[APD] Backflow into CO2 systems

["Brown, Diane" <Brown_D at kids_wustl.edu>]

The recent discussions of CO2 accidents reminded me that I'd been meaning to ask about the following phenomenon:  

I am now running 6 planted tanks of a 20# CO2 cylinder:  my regulator from the tank pressurizes the system to about 15 psi; then I have t-connectors running tubing to a separate needle valve for each tank.  From the needle valve, there is an in-line step-down connector to the thinner tubing that connects to an Eheim diffuser for each tank.  

The problem:  when my CO2 tank runs out, water backs up in the system, sometimes past the needle valves, but never back to the CO2 cylinder. Still, it's a pain in the behind to have to undo connections and blow out the water to get the system going again. 

When I put the check valves that came with the diffusers back into the system (between the needle valves and the diffusers), I wasn't able to see any CO2 delivery--the bubble counters stopped, and there was no bubbling even after I waited overnight--I thought that should be plenty of time to repressurize the system in case that was the problem.

What can I add to the system to keep the water from backing up?

Diane Brown in St. Louis

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