I have been reading the various comments on transducers for water based =
seismometers.

 I initially planned to use a capacitive sensor taking advantage of the =
high dielectric constant of water. I am very familiar with precision =
capacitive bridge circuits such as those used in the MKS capacitance =
manometer and other high sensitivity pressure sensors. In fact when I =
started this investigation I had tried the MKS manometer as the sensor =
to sense the motion of the water .=20

 The problem is that these sensors must work in a gas not a liquid. I =
have also built capacitive sensors for use in water. The water must be =
very pure or the conductivity becomes a contaminating factor in the =
bridge causing a phase shift .The water/oil interface seems like a good =
configuration but it has problems if you want to have some portability. =
You have to fill the thing where you want to use it. The  coaxial =
configuration I currently have in my sensor was originally intended to =
be capacitive but the conductivity of even distilled water was an issue =
particularly if you add something to keep the water from "sticking " to =
the electrode. The use of detergents to eliminate some of the surface =
tension effects also increased the water conductivity. I finally said if =
you can't beat it -- exploit it so, I made based my system on variable =
contact area in a conductive fluid in a vertical coaxial configuration. =
The water is mixed with dishwasher sheeting agent like made by "Cascade" =
in a 100 to 1 ratio. This give the right conductivity and make the =
meniscus ride up and down on the brass pipe  electrode surfaces without =
sticking . The meniscus stays at a fixed contact angle . I have also =
used much lower concentrations of various detergents and very small =
amount of salt to add the ions needed for conductivity. I built a test =
cell to evaluate different mixtures.                              =20

The transducer is of course the core of the instrument -- I have tried a =
lot of things and do not claim to have found the optimum  but it does =
work pretty well and is easy to deal with. I can pick the whole =
instrument up ,not worry about mixing different fluids move it without =
any concern about handling it an any special way. It make some time to =
restabilize but in a day or so it will be back to normal operation. =
Bubbles are a big problem on initial fill and following rough handling =
but they go away .=20

I think there is room for a lot of work in this area ,so keep the  =
discussion going.

Regarding the LTC 1043  -- I buy them on-line from Linear in lots of 10 =
for about $3.50 each.  No minimum order. I have had no problem getting =
them. The can be used in many different configurations including some =
very good capacitive bridge circuits. It is truly a universal building =
block for instrumentation systems. I first discovered it in a =
capacitance pressure sensor made by Modus.

Regarding pressure sensors and  water based seismometers . I tried lots =
of configurations with some limited success but the usual problem was =
noise from atmospheric pressure changes . Even if the system is =
completely closed the tubing or pipe will be compressed by the pressure =
change. when you are working with pressure changes of 10e-6 torr in a =
1000 torr atmosphere the noise can be overwhelming no matter how you try =
it isolate it. I gave it up after trying many different configurations. =
For those interested in infrasound some of the sensors I investigated =
would be ideal.

Dave Nelson








I have been reading the various =
comments on=20
transducers for water based seismometers.
 
 I initially planned to use a =
capacitive=20
sensor taking advantage of the high dielectric constant of water. I am =
very=20
familiar with precision capacitive bridge circuits such as those used in =
the MKS=20
capacitance manometer and other high sensitivity pressure sensors. In =
fact when=20
I started this investigation I had tried the MKS manometer as the sensor =
to=20
sense the motion of the water . 
 
 The problem is that these sensors =
must work=20
in a gas not a liquid. I have also built capacitive sensors for use in =
water.=20
The water must be very pure or the conductivity becomes a contaminating =
factor=20
in the bridge causing a phase shift .The water/oil interface seems like =
a good=20
configuration but it has problems if you want to have some portability. =
You have=20
to fill the thing where you want to use it. The  coaxial =
configuration I=20
currently have in my sensor was originally intended to be capacitive but =
the=20
conductivity of even distilled water was an issue particularly if you =
add=20
something to keep the water from "sticking " to the electrode. The use =
of=20
detergents to eliminate some of the surface tension effects also =
increased the=20
water conductivity. I finally said if you can't beat it -- exploit it =
so, I made=20
based my system on variable contact area in a conductive fluid in a=20
vertical coaxial configuration. The water is mixed with dishwasher =
sheeting=20
agent like made by "Cascade" in a 100 to 1 ratio. This give the right=20
conductivity and make the meniscus ride up and down on the brass pipe=20
 electrode surfaces without sticking . The meniscus stays at a =
fixed=20
contact angle . I have also used much lower concentrations of various =
detergents=20
and very small amount of salt to add the ions needed for =
conductivity.=20
I built a test cell to evaluate different mixtures.=20
            &=
nbsp;           &n=
bsp;    =20

 
The transducer is of course the core of =
the=20
instrument -- I have tried a lot of things and do not claim to have =
found the=20
optimum  but it does work pretty well and is easy to deal with. I =
can pick=20
the whole instrument up ,not worry about mixing different fluids move it =
without=20
any concern about handling it an any special way. It make some time to=20
restabilize but in a day or so it will be back to normal operation. =
Bubbles are=20
a big problem on initial fill and following rough handling but they go =
away .=20

 
I think there is room for a lot of work =
in this=20
area ,so keep the  discussion going.
 
Regarding the LTC 1043  -- I buy =
them on-line=20
from Linear in lots of 10 for about $3.50 each.  No minimum =
order. I=20
have had no problem getting them. The can be used in many different=20
configurations including some very good capacitive bridge circuits. It =
is truly=20
a universal building block for instrumentation systems. I first =
discovered it in=20
a capacitance pressure sensor made by Modus.
 
Regarding pressure sensors and  =
water based=20
seismometers . I tried lots of configurations with some limited success =
but the=20
usual problem was noise from atmospheric pressure changes . Even if the =
system=20
is completely closed the tubing or pipe will be compressed by the =
pressure=20
change. when you are working with pressure changes of 10e-6 torr in a =
1000 torr=20
atmosphere the noise can be overwhelming no matter how you try =
it=20
isolate it. I gave it up after trying many different configurations. For =
those=20
interested in infrasound some of the sensors I investigated would be=20
ideal.
 
Dave Nelson