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Old 12-26-2005, 04:54 PM   #1
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In another forum, I got my tongue wrapped around my axle when I was trying to discuss propane quantity. I'm hoping someone here can help me figure things out correctly from a propane quantity stand point.

Our MH has a propane tank clearly labeled 38 gallons. The guy who filled it last we showed it to me - it is stamped right into the metal. My GBM spec sheet shows that tank as "136 pounds".

Since I can only fill it 80%, I'm assuming that it has a usable capacity of 30.4 gallons and and 108.8 pounds, assuming that I'm willing to run it dry. Running our propane generator (Onan 6.5 Marquis) from 4:00pm one day until 11:00am the next, I put 17.4 gallons in the tank to fill it. I figure that as .9 gallons an hour. Somewhere, I found that one gallon of propane weighs 4.25 lbs. Can I assume that I used 73.95 lbs of propane in that period?

Lastly, assuming that I'm starting with an an 80% full tank and I use 17.4 gallons out of it, the gauge should have read nearly 1/3 full, right? It actully showed 1/4 full.

What am I missing?
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Old 12-26-2005, 04:54 PM   #2
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In another forum, I got my tongue wrapped around my axle when I was trying to discuss propane quantity. I'm hoping someone here can help me figure things out correctly from a propane quantity stand point.

Our MH has a propane tank clearly labeled 38 gallons. The guy who filled it last we showed it to me - it is stamped right into the metal. My GBM spec sheet shows that tank as "136 pounds".

Since I can only fill it 80%, I'm assuming that it has a usable capacity of 30.4 gallons and and 108.8 pounds, assuming that I'm willing to run it dry. Running our propane generator (Onan 6.5 Marquis) from 4:00pm one day until 11:00am the next, I put 17.4 gallons in the tank to fill it. I figure that as .9 gallons an hour. Somewhere, I found that one gallon of propane weighs 4.25 lbs. Can I assume that I used 73.95 lbs of propane in that period?

Lastly, assuming that I'm starting with an an 80% full tank and I use 17.4 gallons out of it, the gauge should have read nearly 1/3 full, right? It actully showed 1/4 full.

What am I missing?
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Old 12-27-2005, 05:55 AM   #3
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You aren't missing anything but you are perhaps expecting a level of accuracy that simply isn't there.

Propane tanks are measured by their water capacity because propane is compressable but water is not. In other words, propane capacity would change with pressure but water capacity is constant.

Your 80% capacity numnbers are right but the weight of propane per gallon is somewhat variable, again because of the compressability. Most RVers figure 4.5 lbs/gallon because that seems to yield a fairly accurate result for small tanks under pressure.

The amount of propane to refill the tank gives you an approximation of what was used but will seldom be exact unless temperature and air pressure are exactly the same at each fill. Even then, there will likely be some variability.

Last, the gauge on your tank is crude and at best an estimate of what is left.
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Old 12-27-2005, 06:06 AM   #4
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It sounds like your numbers and reasoning are pretty much on track. I use 4.24 lbs per gallon as the weight of propane but 4.25 is close enough.

Your 38 gallon tank is theoretically capable of holding 38 gallons. However, you want to burn the propane gas, not the liquid and you need some room for the gas to be held as it evaporates off of the liquid. That's why you only fill to 80%. Assuming that your tank is perfectly installed (most are close, but not exact) the bleeder valve should be able to detect when the liquid gets to the 80% mark so that the refueler can shut off the propane flow into the tank. At that point you would have 80% of 38 gallons, which is 30.4 gallons. At 4.24 pounds per gallon that should give you 128.9 pounds of propane that you can burn.

The propane gas is pressurized as it boils off the liquid propane. Just as in a radiator, increasing the pressure raises the boiling point. So, as the gas leaves the tank, the pressure reduces, and more liquid propane evaporates into gas. The problem occurs when your LP tank gets down to around 10-20% full. There's plenty of room for gas but not a whole lot of liquid left and the gas pressure drops off, especially when the tank is cold. You will find that your LP furnace, genset, or other applinaces aren't burning too well when the propane gets low. So, in theory, you have 80%, or 30.4 gallons in your case, to use. But, in actuality you won't want to take it down that far so I wouldn't count on that much runtime.

Your calculations on propane usage in your genset appear to be correct. Remember that the propane consumption will vary as to how much load you are running on the genset but that appears to be a good average.

Tank gauges are not all that accurate. They're based upon a float lever, not that unlike a regular gas tank gauge, so they are subject to slight variances. I've refilled and did the math and sometimes what I expected to put in, based upon the tank gauge's reading, wasn't what the pump meter came up with.
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Old 12-27-2005, 09:45 AM   #5
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Thanks to both Mark and Gary for your reassurance. I was beginning to think that I'd lost what little I had left of my mind!. Your statement, Gary, about the variabilty of propane at different temperatures, etc. makes perfect sense. I thought that because it was dispensed as a liquid, that it would follow a similar pattern to water. I was also assuming that the gauge on the tank had to be more accurate because of the 80% requirement.

Please consider me better educated as a result of your responses.

Charlie
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