Any Math Professors Out There? Brain Teaser - Propane Tank Capacity: Gallons to Pound

[The_Breeze]

Here's are the knowns (or trusted)
- The spec sheet for my RV cites 41lbs. and 9.6 gallons.
- The tank's label cites 12.2 gals.
- Only fill propane tanks to 80% capacity (12.2*.8=9.76)
- Propane weighs 4.24lbs/gal
- 9.76g * 4.24lbs = 40.704lbs (almost the cited 41lbs)
- The tank was approx 3/8 full and took 5.7 gals to reach 80% or complete the remaining 5/8.

I tell the guy at the propane station it’s a 41lb tank. He scoffs and says the 12.2 is water capacity – the tank is a 10 or 12lb tank . I refer to bullet #3, scoff at him, and figure I’ll try to do the math.

I tried backing into the number using the last two bullets to no avail. It should be simple algebra, solving for one unknown. I don’t know why I can’t solve for it.

5.7(4.24)*5/8 = X
24.168 *.625 = 15.105 (5.7 gal equals 15lbs or the 5/8 it took to fill the tank to 80% (i.e. 9.6 gallons)
15.105/8 = 1.888 (break it down into eighths [of a tank])
8*1.888 = 15.104 (8 eighths of a tank [or full])

I’m back where I started @ 15lbs. Even if I fudge and say another 3/8, that's only about 20lbs? Where am I going wrong?

[10scDust]

You say it took 5.7 gallons to reach the 80% mark.
How much was in there to start with?

Where do you get these figures of 5/8 & 3/8?
4/5 = 80%
5/8 = 62.5%

[TyCreek]

Not sure what unknown you're after ... seems the 3/8 was the only unknown and results you got seem ok to me?
If you know the tank holds 9.6 gallons and you were charged for 5.7gal, that's 60% meaning the tank was 40% before filling. The approximation of 3/8ths full is .38 and rounded up to .4 effectively confirms 40%.

[Bob Denman]

I see your problem... You're trying to do Math on a Wednesday evening.

But if the tank's capacity is 41 pounds (9.6 gallons): that's close enough to call it a 10 pound tank.

[The_Breeze]

Originally Posted by Bob Denman

I see your problem... You're trying to do Math on a Wednesday evening.

But if the tank's capacity is 41 pounds (9.6 gallons): that's close enough to call it a 10 pound tank.

I'm pretty sure you meant 10G, not 10LB?

[The_Breeze]

I'm trying to go with TyCreek's numbers for now but still can't verify 41lbs potential propane capacity. There's no mathematical conversion from percent to gallons or pounds - but fear not, I'm working on it. I want to know that I have 41lbs potential capacity. That's more than the conventional 30 (which supposedly lasts a long time) but less than the 60 in the 5th wheels (which lasts even longer).

[10scDust]

As Vinny, on Welcome Back Kotter, always said: "I'm so confused!"

[JamieGeek]

Originally Posted by The Breeze

Here's are the knowns (or trusted)
- The spec sheet for my RV cites 41lbs. and 9.6 gallons.
- The tank's label cites 12.2 gals.
- Only fill propane tanks to 80% capacity (12.2*.8=9.76)
- Propane weighs 4.24lbs/gal
- 9.76g * 4.24lbs = 40.704lbs (almost the cited 41lbs)
- The tank was approx 3/8 full and took 5.7 gals to reach 80% or complete the remaining 5/8.

I tell the guy at the propane station it’s a 41lb tank. He scoffs and says the 12.2 is water capacity – the tank is a 10 or 12lb tank . I refer to bullet #3, scoff at him, and figure I’ll try to do the math.

I tried backing into the number using the last two bullets to no avail. It should be simple algebra, solving for one unknown. I don’t know why I can’t solve for it.

5.7(4.24)*5/8 = X
24.168 *.625 = 15.105 (5.7 gal equals 15lbs or the 5/8 it took to fill the tank to 80% (i.e. 9.6 gallons)
15.105/8 = 1.888 (break it down into eighths [of a tank])
8*1.888 = 15.104 (8 eighths of a tank [or full])

I’m back where I started @ 15lbs. Even if I fudge and say another 3/8, that's only about 20lbs? Where am I going wrong?

Your problem is here:
15.5/8
You are trying to break down the entire tank's capacity into 8ths but using the 5/8 fill value instead of the full 40lbs.
1/8th of the tank would be 40.704/8 or 5.088lbs.

So 5 of those is ~25lbs and 3 of those is ~15lbs (your 15.5 is right around that value).

