Propane to butane ratio in the US?

[Laurie 1799]

I wouldn’t think so. If you open the screw vent, the remaining propane vapor will vent until the tank pressure equals outside air pressure. No air would go into the tank unless the outside air pressure ever becomes greater than the tank pressure. This could only happen if you leave the screw vent open for an extended time (days) and allow changing barometric pressure and changing temperature, up and down, to put a bit of air in the tank. If you close the vent shortly after relieving all of the propane pressure, no air will go into the tank and no purging needed.

It happens fairly quick four litres of liquid every hour out of the 80% screw. Two or three lbs would be under one hour. At that point it would equalize and you introduce air to the tank.

[huntindog]

It happens fairly quick four litres of liquid every hour out of the 80% screw. Two or three lbs would be under one hour. At that point it would equalize and you introduce air to the tank.

Only if you turned the tank upside down.
Propane is heavier than air. So if the tank is upright the propane even at zero pressure will stay in the tank

[lorend]

How about an experiment? My tanks are full and in storage so I can't try this any time soon.

Once a tank is "empty" and reading whatever minimal level. Disconnect it, vent, and see if the reading changes.

Chad from Changing Lanes did the experimentation with the Mopeka sensors by weighing the tanks and then discharging some propane, then re-weighing, repeat. He found the sensors to be fairly accurate. Enough so that you shouldn't run out if you are checking occasionally.

[Laurie 1799]

Only if you turned the tank upside down.
Propane is heavier than air. So if the tank is upright the propane even at zero pressure will stay in the tank

No it won’t liquid propane turns to vapour at -57 degrees Celsius. The liquid left turns to vapour and comes out the bleed valve. The warmer out the quicker this happens. 25 years in the propane industry 2 gas tickets.

[Tom Traveler]

Only if you turned the tank upside down.
Propane is heavier than air. So if the tank is upright the propane even at zero pressure will stay in the tank

No it won’t liquid propane turns to vapour at -57 degrees Celsius. The liquid left turns to vapour and comes out the bleed valve. The warmer out the quicker this happens. 25 years in the propane industry 2 gas tickets.

Well, yes it WILL stay in the tank. The boiling point of propane, at normal atmospheric pressure, is -42°C, not -57°C (Wikipeida, The Engineering Toolbox, Marks Standard Handbook for Mechanical Engineers, etc.). At any temperature above -42°C, the propane will begin to vaporize. If one were to start with a full tank of propane, at say +20°C (+68°F), and open the screw vent, the liquid propane in the tank will gradually boil off and the pressure in the tank will decrease. The last of the liquid will boil off as the tank equalizes to atmospheric pressure (air pressure outside of the tank). At that point, the tank will still be filled with propane vapor equal in pressure to air outside of the tank. Propane will then stop flowing out of the valve and no air will flow into the valve.

Propane is 56% heavier than air so no propane will flow out of the tank (a tip of the hat to "huntingdog") and no exchange will happen without a change in either atmospheric pressure or temperature. If the atmospheric pressure outside of the tank goes up, some air will go into the tank. If the temperature rises, some of the propane will flow out of the tank and when the temperature goes back down, some air will be drawn into the tank. But, this will only happen if the screw vent is left open for a period of time long enough to have significant changes in pressure or temperature.

In normal use, this is not what is happening. A propane tank is hooked up to a regulator, let's say to a barbecue. When you open the gas valve on the barbecue, the pressure on the downstream side of the regulator decreases and the regulator allows propane to flow from higher pressure to lower pressure. When the tank goes empty, that is, all the propane has boiled off, the regulator sees equal pressure on both sides and stops all flow. Also, regulators are like check valves. Air cannot flow back into the tank, no matter what the atmospheric pressure or temperature happens to be. Also, the main value on modern, legal propane tanks has a mechanical check valve. Air cannot flow back into the tank through the main valve even if atmospheric pressure is greater than the tank pressure.

You can run your tank empty in your RV and you do not need to get it purged. If the gas supplier tells you otherwise and charges you for it, he is stealing your money. Go to a different vendor. Unless, of course, you are in the habit of opening the screw vent and leaving it open for days or storing or transporting your propane tanks upside down with the screw vent open. Then it's on you.

Physics is a wonderful thing; it is not subject to opinion. It either is or isn't. Sleep easy, many things are simpler than people trying to take your money would have you believe.

Sorry for being pedantic and repeating things most of you already know. I used to teach journeyman apprentice classes and can't help it.

[ajg617]

How much propane do you put into a 30lb tank?

I keep a 1000 gal tank filled at work for a Sat Dish and supplier will normally fill it up to 80-90% depending upon weather forecast and the individual the supplier sends to fill it (they have different opinions on max fill). Last fill was 447.4 gallons at close to 50%.

Most places we stopped at this year (about 6 or 7 tanks filled) would fill by weight and the gallons would be roughly 7.3ish. But I had two stops where they filled by gallons which ran up to 8 according to their meters. About 45 degrees when filled but temps went up to 70s during the day. I was smelling propane seeping out (tank was on the sun side) all day long which had me checking for leaks. Even the dealer puts in 8 gal (or so his fuel flow reads). I don't think 8 gallons would fit if I'm calculating correctly. Either way, even 7.3 seems on the high end.

[Tom Traveler]

How much propane do you put into a 30lb tank?

I keep a 1000 gal tank filled at work for a Sat Dish and supplier will normally fill it up to 80-90% depending upon weather forecast and the individual the supplier sends to fill it (they have different opinions on max fill). Last fill was 447.4 gallons at close to 50%.

Most places we stopped at this year (about 6 or 7 tanks filled) would fill by weight and the gallons would be roughly 7.3ish. But I had two stops where they filled by gallons which ran up to 8 according to their meters. About 45 degrees when filled but temps went up to 70s during the day. I was smelling propane seeping out (tank was on the sun side) all day long which had me checking for leaks. Even the dealer puts in 8 gal (or so his fuel flow reads). I don't think 8 gallons would fit if I'm calculating correctly. Either way, even 7.3 seems on the high end.

We had ours filled about 4 times in Nov, Dec. The tanks were empty and the fill was 6.8 to 7 gals. Some places still open the vent and stop filling when liquid starts vaporizing at the vent. Others don’t open the vent and fill by wight or volume (gallons). Seems kind of random and definitely depends on who is doing the filling.