OPD valves are set to prevent the tank from overfill as a percentage of the tank volume. That set limit is 80% of the volume of the tank it is designed for. A valve for a 20# tank is designed for that tank and would not sense the correct 80% full level in a 30# tank. They even look the same - I almeost walked out with the wrong valve from my propane dealer and he stopped me and sent me back to the rack for the right one.
A 20# tank would fill to 16# (80%) leaving room for 4#. The same valve in a 30# would still leave room for 4# filling it with 26# or 87%. The cost of the valves is almost the same, use the correct valve for the tank you have.
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Richard Wally Byam Airstream Club 7513
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