PSN-L Email List Message

Subject: Re: water seismometers
From: Gordon Couger gcouger@..........
Date: Wed, 31 Aug 2005 03:36:04 -0500

Hi Chris,

Two coats of Epoxy swimming pool paint will absorb about 2% 
moisture and works very well with capacitive sensor. I built a 
soil moisture senor that way that the condenser was part of the 
oscillator that determined the frequency. There are better ways.


ChrisAtUpw@....... wrote:
> In a message dated 29/08/05, davefnelson@....... writes:
>> I initially planned to use a capacitative sensor taking advantage of 
>> the high dielectric constant of water. I am very familiar with 
>> precision capacitative bridge circuits such as those used in the MKS 
>> capacitance manometer and other high sensitivity pressure sensors.  
>> The problem is that these sensors must work in a gas not a liquid. I 
>> have also built capacitative 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.
>       Can you operate an MKS sensor in oil? It would have a different 
> sensitivity.
>    You need to boil distilled water to get the air out and then cool it 
> in a sealed container. If you leave pure water open to air, it absorbs 
> CO2 (as well as O2 and N2), which gives you very dilute carbonic acid. 
> You fill the tiltmeter / seismometer etc and then either cover the ends 
> with oil or seal in a Nitrogen atmosphere.
> 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. 
>       You can get vacuum valves which fit water pipe sizes. These have a 
> thick rubber diaphragm compression closure and O ring clamp fittings for 
> the pipes. If you fit one in the centre of the apparatus, you can close 
> off the middle and prevent flow to either end for handling.
> The coaxial configuration I currently have in my sensor was originally 
> intended to be
>> capacitative 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 gives the right 
>> conductivity and makes 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. 
>       Another capacitance method is to coat the metal surfaces with 
> marine varnish to prevent conduction. It can be dusted with Aluminium or 
> Titanium Oxide while wet.  Alcohol is quite effective at reducing 
> surface tension.
>       In the traditional tiltmeter applications you have a larger 
> diameter pot at either end of the horizontal tube, which increases the 
> period, a horizontal disk electrode just below the water surface and a 
> horizontal disk electrode either in free gas or submerged in oil just 
> above the water surface. You may use 10 to 50 pF total. Due to the high 
> dielectric constant of water, this arrangement is not too sensitive to 
> changes in conduction. If you use oil with a dielectric constant of 2, 
> you can allow a greater physical separation of the water surface from 
> the bottom of the top electrode for a comparable capacity.
>       Have a look at 
>       Another method is to use the capacitance as part of a high 
> stability RC oscillator and measure the change in frequency at both 
> ends. This can give a linear position readout. Some frequency meters 
> have a RS232 digital output which can be hooked up to your computer.
>       I would advise against adding salt or any other halide to water to 
> increase the conductivity. They are much too corrosive. Have you tried 
> bicarbonate of soda?
>> 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. 
>       You can buy a water jet pump to provide a near vacuum for the 
> initial fill. Even pumping out by connecting the suction end of a small 
> paint spray compressor will considerably reduce the volume of any 
> trapped air.
>> 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.
>       I am puzzled as to why you are seeing ANY signals due to 
> atmospheric pressure / noise changes.
>       The more usual arrangement is to provide a central inverted U tube 
> half filled with oil and a /differential pressure sensor/ at the top. 
> The oil floats on the water and the sensor is only in contact with oil. 
> If you have sealed end pots on the ends of the two arms but connect them 
> together with a plastic tube, while the gas pressure may change with 
> time and temperature, it is the same for both arms and the differential 
> sensor will not see it. It is an advantage to use silicone fluid, since 
> it is inert. You can use 'transformer' oil. It comes in several 
> viscosities. Lubricating oil may have a lot of quite nasty additives and 
> is best avoided. You can use a large piezo sounder disk as the sensor if 
> you are only interested in periods up to about 5 to 10 seconds. These 
> also make quite good infrasound sensors, but they need a very stable 
> temperature environment.  
>       Another type of diaphragm sensor may be made by sticking discs of 
> thin Permalloy or similar onto the centre of a circular stretched mylar 
> membrane, with two coils in ferrite transformer cups on either side. The 
> coils are connected up as a LR relaxation oscillator and you detect the 
> Mark/Space variation as the discs move toward or away from either coil. 
> These are used to measure very small air pressure changes as a glider 
> gains or looses height.
>       Regards,
>       Chris Chapman


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