Allow me to jump in here with a bit of clarification. Most generally, there is a difference between filling practices (safety) and billing practices (money).
Filling Practices
The National First Protection Association bible for propane handling in the USA (NFPA 58) specifies either of two safe filling practices:
1. Fill by weight. Using the TW and WC figures stamped into the collar of the cylinder at manufacture, set the scale for the full weight and fill to that point.
2. Fill by volume. Open the small screw valve on the side of the OPD and fill until propane spews out.
Either method is a safe way of filling, but some states -- this is a state-regulated matter -- prefer one over another. Here in NM, for instance, they recommend filling by weight unless volume filling is required. (The reasoning is environmental, not safety related.)
Adding in here, note that filling by the gallon is not permitted. This is so because the notion of "gallon" is a variable one that depends on ambient pressure and temperature.
Similarly, filling a cylinder until the built-in OPD valve stops the flow is also not permitted. This is so because OPD valves are known to fail. After they are installed at manufacture, they are never inspected (for obvious reasons), and are therefore suspect as a standard filling practice.
Billing Practices
Billing has nothing to do with safety and is not addressed by NFPA 58 at all; states, villages, cities, or just fillers are free to make up their own rules. A few notes here:
1. Most charge (but not fill) by the gallon, I suspect. (That's what we do here.)
2. Others may charge by the pound if they wish.
3. Most critically for consumers is whether fillers charge a
flat rate for cylinders or they just
base their charges on amount of propane dispensed (either gallon or pound). Most often, your best deal is when fillers base charges on amount of propane dispensed, kind of like at a gas station.
Lynn
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