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Biogas slurry often contains sand and grit and organic matter that eventually sediments. Clearing thetank from sedimentation is expensive (plant downtime, additional equipment) and dangerous. So it would be interesting to know how much sediment there is.

Typical tank heights are around 8m, so I think opening a hatch in the roof and poking with a long stick is not an option. What are other ways to do this?

Liquid level gauging is done with a pressure sensor mounted in the lower part of the wall, or a radar sensor on the roof.

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Buy a depth finder!

As @SF pointed out, you can use an ultrasonic device to measure the depth of the water. These devices work by sending out an acoustic impulse and recording the return signals. The acoustic impulse reflects off of any density boundaries in the medium. By measuring the time for the signal to come back the distance from the transducer can be inferred by knowing the acoustic speed in the medium. If the sediment is very dense then you will only get a signal from the sediment boundary, but if the sediment is not so dense (like muck on the bottom of a lake) then you will get a signal from both the sediment boundary and the bottom of the tank.

You might think that this sounds like an expensive device to design and build, but they are mass produced for boaters. Almost every lake-going boat I've ever been on has one. They range in price from \$50 to \$350 on Amazon depending on features and accuracy.

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You can put a rod with a blades on the bottom rotate it along the bottom. when the resistance is high then the sediment has reached above the blades and it is time to clean the tank.

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  • $\begingroup$ would work, but would essentially introduce a small longshaft mixer into the tank. sounds expensive. $\endgroup$ – mart Feb 2 '15 at 11:21
  • $\begingroup$ @mart a stainless bar and a flat bar welded to the end and a crank to the other? $\endgroup$ – ratchet freak Feb 2 '15 at 12:19
  • $\begingroup$ ... plus mountings, gas tight conduit. but you're right, it need't e more complicated. $\endgroup$ – mart Feb 2 '15 at 12:20
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If you want this automated, probably Rachet's solution is better; for a more low-tech approach for periodic manual testing, a wide, flat weight on a rope will work; lower it below water surface, let it rest on top of the sediment; the moment the rope goes loose, you've touched the sediment surface.

For a more hi-tech solution, an ultrasound distance sensor will be helpful, submerged under water surface.

If you're not interested in measuring the level so much as determining whether the sediment has crossed a certain critical level, a valve installed in the wall at given height will allow you to pick a small sample of whatever is at the valve level, helping you determine if it's covered by the sediment yet.

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It sounds like the problematic solids level is/can be known ahead of time.

Assuming this is the case, you could install a vibratory apparatus at the bottom of the tank, and some sort of acoustic sensor at the level of interest (probably several of them positioned around the tank wall would work best). Since tranverse/secondary waves don't travel through liquid, you will know the sensor is covered by solids if it detects these kinds of waves.

CHALLENGES:

The sensor would have to be sufficiently separated or damped from the tank itself so it does not detect waves propagating through the tank.

The sensors may not work well when the sedimentation layer is very very loose (as it probably would be near the very top); the vibrating might help with this by "compacting" the sediment, however.

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For a low tech and somewhat primitive method, given a metal tank with access to the bottom you could simply tap on the outside of the tank with a metal or other hard object (not hard enough to dent or damage it, just enough to generate a sound.)

Start at the bottom and begin tapping repeatedly moving upwards until you hear the echoing sound you are making change pitch. I'm thinking that it will be higher pitched while tapping at the sediment layer and drop to a lower pitch when you hit the liquid layer.

It is similar to how a carpenter will tap along a wall until the pitch is higher. That's how you can tell when you have found a stud for hanging items on instead of just the dry wall.

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