Water level measurement.

capnruss

I have a boat with 2 fresh water tanks. I would like to measure the level of the tanks using a non invasive method such as capacitive sensors mounted on the outside of the tanks. Any one out there that has done this. The tanks are non metalic.

Chris Savage

I don't have a suggestion for a non-intrusive solution, however if you can't find something here's a product to consider. We have a few Liquid Level Sensors that won't affect the water.

https://www.parallax.com/catalog/sensors/pressureflexrpm

capnruss

Thanks but I can't get to the inside of the tank. Need some to attach to the outside of the tank.

Tracy Allen

Welcome to the forums. You might try two wide strips of adhesive-backed metal tape on the outside of the tank. The capacitor is formed primarily by the plastic of the tank walls, and the water acts primarily as an electrical conductor. Attach the strips into an oscillator circuit using a 555 timer, or, for a first cut, you could try RCtime with the Stamp. The Stamp would have to be really close to the strips. The whole environment would have to be well controlled to reduce outside influences. Strip orientation vertical for analog level, horizontal for a specific level.

W9GFO

Tracy Allen wrote: »

You might try two wide strips of adhesive-backed metal tape on the outside of the tank. The capacitor is formed primarily by the plastic of the tank walls, and the water acts primarily as an electrical conductor. Attach the strips into an oscillator circuit using a 555 timer, or, for a first cut, you could try RCtime with the Stamp. The Stamp would have to be really close to the strips.

This sounds like something that I could use on my solar boat to sense heel and pitch angles without having any external components. I'd like to know more about it. Can you point me to any examples?

Tracy Allen

Rich,

I'm sure this method has come up DIY before here and elsewhere, but I can't put my url on it right now. My own experience with this has been with stream level and tidal monitoring. That uses a metal tube with a teflon insulated wire running down the center. The capacitor is formed across the insulation of the wire. They work okay, but not great accuracy. As the wire becomes coated with crud it forms a spurious conductive path and makes it seem like the water is deeper than it actually is. (salt especially on a marine tide gage) It is good for maybe 1% or 2% of repeatability.

External components? You mean a sensor inside the hull only?

W9GFO

Tracy Allen wrote: »

You mean a sensor inside the hull only?

Yes, this is the part I don't quite understand. You said; "You might try two wide strips of adhesive-backed metal tape on the outside of the tank". Maybe that was a typo. If both strips are on the side opposite the liquid, how does it work?

Tracy Allen

The plastic of the tank (or hull of the boat?) acts as the dielectric of a capacitor. With two strips on the tank it amounts to two capacitors in series
metalStrip#1 ---- plasticInsulation ---- water ---- plasticInsulation ---- metalStrip#2
The electrical connection is made to strip#1 and strip#2. The water acts as an intermediate conductor, not as a dielectric.
It is a sliding plate in relation the the strips. The strips should probably be wide relative to the thickness of the hull or tank wall.

W9GFO

Do you mean two strips, say a quarter inch apart on the same side of the tank, or one strip on each opposite side of the tank - which would mean a very large separation. If the former then that is unexpected, a neat solution that I will have to try.

Tracy Allen

Yes, two strips side by side. It really does not matter where on the tank they are placed, just remember, the water acts as a conductor, not a dielectric, so the "thickness" or orientation of the water layer has nothing to do with it. I believe it is important to have the front-end electronics mounted directly on the strips and to transmit the data back to the processor and display. The tank or any large body of water acts as an antenna, so the electronics may have to deal with common mode effects. You don't want waving a hand near it to affect the reading.

I did find this commercial application...
https://garnetinstruments.com/rv-shop/

capnruss

Thanks for the ideas. As it turns out I have already started the 2 strip and 555 timer circuit. I have the astable 555 running at 100 khz with the tank empty. It is about 50 khz with the tank full. I have ordered an LM331 frequency to voltage chip and will let you all know the results of that. If anyone has experience with the LM331, pros or cons, problems, successes please let me know.

L_Gaminde

I would like to know what kind of resolution you get using the count command