Originally posted by Chipmaker
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There is no air in a propane tank. All air is purged out in the initial tank filling process because air and propane together inside the tank could create a combustible mixture. When the referenced valve is opened, what is coming out is raw propane vapor. Use of this valve is not permitted in some jurisdictions. See "spit valve" discussion below.
A propane tank can be filled with liquid propane to 80% of its total volume. This is a "full" tank. The remainder of the space will be filled with propane vapor and must be there for safe expansion of the liquid propane with temperature change. All modern removable propane tanks have a float that shuts off the fill at 80% liquid level (the OPD or Overfill Prevention Device).
Before the OPD, there were two ways of knowing when the tank was 80% full of liquid. One was by weighing the tank and its contents. (Thus a "20 lb" tank or a "30 lb" tank) For tanks too large to weigh or perhaps permanently mounted in a vehicle, there is a "spit" valve with a tube inside the tank to the 80% level line. The process used to be to open this valve, releasing propane vapor until the liquid level reached this tube and liquid propane would begin to "spit" out this valve. Obviously either of these fill methods is highly dependent on the person doing the filling to stop the fill at 80%. . . . thus the development of the OPD for a more reliable fill to 80%.
Agreed that "exchange" propane tanks are usually filled to less than their allowable 80% or 20 lbs of propane. I presume that this is done for an extra level of safety WRT temperature expansion.
Rob
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