In article ,
Don Foreman wrote:

On Sun, 05 Apr 2009 13:42:14 -0400, 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.

Joe Gwinn

Have a look at
http://www.spectronsensors.com/datas...S-105-2704.pdf

Not bad. The resolution is up there, but the repeatability is 0.0008
degrees, or 2.9 arc seconds, versus 0.1 arc seconds. Better than 5 arc
seconds, but still. This is why people bother with pendulum tiltmeters.

Joe Gwinn