PSN-L Email List Message

Subject: water seismometers
From: "Dave Nelson" davefnelson@.......
Date: Mon, 29 Aug 2005 09:17:13 -0700


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
 
 

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