In article ,
Jon Elson wrote:

Joseph Gwinn wrote:
In article ,
Spehro Pefhany wrote:

On Sat, 04 Apr 2009 19:50:29 -0400, the renowned Joseph Gwinn
wrote:


As for manufacture by a HSM, either approach is practical, but the Wyler
approach is far simpler mechanically. In either case, the electronics
part is dead simple by present-day standards, but was a big deal back in
the 1960s and 1970s.


Joe Gwinn
Have you considered the liquid type? They claim 5 arc second
repeatability.

Yes, I did consider electrolytric types (a bubble level vial with
electronic readout). The pendulum units are a factor of ten better in
resolution and repeatability, 0.1 or 0.2 arc seconds versus 5 arc
seconds. Long term drift is also an issue. It sure would be easier if
the electrolytic sensors were better than they are.

Believe me, 0.1 arc seconds is AWFULLY sensitive.
I can walk from one end of my Sheldon lathe to the
other and tilt it a full arc second by deflecting
the concrete floor.

I like it! I bet you can detect the house swaying in the wind as well.


So, unless you are aligning turbo-alternator sets
in nuclear power plants, you have no need for such
sensitivity.

Probably the turboalternator twists the thicker floor of the power
station just as much as your lathe does to your basement floor.

One wants instruments to be at least ten times more accurate than what
is being measured, so the instruments don't limit the measurement.


Possibly because my Talyvel is old
and hasn't been to the factory in 20+ years, it
also drifts. If I am going to be doing anything
precise, I turn it on and let it "warm up" for
5-10 minutes while I am setting up. Then it is
pretty stable. It may still drift a full arc
second or so per hour, may be temperature changes
as I handle it a lot when checking surfaces.

No, the drift is built in, a matter of temperature coefficients and
residual deviations from symmetry. Aging has little to do with it. I'm
glad to know the rough magnitude.

While handling, temperature gradients may be the issue, as gradients
cause transient asymmetry. A wooden handle may be in order to cut down
on handling-induced thermal drifts. Or at least cover the existing
handle with the cloth tape used on bicycle handles.

One thing the fused-quartz Wyler unit is used for is long term
monitoring of such things as bridges, where the temperature drift of the
Taylvel would preclude this use.


If my only reason were to measure tilt, I would just monitor evilbay for
a Taylvel. I may yet do this. (I saw three Taylvel electronics boxes
yesterday, but "as is" and no sensor.)

But this is a Project, and if I am going to go to that much effort, the
pendulum units are far more interesting, and accurate.

Joe Gwinn