Sorry to revive an old thread, but I just found it, and I might be able to add something, as I've been in the LP gas dispensing business for a number of years.
It's the dispenser's choice whether to charge by the pound or by the gallon. Two reasons for charging by the pound are to obscure the price ($0.50/pound sounds better than $2.10/gallon); and because scales are always accurate, but meters tend to wander a bit. In our area, scales are checked every year, but meters are no longer checked. They used to be checked every 3 years. With that said, I charge per gallon because it's easier on motor homes and large tanks that can't be weighed.
The rest of this is about small portable cylinders, up to 40# capacity. About cylinders which can no longer be filled - there's a fair amount of misinformation out there. Until 1994, all cylinders had internal left hand threads, and usually required a wrench to tighten the attachment. People forgot about the "wrong" threads and bungled up the fittings pretty good; left the fittings loose if they couldn't find a wrench; or used the regulator for a "wrench", which, while tightening the POL fitting to the cylinder, actually loosened the RH threads to the regulator. All of those contributed to lots of fires from leaks.
In 1994, the QCC-1 (Quick Connect Coupler) fitting was introduced on small cylinders. These used an external, acme-like thread to attach to the cylinder. They use "normal" RH threads, are large enough to tighten by hand, and use a rubber seal to prevent leaks. They also have a check valve that prevents gas from flowing unless something is attached. The QCC coupling nut has a thermal ring that will melt at the temperature of a grease fire, allowing the fitting to pop out, thereby closing the check valve and cutting off the flow of gas. Great safety, at the price of converting all the cylinders. By the way, all cylinders, including the newest ones, still have the internal, left hand thread attachment.
But, there was still a significant safety problem. Many cylinders were being inadvertantly overfilled, often by untrained or careless dispensers, but even occasionally by the automated stations used by exchange services. LP Gas is very sensitive to temperature changes, and an overfilled cylinder can build up pressure to the point where it blows off at the pressure relief valve. This free release of flammable gas can come at very incoventient times, often causing a "floomph". A properly filled cylinder is only filled to 80% capacity, allowing a 20% expansion space.
So, the federal D.O.T. (which regulates all portable cylinders) mandated that all cylinders be fitted with Overfill Prevention Devices (OPD). What this consists of is a float and valving inside the cylinder that works a lot like a toilet tank valve - when the cylinder is 80% full, the float comes up, shuts off the flow and prevents the cylinder from being overfilled. These were required on all new cylinders in October, 1998, and in April, 2003, it became illegal to refill any cylinder without OPD. (This applies to all cylinders up to and including 40 pound capacity - cylinders 50# and up are not required to be OPD. Also excluded are cylinders used in welding applications, and fork lift cylinders, which use a different attachment, anyway).
The only way to convert a cylinder to OPD is to replace the entire valve assembly - there are no conversions to the valves, themselves. OPD is internal. It's confusing, because it was just a few years before that QCC came into being, and some people see the two things as one. If you have expensive 40# cylinders, or expensive aluminum cylinders of any size, it's worth taking to a gas company and having new valves installed. For 20# grill cylinders, the cheapest thing to to is just to exchange the old tank for a proper new (or converted) one from one of the exchange racks. It's also more cost effective to buy a new cylinder than to install a new valve in a 20# cylinder.
Which leads me to the final tip - avoid the exchange racks if possible. First of all, you'll pay more (often a lot more) for an exchange than for a refill. Second, you probably won't be getting a full fill. Read the small print on the exchange rack - it probably says 17# net weight, meaning that you're getting 3# less (almost 3/4 gallon) than your cylinder will safely hold. And finally, unless you don't have a dispensing station pretty close by, the exchange racks are actually less convenient. You have to wait for a clerk to make time to open the rack, often only to find that all their full cylinders have been sold. I've known customers to go to 3 or 4 stores on a holiday weekend.
You can have any OPD cylinder refilled at any dispenser, even if it is an exchange cylinder, no matter what it says on the wrapper. You owned the cylinder you traded in, so you now own the cylinder you got from the exchange rack.
