Capacitive water tank level determination

[twesthoff]

I would think just the center would be good enough, especially if you just want to know when it is almost full.  Maybe there is "center of the boat" where it might be more accurate than the center of the tank.  Also, averaging a number of readings may give you a better reading.  You could possibly put an simple "attitude" sensor (pendulum attached to a pot) to adjust the reading.

On 9/12/2011 12:17 PM, General wrote:

[GTBecker]

I'm still working on the tank measurement scheme. One tank is taped and ready, but the other - the oddly-shaped one in the bow - has proven to be difficult to gain access to the aft vertical surface where I intended to place aluminum tape. That got me started thinking about a dip tube, a dry thin-walled PVC pipe with sensor tapes on its inner wall, penetrating the tank from the top - and that lead to an interesting experiment.

I have long wanted to measure the speed and wakes of passing boats on our canal. I tried an acoustic ranger for the wave heights, which worked - for a month; the environment is harsh. A sealed capacitive dip tube should be ideal - except that anything submerged near the surface will normally be coated with marine growth in short order. A coating of algae and barnacles would surely interfere with the device.

I thought about boat bottom paint, which is commonly composed of a copper-based pigment that is deliberately conductive; that wouldn't allow capacitive sensing to work. I researched a few depth-sounder transducer spray paints, and found that several are composed of arsenics or cyanides that I'd prefer to avoid - but one is simply zinc. A coating of Pettit's Transducer Paint ($28 for 16 ounces!) is 93% zinc after curing.

That sounded familiar and, sure enough, a can of Rustoleum Cold Galvanizing Compound ($5 for 20 ounces!) that I had in the garage appears to be the same product after comparing the MSDS data. The resulting coating, despite being essentially pure metal, is not conductive. I sprayed half of a length of PVC pipe and hung it off the dock to see if the Rustoleum would prevent growth. So far (a week), it seems that it does; the white PVC is now green and the gray zinc is unchanged.

I tested to see if the zinc coating would interfere with capacitive sensing. I coated the interior of the first test bucket with several layers and let it dry. It is absolutely opaque, a thick coating, but it remains non-conductive and strangely, to my mind, has no effect on the capacitive function. The measurement scheme continues to work fine.

So, soon I expect to have another application for this method of water level determination - wave and wake measurement.

FYI.

[GTBecker]

Well, the zinc coating appears to work to prevent marine growth, and it is not conductive - but it doesn't shed water; it stays wet - surprisingly well, in fact. That means capacitive sensing will always see a wet surface, an apparently full tank or high wave.

Alas.

[dlh]

Zinc is used in medical ointments and wound dressings for its hydrophilic properties.

[DougWong]

You are on to a good sensor application here...
As mentioned previously, the capacitance is proportional to electrode surface area. If you want to maximize the capacitance, make the electrodes larger. The largest you can get is to cover the two opposite sides of the tank with semi-cylindrical electrodes. You should leave a half inch gap between electrodes in a strip up each side. The gap should be much larger than the thickness of the tank wall to minimize the empty tank capacitance.
To make a linear output voltage, look up capacitance-to-voltage circuits on the net.
To prevent the electrodes from being susceptible to external influences, look up driven shields. You would need to cover the sensing electrode with an insulation layer and then a driven shield layer - on the outside only, not the side facing the tank. This driven shield should also not cover the electrode connected to your oscillator.
If you want to minimize radiation, the whole tank and sensor assembly could be covered with insulation tape and then with grounded conductive tape.
Good Luck.

[GTBecker]

Active shielding might be necessary, you're right, to minimize the otherwise large stray capacitance that would result from simple shielding. But you say

[GTBecker]

I had another ill-conceived notion. I wonder what would result from wrapping the tank in a coil (or immersing an air coil in a dip tube or, perhaps, a ferrite rod with a coil at one end) in the liquid and using that as the variable resonant component? As with capacitance, if the effective inductance of the coil is changed by the liquid, level could be inferred from it.

Wrapping a coil around a plastic tank might well be difficult or impossible in the conventional sense, but slipping a turn or two of 40-conductor ribbon around it might well be feasible, with loop connections made at headers on a circuit board.

[DougWong]

If you are willing to stick something in the tank, you could simply wrap the entire tank with conductive tape connected to ground and connect your oscillator to a vertical stainless steel rod in the tank. The rod should be spaced off from the tank wall a little bit but there is no need to isolate it from the water, as long as the water can't contact your ground.
If the water is grounded, the electrode leads could be reversed, but generated EMI would be worse.
If the active element is driven from a low impedance source, it won't be affected much by stray influences, which are generally high impedance.
If the tank is metal, it could be grounded and the oscillating electrode could be isolated from the water. For the oscillating electrode, you could coat a rod with RTV or simply run an insulated wire to the bottom of the tank an back up so there is no possibility of the wire getting wet. To get higher capacitance use a longer wire - run it up and down a few times. I have tried this and it works well.
I don't think water affects inductive coupling dramatically, but it might be worth testing the coil idea.
I have tried the ribbon cable coil with staggered header concept, but only for a metal detector antenna, where it works fine.

[GTBecker]