Hi Chuck
I've designed tiltmeters for many years for use in the
oil industry. My designs typically can resolve 5
nanometers of tilt when they are deep in the earth
away from surface noise. You need to be down about 40
feet (or on the surface way out in the middle of
nowhere) before you can see earth tides. The two most
prominent electrolytic sensor manufacturers are
Fredericks and Sensitron. I prefer the Sensitron units
myself. They have three platinum electrodes in a
liquid filled tube with an ever so slight curvature to
it. The liquid is a   secret formula that is a solvent
with a small amount of organic salt dissolved in it to
make it conductive. They add another ingredient to
stabilize the liquid over time.
It would be hard to make one yourself. See if one of
the manufacturers will give you a sample.

The sensor bridge must be driven with an AC signal at
something like 300 to 700 Hz. If you used DC on the
bridge, one electrode would plate over to the other
and be no good in short order.
Sense the bridge with an instrumentation amplifier
with a gain of 10 or so. Then AC couple the output to
eliminate DC drift. Run into a switchable gain stage
of 10, 100, 1000. Then into a gain of 4 low pass
filter with a 3dB cutoff at 30 seconds (NOT 30Hz).
From there level shift with a good voltage reference
to go to your A/D.

You'll need a solid base and very fine pitch screws
(or better yet, micrometers) to level the mechanism.
Start out with gain =10 and level, switch to gain= 100
and level again. If you have a very stable base, you
can try a gain=1000 and level again.

This isn't hard to duplicate but will take some time.
I can give you tips on amplifier selection but,
unfortunately, I can't give you a schematic because it
is proprietary to my company.

I'm in San Jose, CA. I'd be glad to have you visit my
lab if you are ever in the area.

Feel free to ask more questions.
Pete Rowe
--- Chuck Burch  wrote:

> 
> I am drafting plans to build a three electrode
> spirit level style tiltmeter to use as a 
> horizontal seismometer.  Any hints on construction? 
> What liquid is best?  Are 
> surface tension and/or viscosity critical?  
> 
> I'm planning on using a Wheatstone bridge detector
> circuit.  Can this be improved?
> 
> Thanks for your comments.
> 
> Chuck I. Burch
> 
>
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