Variable Capacitance Testing

As per Rylan request, I have started this new thread to show my progress in testing Variable Capacitance as a viable means of real time Z axis feed back.

The variable Capacitor I used was made out of a length of 1/2" PVC pipe with a strip of Aluminium tape on the outside then wrapped with electrical tape to prevent shorting, the tape was then sealed with several layers of spray paint, the cathode is made up of 1/4" steel rod that has been covered with heat shrink tubing again to prvent shorting. the whole thing is held together by two custom made end caps made out of Poly-Styrene.

the experiment shown in the video consists of:
-6 Dixie cups with 50 ml of water
-3 of the cups had water that was left out for one and a half weeks to evaporate any Chlorine and to allow any disolved solids to settle
-the other 3 cups had water that was drawn straight from the tap just prior to the experiment only allowing enough time for it to reach room temperature
-each set of 3 cups had one cup refrigerated for an hour and a half, one cup that was microwaved for 40 seconds, and one that was kept at room temperature
-Each sample was tested for two minutes each (video has been sped up 2x to decrease video run time)

the purpose of this experiment was to show that simply making a variable capacitor will not make a reliable Z axis feedback system, however over the course of time I plan to build a sensor array that will allow the system to compensate for all the variables, making it 100% reliable

http://youtu.be/xpPafpqoWJI

Why use capacitance when resistance will probably give you better, more repeatable and stable results?

If you use resistance and a Wheatstone bridge with one fully submerged for a reference and one that gets progressively covered as the depth increases then you have it sorted and stable. Capacitance will vary with location and even your proximity but resistance will not.

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Originally Posted by Mjolinor

Why use capacitance when resistance will probably give you better, more repeatable and stable results?

If you use resistance and a Wheatstone bridge with one fully submerged for a reference and one that gets progressively covered as the depth increases then you have it sorted and stable. Capacitance will vary with location and even your proximity but resistance will not.

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I had originally suggested a variable resistor, however much of the community were more in favor of a variable capacitor

I have no idea why you are wanting to do this as this is the first thing I have read but if I wanted to measure the level of liquid in a vessel one of the easier ways is to use an ultrasonic TX/RX above, pointing down.

Aztecphoenix Thanks for taking the time to document this so well!!
Question, dose your cathode make electrical connection with the water ? or is it well insulated from the water via the heat shrink tubing?

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Originally Posted by rylangrayston

Aztecphoenix Thanks for taking the time to document this so well!!
Question, dose your cathode make electrical connection with the water ? or is it well insulated from the water via the heat shrink tubing?

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There is no electrical contact with the water at any point, even the base ot the cathode is embedded in the bottom cap.

I think you may be going about it the wrong way. The water itself should act as one electrode. I think a fairly usable capacitor could be made using a strip of copper foil, as one capacitor plate, along the inside wall of your build container, insulated from the water, perhaps with some kapton tape, or laquer, making sure to get a good seal around it. For the other "plate", place an electrode that won't corrode, close alongside it in direct electrical contact to the water all the way to the bottom. The water would be salinated, which of course it needs to be for the peachy anyway.

I don't beleive the salt level will matter much as you'd be relying on the capacitance, which is based on the thickness, and dielectric constant of what you use to insulate the strip. It should be unaffected by temperature, or resistance of the water. You could simply use this as the capacitor in a 555 timer based oscillator (astable multivibrator) if you keep it in the audio range you can read it on the mic input of a sound card. Sound cards are very good with frequency accuracy.

I could whip something up. I've been thinking about it a while, I've just been lazy and unmotivated :-P

With regards to having the saltwater act as an electrode, we already have at least one other metal object in the tank(build mesh), and a metal object in the dripper assembly - if we keep putting dissimilar metals in the saltwater, we will end up with galvanic corrosion occurring.

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Originally Posted by jsondag

I think you may be going about it the wrong way. The water itself should act as one electrode. I think a fairly usable capacitor could be made using a strip of copper foil, as one capacitor plate, along the inside wall of your build container, insulated from the water, perhaps with some kapton tape, or laquer, making sure to get a good seal around it. For the other "plate", place an electrode that won't corrode, close alongside it in direct electrical contact to the water all the way to the bottom. The water would be salinated, which of course it needs to be for the peachy anyway.

I don't beleive the salt level will matter much as you'd be relying on the capacitance, which is based on the thickness, and dielectric constant of what you use to insulate the strip. It should be unaffected by temperature, or resistance of the water. You could simply use this as the capacitor in a 555 timer based oscillator (astable multivibrator) if you keep it in the audio range you can read it on the mic input of a sound card. Sound cards are very good with frequency accuracy.

I could whip something up. I've been thinking about it a while, I've just been lazy and unmotivated :-P

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the problem with what you're suggesting is the chance of electrolysis, in other words, the disolved metals electroplating themselves onto the contacts obscuring the readings, this is why both the anode and cathode are isolated from the water

Other that the possibility of it becoming a galvanic cell if you use dissimilar metals both contacting the water, which would be minimal, and could be eliminated by using similar metals between the build mesh, and the contact, you won't have any electrolysis if only one contact is in the water. There won't be any actual current flow, just an electrostatic charge building up in the insulator between the water and the strip. It's a not uncommon type of homemade capacitor, and it's capacitance varies with the water level.

hmm- i wonder how this project gets away without liquid temperature sensors

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Originally Posted by 3dspider

hmm- i wonder how this project gets away without liquid temperature sensors

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most likely because it isn't using capacitance as it's measurement system

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Originally Posted by Aztecphoenix

most likely because it isn't using capacitance as it's measurement system

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It is, actually. the creator says as much in the comments page... anyway, it doesn't appear to be patented, so might be worth picking one up to see how it's gotten around the temperature problem (my bet is it has something to do with the dry+wet calibration it uses.

Did I miss some posts. When did Peachy basic start using capacitance????? Bewildered of Tamworth.

Mike, the Peachy Basic doesn't. It uses the drip sensor. All the sample prints so far are using the drip sensor, and the Peachy is planned to ship with the drip sensor. What this thread is about is experimental stuff that might eventually become some kind of add-on for the Peachy.

As far as I know Aztecphoenix isn't a Peachy betatester, just someone helpful doing experiments for the advancement of science.

Feign, thanks for the 'heads up'. I should log on here more often !!!!!!!