In a message dated 2007/04/07, tchannel@.............. writes:

> Hi George,  This is interesting.  Where can I find articles to read more 
> about the basic principle. etc. 

Hi Ted,

       See http://ntweb.mcn.org/gbush/Tiltmeter/tiltmeter%20Main.html for 
several references.
       This tiltmeter has two Perspex cups 2 ft apart on a baseplate, 
connected by plastic tube and part filled with Mercury. Two flat plates are suspended 
just above the Hg surfaces to form capacitative level detectors. The cup 
system is sealed to contain the Hg vapour, which is very poisonous and attacks 
electronic components, Cu wire, pcbs etc like fun.
       The original SciAm circuit used valves, a crystal oscillator and 
resonant tuned detectors followed by diode rectifiers. The oscillating voltages 
were quite high. The Nuts and Volts version was an adaptation of this to 
semiconductors, with lower voltages. The new N&V board seemed to suffer from AC hum 
from the on board PSU. A major problem with the whole concept is that the 
foreward voltage of rectifier diodes drifts by about 2.5 mV / C deg and the resonant 
circuits also drift with temperarure. So, unless you are very determined, you 
may end up with an expensive differential thermometer sensitive to random air 
movements and to any stray capacitance going. The work/satisfaction ratio may 
be discouraging.
       A good way around this ''problem'' is to redesign the circuit using a 
capacitative detector working at high audio frequencies, not at RF and to 
provide a FET phase sensitive detector. The LTC1043 IC has all the switching 
components on it. You can use the on chip oscillator, but I found that an external 
Crystal oscillator gave significantly better results. I used a 2.4576 M Hz AT 
cut crystal in a 4060 oscillator divider circuit, giving a 9.6 kHz drive. 
       If you want to sense only earth tides and similar, severely damping 
the Hg flow will tend to reject any passing earthquakes, local delivery trucks, 
etc. Clamp a piece of cleaned iron wire centrally in the plastic tube 
containing the Hg, using an external clamp maybe 1" wide. Hg 'wets' the iron, but does 
not dissolve it, leaving two narrow triangular passages bounded by the 
plastic and the wire.

       Regards,

       Chris Chapman   
In a me=
ssage dated 2007/04/07, tchannel@.............. writes:



Hi George,  This is intere=
sting.  Where can I find articles to read more 

about the basic principle. etc. 




Hi Ted,



       See http://ntweb.mcn.org/gbush/Tiltmete=
r/tiltmeter%20Main.html for several references.

       This tiltmeter has two Perspex cups 2 f=
t apart on a baseplate, connected by plastic tube and part filled with Mercu=
ry. Two flat plates are suspended just above the Hg surfaces to form capacit=
ative level detectors. The cup system is sealed to contain the Hg vapour, wh=
ich is very poisonous and attacks electronic components, Cu wire, pcbs etc l=
ike fun.

       The original SciAm circuit used valves,=
 a crystal oscillator and resonant tuned detectors followed by diode rectifi=
ers. The oscillating voltages were quite high. The Nuts and Volts version wa=
s an adaptation of this to semiconductors, with lower voltages. The new N&am=
p;V board seemed to suffer from AC hum from the on board PSU. A major proble=
m with the whole concept is that the foreward voltage of rectifier diodes dr=
ifts by about 2.5 mV / C deg and the resonant circuits also drift with tempe=
rarure. So, unless you are very determined, you may end up with an expensive=
 differential thermometer sensitive to random air movements and to any stray=
 capacitance going. The work/satisfaction ratio may be discouraging.

       A good way around this ''problem'' is t=
o redesign the circuit using a capacitative detector working at high audio f=
requencies, not at RF and to provide a FET phase sensitive detector. The LTC=
1043 IC has all the switching components on it. You can use the on chip osci=
llator, but I found that an external Crystal oscillator gave significantly b=
etter results. I used a 2.4576 M Hz AT cut crystal in a 4060 oscillator divi=
der circuit, giving a 9.6 kHz drive. 

       If you want to sense only earth tides a=
nd similar, severely damping the Hg flow will tend to reject any passing ear=
thquakes, local delivery trucks, etc. Clamp a piece of cleaned iron wire cen=
trally in the plastic tube containing the Hg, using an external clamp maybe=20=
1" wide. Hg 'wets' the iron, but does not dissolve it, leaving two narrow tr=
iangular passages bounded by the plastic and the wire.



       Regards,



       Chris Chapman