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Is it possible to measure the amount of water in a rubber water tank by using sound waves?

I want to make a non intrusive measuring device that attaches to the outside of a tank, and is able to approximate the amount of water in the tank however I don't know how to calculate what sound of a specific frequency will "sound" like given different mediums (e.g. sound travels faster in water than air).

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How about a force sensor to measure the weight of the tank? – W5VO Apr 8 '11 at 15:51
That's not a bad idea. But the tank weighs 100's of pounds. – kurtnelle Apr 8 '11 at 19:24
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And they make load cells capable of weighing 80,000 pound trucks. – W5VO Apr 9 '11 at 1:40
I'm guessing the problem is placing the cells under the tank as much as sourcing them. But still, it's a proven and dependable solution if you can swing it. – drxzcl Apr 9 '11 at 9:12
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3 Answers

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Also consider using a capacitive sensor.

If you glue two plates side-by-side on the outside of the tank, they will act as a capacitor with the water (or air) in the tank as the dielectric. Because water has a dielectric constant that is 100x that of air, you can easily and accurately relate the capacity of the plates to the water level in the tank.

See Measuring Water Level Without Getting Wet for more information.

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I think I'll accept this method. – kurtnelle Apr 8 '11 at 19:25
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up vote 8 down vote

You could place an ultrasonic transducer (transmitter-receiver pair) in the top of the tank, sending its signal down to the water, and measure the time between sending a pulse and receiving its reflection.

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I've done this for a fuel oil tank, and it worked well. – blalor Apr 8 '11 at 14:08
That's awesome; hadn't thought of that. The surface of the water will reflect the wave. – kurtnelle Apr 8 '11 at 19:25
This is probably the easiest method that requires the least calibration and minimal installation. – freespace Apr 10 '11 at 4:56
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up vote 0 down vote

This is not a method I've tested myself, I am just offering a potential solution.

You can place a speaker on top of the tank that outputs at a tone at a certain frequency, and place a waterproof mic on the bottom of the tank.

If there is more water in the tank, the mic will pick up less volume from the speaker. To determine the amount of water, you just need to calibrate the device by recording the mic level at each water level.

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