I believe that a short time ago there was a discussion about this. but depending on the head of the tank one could use an MPX5000 series pressure sensor with a dip tube or am I too late?
I have a number of 55 gal drums of water that I would like to know the depth of in my rain water gathering system so that I don't pump it dry and ruin a sprinkler pump. I am going to use one of them per drum to monitor the levels. I believe that the pressure of one foot of head is .43 PSI and that a drum is about 3 feet deep, giving a pressure of 1.29 PSI. Does this sound correct?
This technique of measuring tank depth (quantity) may not work well over a long period of time. In a washing machine, for example, the dip tube only needs to maintain a reading for the duration of the fill cycle. I suspect that slow leakage or air in the dip tube dissolving into the water over longer periods (days, weeks) may reduce its accuracy.
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Paul Hovnanian mailto:Paul@Hovnanian.com
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It may be much easier to simply detect the tank going empty. Maybe a float switch near the bottom or a short piece of pvc pipe on the output to the pump with a pair of electical contacts.
On ships I've worked on, they used a "bubbler". That's a tube that enters a tank at the top and ends on the bottom. A needle valve admits compressed air to the top, and it slowly bubbles out of the bottom, and the back pressure indicates depth. It's especially used for petroleum products and other explosive liquids.
Then I'd have to re-prime the pump. I would like to also be able to determine the amount remaining in the system if it isn't all pumped in the time allotted.
I wouldn't depend on a pressure sensor to be that accurate at the kinds of pressures we're talking about here. What's wrong with, say, a float switch, or maybe something like a sight glass with an optical coupler...
Now that's more luxury than you mentioned at first. Some ideas that might or might not work in your case:
a. A gas gauge mechanism from a car. Might corrode out over time.
b. Sound transducers (old piezos, whatever) fastened to the side of the barrel at equal distances. You could use a uC such as the MSP430, a version with enough ADC muxes to excite and then scan them sequentially. This could also calculate the approximate contents and report in gallons, liters, jars, buckets, number of remaining toilet flushes or whatever is preferred.
c. PVC pipe with end cap, must be absolutely water tight. Copper pipe in the middle that has an OD almost as large as the ID of the PVC pipe. Connect oscillator circuit to it. This forms a capacitor where the capacitance changes with the water level. This in turn changes the frequency but it won't be linear. Follow with a little F/V detection and you'd have a nice analog readout. Add a comparator that shuts off the pump below a chosen threshold.
d. Stainless contacts inside, equidistant, with the lowest being the level where you want to pump to cut out. Connect to a stack of comparators a la LM339 and then to a row of LED. The lowest one deactivates the pump when the contact goes open.
Of course, some of this depends on your water and other things.
Do you mean, the copper middle electrode is completely insulated from the water? If that's all you need, you could paint it. How about a stainless steel rod inside a stainless steel tube, where the water is the dielectric between these two - i.e., the level on the inside is the same as the level on the outside.
That is, I'm thinking that the water goes up inside the capacitor tube, but your description of an "absolutely water tight" construction makes me wonder what you have in mind. ?:-/
Even rain water can be aggressive. When I lived in Europe there were times when I got rain in the eyes and they started buring. Same back east. Has gotten better though.
Paint, exposed metal, all that will rot over time. I have even seen steel corrode although it was sold as stainless. In an application like this ideally you need something that you can build and forget for a few years.
Not up there near my alma mater. We were surrounded by power plants and other plants that burned surface-mined coal. High in sulfur content and all that. Depending on where you parked it could etch the paint on a car.
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