Broadback wrote:
How about lowering a fairly large diameter transparent tube, then seal
the end (the top end in case you are squeamish) then bring the tube out
and see what is there?

Ah, now, that's an interesting idea! Am I right in thinking that you're
imagining that if the tube is wide enough and rigid enough it will cut
through the sludge like an apple-corer and that one would keep pushing
until one hits the bottom of the tank, then by making the top airtight
and then retracting the tube out of the tank one could see the exact
depth of the sludge layer? And if you were really interested you could
let the contents flop onto a bit of out-of-the-way garden and see just
how "solid" they are! (And I think the examination ought to stop
there!)

Good thinking! There would be quite a gravitational pull on the
contents though as one retracted the pipe. The airtight seal at the top
would have to be STRONG as well as absolutely airtight. That sounds a
bit challenging. Any ideas?

Eddy.