I am not an expert on gas physics, however I do have an observation about the illusive EOTD. CO2, as I understand it, is not like other gasses - like the tanks you get filled with air for scuba diving. Those are filled according to PSI as air remains a gas under pressure. CO2 and Propane however are measure in lbs. This is because the gas turns to liquid under pressure. You can observe this by pouring really hot water down the side of a half-filled propane tank and you can feel exactly where the level is because the liquid will absorb the heat from the water below the gas line. Hot above, cold below. CO2 remains at a steady pressure somewhere around 870psi as it is "boiling" off within the cylinder. You can measure that the overall weight of the cylinder will decrease as you use it, but the PSI will remain steady. At the end, the PSI will decline to your setpoint ( I run about 30psi on the low side) on the reg, but I am uncertain if any "extra" gas will get past it. I have read, but cannot confirm that some regs, if there is not a certain minimal amount of pressure on the high side, will vent to the low side at that minimal pressure. If that "minimum" is something like 60psi, your bubbles at the counter I suppose could be 2x as large because of the added pressure at the needle valve. But, I would think that the needle valve itself provides a pressure drop across it as well.
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