Filling Propane Tanks

The little vent they open with the screwdriver ("spit valve" it's sometimes called) will tell when the tank is filled to the proper capacity, with appropriate headspace, by starting to dribble out liquid propane rather than just a bit of gas.

For DOT cylinders, which I believe is what you're talking about, there are three basic ways to tell when they're full. One is using the vent. Another is using a scale, set to the tank tare weight plus the weight of propane plus the weight of the filler adapter. A third is to rely on the OPD valve which has a float mechanism to shut of the flow when it reaches a certain level, sort of like a toilet tank valve. I'm not an expert, but I'm under the impression that relying solely on the OPD valve is not generally permitted; it's more of a safety backup.

For built-in tanks, the same basic ideas apply except it's obviously not practical to fill by weight.

The "vent" that they open while filling tanks and cylinders is not actually a "vent" at all. It's official name is the "Fixed liquid level gauge", and it's connected to a calibrated tube that vents gas until the liquid propane reached the tube. The difference is readily apparent when that happens. Refillers that fill DOT cylinders by weight often don't open the gauge valve, and most of the time, the correct weight will be reached just before the OPD valve trips. The fixed ASME tanks on motorhomes can't be filled by weight of course, so the level gauge should always be open while they're filling.

Seen it both ways. I just stand back.

profdant139 wrote:
OK -- I am once again revealing my ignorance. I have always wondered about the venting and the screwdriver. Why is that necessary? What does it accomplish?

Thanks for enlightening me -- sorry if this is an obvious question.

The danger lies in overfilling the tank and not allowing for thermal expansion of the liquid inside the tank when the tank gets warmer. The tank could then vent (if the vent works) in an enclosed area and cause an explosion if something ignites the fuel air mixture. Venting propane is heavier then air and will travel along the ground and could happen upon an ignition source. In other words venting propane can be dangerous so they do not want to overfill the tank.

OK -- I am once again revealing my ignorance. I have always wondered about the venting and the screwdriver. Why is that necessary? What does it accomplish?

Thanks for enlightening me -- sorry if this is an obvious question.

This guy is relying on the OPD valve to vent when the tank is full....
I prefer the old method of using the manual bleeder with a screw driver...

I always ask them to fill using that method.