You should make it clear that the float has to extend above the highest level of the tank. If the float is completely submerged, it will not experience any more buoyancy as the water rises.

For that matter, you could have a "float" that actually sinks, hanging down into the water, most of the way to the bottom. The buoyancy change only depends on the volume, so the amount by which it gets lighter tells you how much is in the water.

You would probably aim for your float to weigh just a little more than its maximum buoyancy, so that your maximum load would be as small as possible, so easier to measure.

(Just for fun, how much lighter is a cubic foot of lead when it is submerged?)

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I would perfer a 3 digit LED 7-segment display to disaply as a percentage.
Its for my mum. A number is easier to interpret than counting the bars on a bar graph.

I actually have 4 tanks.

1 off the front of the house (3000L), 2 off the back of the house (4500L) and another off the back shed (2270L)

So I would like to design a system, x4 (mount to each tank), and i would like to link them and send the results to a reciever in the house that gives an average of the 4 tanks as a percentage.
If two are empty, and 2 are 50%, the display in the hosue will say, 'overall capacity is 25% (and on an LCD maybe, it can show the overall capcity in litres, and of each tank).

I have work to do haha

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ummmmmmmmm...
It isn't any lighter...
The actual masss of the object doesnt become lighter.

am i right?

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True, but how much less force does it take to support it?

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I'm not sure.
How do you work it out?

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buoyant force = volume of water displaced * density of water
= about 62.4 lbf/ft³

assuming the cubic foot of lead is completely submerged.

So weight of submerged cubic foot of lead = 707.9-62.24=645.66 lb force

Interestingly .... if that cubic foot quantity of lead were somehow shaped into a sphere of about 1.5 ft. radius, the volume would be roughly 12 ft³, and it would float.

 

Wow!!!

Now we can make boats out of metal!


Now you can make a 10 foot radius metal sphere, from 300 lbs of metal, filled with a vacuum... (i bet you know where this is going)

 

Hi try the following link for pic based water tank level meter
Regards

 

link

Silicon Chip Online - PIC-Based Water-Tank Level Meter; Pt.1

 

The point I was making was that the change in force, 62.4 lbf/ft³ is the same whatever you make your "float" out of. The reason not to use lead is that you have to support and weigh 700 lbs to get a change of 62 lbs. If you use a less dense material of the same volume, (and make it hollow), the change in force is just the same but you have to support and weigh less weight.

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The air pressure method works great, but I've always seen them with the water bubbling constantly. All that is required is a cheap aquarium air pump throttled with a needle valve. Just enough to keep a couple bubbles a second or so. Then use one of these ==> PTD25-10-0100WCH Products
teed off the line, 0-10V output. Simple and rugged.

We have used ultrasonics ==> Level Measurement: Continuous Ultrasonic Level Transmitters
for big (30' tall) fuel tanks. They work good but are $$$$

 

The more I think about it, the air would definitely have to bubble constantly for satisfactory operation.

 

One cubic foot of the liquid it is submerged-in lighter.

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Replace the pointer with a 100 ohm potentiometer on one of these installed at the top; or make your own for the proper lenght. Then connect a digital ohmeter and a four pole selector switch to read each tank.

Gas Caps

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Kinda what I was talking about, but you still need the air pump to get the water out of the line. But the electronics are nicely done.