Digester level...an interesting problem for Sherlock Boyes...

"Daniel M." wrote to the Automation List at Control.com: "...we have a waste water treatment plant, and some anaerobic digestor (height 10 m, diameter 13 m). We need to measure level AND INTERFACE in these digestor. Ex zone due to biogas presence. If someone already faced this problem, I would appreciate any comments regarding the apropriate sensor or sensors (I personally think is not possible)." Here's what I wrote back: What you need depends on what you want to measure. Is there foam in the digester? Do you need sludge blanket interface or liquid/foam interface? Foam/liquid interface can be measured by guided wave radar (also sometimes called TDR, time domain reflectrometry), and sometimes by RF admittance (Drexelbrook, Princo, E+H, and hordes of others). If you need to measure the sludge blanket in an anaerobic digester, you need to understand that nothing works really well, even differential pressure or hydrostatic level. The reason that nothing works really well is that there is not a sharp, well defined interface. There is always a "rag layer" which goofs up (that's the correct technical term, my wife's orthopedic surgeon assured me of this) the reading on radar and RF admittance. You can't use ultrasonic, even uplooking ultrasonic, for the same reason. The thing that I've seen work the best is a float that has a custom specific gravity that is calculated to be approximately (and I use the word advisedly) the specific gravity of the interface. This can get interesting if the sludge blanket is almost the same specific gravity as the liquid, say, 1.02 for the supernatant liquid and 1.1 for the sludge (this can often happen with organic sludges). Check with one of the good old time float gauge manufacturers, like Magnetrol, or Krohne. If you can pull the digester down, you can also try an armored sight gauge, with optical alarms. But this requires modifying the vessel, and your codes may make this ugly. Good luck!