[halfprice]

Just weigh your tank. THe tank should have a tare weight on it. Subtract tare weight from your now full weight and youll know what your tank holds


jerry

[16ACE27]

Originally Posted by halfprice

Just weigh your tank. THe tank should have a tare weight on it. Subtract tare weight from your now full weight and youll know what your tank holds


jerry

So you want him to unbolt/remove his tank from the coach?

[Bob Denman]

Originally Posted by The Breeze

I'm pretty sure you meant 10G, not 10LB?

That's why Math is off-limits on Wednesdays...

[halfprice]

Originally Posted by Tfryman

So you want him to unbolt/remove his tank from the coach?

Ha disregard my post. Was thinking i was on the pizza forum

[TyCreek]

Originally Posted by The Breeze

...
I want to know that I have 41lbs potential capacity.
...

I thought you sort of proved that in the first set of bullets?

Originally Posted by The Breeze

...
- 9.76g * 4.24lbs = 40.704lbs (almost the cited 41lbs)
...

Looks like 9.6 was used instead of 9.76 as typed in that bullet result...
Using the 9.6 gallons specification is probably most appropriate anyhow and 40.7 nicely rounds to 41lbs. Maybe I'm just missing what doesn't add up?

[The_Breeze]

Originally Posted by JamieGeek

Your problem is here: 15.5/8. You are trying to break down the entire tank's capacity into 8ths but using the 5/8 fill value instead of the full 40lbs. 1/8th of the tank would be 40.704/8 or 5.088lbs. So 5 of those is ~25lbs and 3 of those is ~15lbs (your 15.5 is right around that value).

Nice. You are where I was in my initial problem. I hit the wall when I tried to proof it. Your 25+15 brings 40 and approximates the answer I was trying prove.


The original problem is inaccurate in it's measurement's contain assumption (eights). Thank you for chiming in.

[The_Breeze]

Originally Posted by TyCreek

I thought you sort of proved that in the first set of bullets? Looks like 9.6 was used instead of 9.76 as typed in that bullet result...Using the 9.6 gallons specification is probably most appropriate anyhow and 40.7 nicely rounds to 41lbs. Maybe I'm just missing what doesn't add up?

You and Jamie seemed to converge on the same conclusion. That's a good sign. I think I'll take the proof off to the side and post it if it proves out.

I have what was printed and what was shared in this thread. I can plan against that. I have a community of smart people I can count on. That requires no further proof.

[bry899]

My propane guy sells propane by the gallon, his scales are too small for me to drive on.
Propane is also sold by the gallon for residences.

Too confusing if I had to convert all my fuel usages (Gas, Diesel, Propane etc) to pounds.

[Chance]

Originally Posted by The Breeze

.....cut..... I want to know that I have 41lbs potential capacity. .....cut.....

If it’s that important to confirm capacity, next time can you run it empty, and then you’ll know when you fill it?

[JimOIB]

Not a math professor but a systems engineer....Going back to the OP, there are some errors. You probably have an Armebe tank like I do. The 12.2 is water capacity like your propane guy stated. If you look at the spec sheet for Armebe it states a 12.2 WC tank holds 42.75 pound of propane. This is the 80% fill capacity weight, it can hold another 10.6 pounds but that 20% space is for expansion. Your 40 pound tank at 80% full holds 42.75 pounds. One gallon of propane weights 4.23 pounds at 60 degrees. So your tank capacity in gallons is (42.75/4.23) = 10.1 gallons. The propane guy looks at 12.2 WC and uses a chart to convert to 10 gallon tank.

So, start here and then do the math. 1) Use 42.75 pounds as the 80% full weight, 2) Use a tank capacity of 10.1 gallons. If it took 5.7 gallons to fill the tank to 10.1 gallons then you started with 4.4 gallons in the tank - half way between 3/8 and 4/8 or 7/16 for the math folks.

[The Gritz Carlton]

1 gallon of propane weighs 4.2 pounds. A "full" 20 lb cylinder should have 4.7 gallons of propane in it.


Not an issue to loose sleep over. Most states require certification of all commercial bulk fuel dispensing equipment. The meter on the LP pump reads out gallons as the refill your tank. All liquid fuel is an even gallon for gallon at 60 degrees. Just read the meter when they are done and calculate for yourself. This will also let you know how much was in the tank before you filled and it gives you a check on how accurate your gauge is. If your tank is permanently mounted to the coach frame...forget pounds...you will never use them and its insignificant. Weight is only a factor when refilling portable tanks.

[revjeffrey]

You can also take your empty bbq thank in for a fill. They can fill it on the scale and you will have both readings, pounds & gallons then compare.