OT: refilling a spirit level bubble.

[N_Cook]

This is on an Abney Level , a simple surveying instrument for taking
levels around a site or off a datum.
Over time the bubble enlarges and so less and less accurate sighting of
the centre of the bubble.
This one , the glass cylinder is cemented in a brass holding barrel. One
end , where molten glass is somehow sealed over the contents of the
cylinder, is accessible. Breaking away the cement at that end , probably
broke away the glass , or maybe had fractured with temp or vibration,
causing the large bubble to form.
Anyway easy to refill vertically using a hypodermic syringe. But how to
seal up, as still recessed inside the brass cylinder. I'm thinking of a
drop of nail varnish over the full to the brim spirit. Then a few more
layers of nail varnish to build up a bit.
I'm hoping there will be enough "dissolved" air to settle out and form a
bubble.
Anyone done this or similar, or any hints?

[pf...@aol.com]

http://www.antiquesurveying.com/precision_ground_glass_spirit_le.htm

Given the need for absolute accuracy - otherwise the instrument is entirely useless - you may want to replace it entirely. Then there is re-installation and calibrating. I 'came up' on on optical transits... fun.

Peter Wieck
Melrose Park, PA

[N_Cook]

Calibration is the easy bit, a 5km professional laser-levelled pair of
fixed points locally about 150m apart.
A few layers of nail varnish worked , for today, but I somehow doubt
itwill be long-term reliable. Surveyed to a benchmark 100m away, to
obtain the height, wrt Ordnance Survey datum. GPS , even dGPS is a waste
of space in comparison to optical or laser levelling to good old benchmarks.

[Tim R]

On Thursday, October 26, 2017 at 2:15:07 PM UTC-4, N_Cook wrote:
> Calibration is the easy bit, a 5km professional laser-levelled pair of
> fixed points locally about 150m apart.
> A few layers of nail varnish worked , for today, but I somehow doubt
> itwill be long-term reliable. Surveyed to a benchmark 100m away, to
> obtain the height, wrt Ordnance Survey datum. GPS , even dGPS is a waste
> of space in comparison to optical or laser levelling to good old benchmarks.

Unless you have a warm air layer in between refracting your line of sight.

[Rheilly Phoull]

I reckon a blob of silicon would complete the seal.

[N_Cook]

I might try that next time. The resultant bubble was a bit larger than
I'd have liked, so next time leave to settle for a few hours, before
topping up with spirit and then sealing over.
I should say I've no access to a pro laser level and sensing staff but
when there were anti-flood measures put in place locally, I got them to
place a nail in the flood wall at a specific height relative to a
distant specific level.
Otherwise presuambly a matter of going to the coast , Abney placed near
a calm sea edge and sighting the horizon.

[bruce2...@gmail.com]

Working near a very, very hot room and opening the doors to the cold, cold outside can cause that visual effect.

[Tim R]

There is a rather famous case where tunnels were supposed to meet from each side of a mountain, but didn't. Analysis showed they'd relied on a laser level, and it had been bent by a warm air layer in the path.

[Ivan Vegvary]

Any surveyor can calibrate quickly with no instrumentation. Set level tripod almost exactly between two objects approximately 200ft apart. Make a mark on each object (rotate 180deg back and forth). Said marks will be exactly at the same elevation. Move level (4-5ft) close to one mark, sight it, rotate to second mark (200ft away) and hope to read the same relationship. Adjust bubble accordingly, and repeat entire procedure.

[bruce2...@gmail.com]

I think that imaging using electromagnetic radiation or waves has come quite a ways, too.