Sometimes you can learn a lot by failing at a project!
I have been thinking that the reported high CO2 diffusion constant published for silicone tubing offers the possibility of using a long piece of that tubing, coiled in the substrate as a diffuser for CO2 that would release the CO2 where it would be most effective. So, I bought a 25 foot roll of Top Fin Silicone Airline Tubing, about $6 worth, and tested it today.
I made a bubble counter out of a club soda bottle to be sure it would be strong enough to withstand up to 40 psi. That was a very conventional bubble counter, with a couple of pieces of silicone tubing pulled through undersize holes in the cap, one deep into the bottle, the other ending near the top of the bottle.
Then I set up for a test, as shown:
I started with a little plastic check valve as a connector, but that leaked CO2 at the connections, so I replaced that with a short piece of rigid tubing:
I put the coil of tubing in a container with about a gallon of water in it, having to weight it down so it wouldn't float up. I purged the air out of the tubing by running CO2 through it for a minute or so, and clamped it shut by folding it over, tieing it with wire and adding a "bag clamp" to squeeze it even more.
Using the regulator, with the needle valve open more than normal, I raised the CO2 pressure to 5 psi, and got a surge of bubbles through the bubble counter, until the tube filled up to the 5 psi pressure. The final bubble rate was about 30 bubbles per minute.
Raising the pressure to 10 psi, 15 psi, 20 psi, 25 psi and 30 psi did nothing but reveal a tiny leak at the tubing connection going through the bubble counter cap - a very small leak, taking minutes to generate a fine foam of soap solution. The bubble rate decreased as the pressure went up until it was about 12 bubbles per minute at 30 psi.
Clearly, little or no CO2 was diffusing into the water. Certainly not enough was diffusing to be able to use the tubing for a CO2 diffuser. So, the idea isn't practical, if it would work at all.
This did prove that we can use cheap silicone air tubing for our CO2 systems, and not lose any CO2 as a result. The normal pressure our CO2 runs at, downstream of the needle valve is about 2 psi, so there should be no measurable leak at that pressure.