20 # Bottle
 

I have been using my BBQ for decades..currently a Weber Spirit. When the bottle is near empty..it weighs in at 18 pounds..maybe slightly less. When the bottle is getting low I weigh it on my digital bathroom scale. Don't want to get caught empty in the middle of a meal.

All's well. The other day I went to my Propane supplier for the last 30+ years . While charging the bottle..The guy doing the recharge put a screwdriver to a little "blead screw" on the side of the bottle valve. Never noticed this procedure before. I asked him about it and didn't get much of an answer.

When I got home I weighed the "Full" bottle. (Never done that before) It weighed 35.2 pounds. My impression was it should weigh 38+ pounds. 18 pounds for the empty bottle + 20 pounds for the propane.

Is this Normal..? :look: NB

I believe it's general practice to fill propane tanks (large and small) to about 80% capacity. Not sure if this is for safety or that it's a law but my twin 250 gallon tanks are never filled more than 75-80%.
As for the 20 lb. grill tanks, I always have a full one on hand as a backup in case I run out while grilling...never cared about how much propane was inside...

Expansion and Contraction
 

You are correct the 75-80% allows for the expansion and contraction of the propane with changing temperatures.

When you think about it, you don't want the liquid level too close to the outlet. You want vapor going out the pipe to the grill, not a slug of liquid, which would provide a sudden surge of fuel to the grill, maybe blowing flame all over your face!

Understand the point of the post is about weights and mathematics.

The place I go to uses both a scale and a unit counter. Gallons I think.

The screw I think is to let excess air out. But let other chime in on that.

Me, I have an extra full tank. So don't have to worry if grille tank runs out while cooking.

seems like when we swap one out at cumberland (blue rhino), there is even less propane in them than when I get the tanks filled back home.

Yup, Blue Rhino only fills it 75% or so to be "extra safe."

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I think you are correct about bleeding off AIR at the top of the bottle. We always keep a spare bottle. I guess using my bathroom scale to weigh the bottle is a bit Anal...that's me..:D NB

PS: NEVER lay the bottle on it's side..Always keep it VERTICAL while transporting the bottle in your car.

Last time i got Blue Rhino they advertised it as 15#. I thought I was getting ripped off but maybe it was just truth in advertising! And it was actually on the 4th of July last year, 2/3 of the way into the cookout! Be prepared! I learned my lesson and keep the spare "full" or at least 3/4!

Tanks are limited to 42% of liquid weight
 

Here is a real thorough explanation. Cant confirm if its correct but it seems reasonable.
http://modernsurvivalblog.com/preps/...ank-really-is/

Let's examine the numbers on that URL Siksukr cited. The WC (water capacity) of the tank shown is 47.6 lb of water, specific gravity 1.0 at room temperature. At 77 F, propane's density is 0.493 (https://en.wikipedia.org/wiki/Propane). Full of propane, the contents would weigh 47.6 * 0.493/1.0 = 23.47 lb. The 42% of WC regulation would allow for 20.0 lb, which is 85% of propane capacity. The numbers all agree.

I had a friend who had Blue Rhino tank exchange at his store. When I said that could come in handy if I ran out when the refill stations were closed (he was opened until 9pm). He told me how Blue Rhino was only filled with 15 lbs, and if I ever did run out, grab a tank at his house because BR was a ripoff. What seems like a good idea on the isn't.
Dave

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The screw the filler opened is an outage gauge. It is connected to a tube in the valve that goes down to the 85% level. Gas will come out at first and when liquid reaches the end of the tube a white fog will appear out of the small hole to the outside of the valve telling the filler that the tank is full. However, all 20# tanks that are legal to fill today have an auto stop fill feature that will stop the filling at 85% or less. Sort of an idiot proof system to prevent overfills which could cause the relief valve to open if the tank became liquid full and the pressure inside reached 250 psi. There is never air in a propane tank except when new. a new tank must be purged of air before filling. If air and gas were mixed it could explode although unlikely as propane's range of flammability is about 1.9 to 2.4%.