Questions about reading capacitance

Hello, we have a water tank for the chickens and I'd like to add a gallons-left sender to our little nRFL2401 home network with the CapSense Library. I found some videos on making the capacitor sensor but need some clarification.

One guy's video showed two flat strips of aluminum each coated with epoxy then held apart a set distance with thin foam. The guy suggested that a tube and rod down the center would be better as the outer tube would provide some shielding properties.

I just happen to have a few feet of 3/4" aluminum tube and a length of 1/4" aluminum rod. I can 3D-print some spacers to center the rod in the tube every 12" but I am unclear on the use of the epoxy in the above design.

Can I leave the aluminum bare or will the water affect the capacitance by conducting between the tube and rod?

Thank you.

Water is conductive and also will rapidly corrode aluminum unless very pure. It cannot be allowed to contact any conductive part of the capacitor.

I’ve attached another design to consider, but a pressure gauge at the bottom of the tank is easier. Pressure is perfectly proportional to water depth.

NV27-Measuring Water Level.pdf (520 KB)

Water is conductive and also will rapidly corrode aluminum unless very pure. It cannot be allowed to contact any conductive part of the capacitor.

In addition, the conductivity of the water will "short" out your capacitor making it unusable. So even if you had Stainless Steel components it would not work. Hence the insulating coating.

I'm sure you will get tons of ideas for making your measurement.

The two I would consider 1st are:

1) Ultrasonic facing down from the top. Ebay is full of ultrasonic breakout boards.

2) Low lever pressure sensor on the bottom of the tank. Pressure would be proportional to water depth.

John

jremington: I've attached another design to consider

Thanks, I will read that tonight. I have actually ordered a 0-10psi pressure sensor but it means making another hole in the piping. I was thinking that using the hole already in the top of the tank for a capacitor tube would be one less potential leak.

JohnRob: 1) Ultrasonic facing down from the top. Ebay is full of ultrasonic breakout boards.

Thanks, I actually tried one of those and it worked fine until it got a lot of condensation on the sensor and froze overnight. Around midday and sunshine it would once again work quite well.

Then we had six days all below 32 and it never un-froze and still has not. :)

While it is rated as waterproof to -40C, they have not taken into consideration the condensation freezing within the sender/receiver horn. It needs some form of waterproof membrane across the front that will pass sound but not allow moisture in.

So, I guess it is the low-pressure at the bottom as those pipes are warmed when the temp drops below 40-F.

If the pressure sensor is located in an outlet pipe, be aware that at a given depth, flowing water exerts MUCH less pressure than still water, so the pressure/depth measurements can be way off. Look up Bernoulli effect for more info.

Make sure to ignore readings taken while water is flowing.

jremington: Look up Bernoulli effect for more info.

Thanks, but I am a retired Mechanical Engineer so pretty conversant with that stuff, but you would not have known that, so your cautionary note was a good idea.

I must however, contest a statement in your earlier post.

and also will rapidly corrode aluminum unless very pure

That is not quite so. Freshly bared Aluminum when exposed to air will very quickly create an Aluminum Oxide layer that is all but impervious to the elements. if it were not so, all the aluminum boats bobbing round the World would only bob for a few years.

The corrosion resistance is based around the pH of the water, but for all intents and purposes, it is safe to assume that the water generally encountered in our daily lives, has a pH well within the non-corrosive limits of Aluminum.

Add a tiny bit of hydroxide or other base to the water and observe how much more rapidly the aluminum corrodes.

Actually aluminum corrosion is caused by the various alloying materials. They create a voltage potential between aluminum molecules and the alloy molecule. I remember seeing a picture of a WWII airplane pulled from Lake Washington, near Seattle. It looked like a sieve. Caused by the copper in the aluminum alloy to make the airplane skin really hard.

Paul

I have actually ordered a 0-10psi pressure sensor

I don't know how high your tank is. A foot of water is slightly less that 1/2 PSI.

I also think Mr Bernoulli will be less that interested unless your chickens drink a LOT of water. Or perhaps this is not for drinking.

If you have a working ultrasound system, then why not use a heating element on/near the sensor. A 100ohm 1/4watt resistor between 5volt and ground could keep it warm enough to stop condensation. Leo..

Wawa: If you have a working ultrasound system, then why not use a heating element on/near the sensor.

A bit late getting back here, but I had tried that (15-Ohm x 10W) but at temps below 15-F there was not enough heat. I could insulate around the top but not the bottom Horn area and that's where the condensation is freezing inside the tank.

Anyway, the 0-10psi sensor has arrived and seems to work on the test pressure rig. As best I can measure it seems fairly linear from 0.5v at 0-psi up to 5.0v at 10-psi. But I am looking at 0 to 2.3-psi total spread and the granularity of the pressure sensor is pretty rugged.

I think I need to find a more sensitive sensor with finer granularity over a smaller spread of pressure.

I have not yet mounted this in the tank piping, so any suggestions for a not too expensive sensor would be greatly appreciated.

And, yes, it is for the chickens drinking water although poop in it, scratch food and dirt into it etc, so sterile water-handling is not a mandatory option. :)