound was thinking very hard :

I agree with the barrel interior shape, but the net result is that the curvature of the 'up side' will still form a segment of a circle.

[Vir Campestris]

I just checked. Both my levels have curved tubes.

If you are about to say "Then they aren't proper" I refer you to the No True Scotsman fallacy.

Andy