LPG tank specs vs reality

[blsmith25]

My specs say my LPG tank is 12 gallons.
I run my fridge all the time and the stove and water heater ocassionally so it lasts awhile.
I've filled it up only twice so far. The first time the fillup was 7 gallons, this time it was 8.2 gallons.
The fridge shuts off automatically, but I figured it would be when the tank is empty.
Does the fridge shut off before it's really empty? Why?
Is my tank really 12 gallons?

 

[Ned]

By law, a propane tank can only be filled to 80% of it's water capacity, so a 12 gal. tank will be filled with 9.6 gal. of propane.  The pickup may not reach all the way to to bottom, so you may not get the entire 9.6 gal.  The refrigerator (and other appliances) require a minimum pressure to operate correctly and will shut off if the pressure drops too low.  Also, if it's cold, propane does not vaporize as readily as when it's warmer, so the vapor pressure drops lower when the tank is depleted.

 

[blsmith25]

So, it sounds as if everything is ok, right?
So basically, when it's cold out, I should be seeing slightly more frequent and cheaper fillups because the pressure will fall faster than in the hot weather.
With the same amount of propane in the tank, in the cold weather the fridge would shut off sooner than in the hot weather because of the pressure? Thereby requiring slightly more frequent and cheaper fillups? But in the summer, the propane will seem like it lasts longer because of the heat keeping up the pressure? So my fillups should be farther between, but more gallons and cost. (In this explanation I'm assuming the price is consistent year-round for proper understanding of the concept).
Am I understanding correctly? I think I am but I'm having difficulty explaining it the way I'm thinking it works based on what you said.

Maybe another way is by a specific example:
I start out with 10 gallons of propane in the tank.
In the summer, the heat will help produce more pressure in the tank allowing it to run until the pressure drops enough, say at 2 gallons?
In the winter, because it's cold, the pressure cannot be kept up as easily so the pressure would drop sooner, say at 3 gallons?
So, my fillups would be 8 gallons in the summer and 7 gallons in the winter.

If this is correct, mine seems backwards. Here are my stats:
August 5 - original fillup of 12 gallons minus 80% = 9.6 gallons starting out.
September 14 - 7 gallons after 40 days. So, tank was not empty but pressure had fallen below the requirement at 2.6 gallons remaining.
Niovember 11 - 8.2 gallons after 58 days. Pressure had fallen below the requirement at 1.4 gallons remaining.
In both cases I waited until the fridge clicked off before filling up.

In any case, my 12 gallon tank is basically reduced to 7 or 8 gallons of useable propane, depending on the weather.  ??? 

 

[Gary RV_Wizard]

You have the concept right, though I doubt if the seasonal difference will be as much as you suggest. 

I'm surprised your fridge shut down with 2.6 gallons in the tank, especially in warm weather.  That doesn't sound like proper operation to me. Will the stove still light and the furnace run after the fridge shuts down?  It might be worthwhile to have a professional check the vapor pressure on the output side of the regulator when the tank is, say, half full and see if it is up to spec (11 water column inches of pressure is the proper amount).  hat's a very tiny pressure, so it takes quite a cold night to drop the vapor pressure to near zero.

 

[Ned]

One correction, the pickup is not on the bottom of the tank, but at the top.  Only gas is taken from the tank, not the liquid.  Sorry about that bit of misinformation.  However, if the temps were quite low, it's still possible that the pressure dropped to low for the fridge, but it's also possible that the regulator isn't doing it's job.  As Gary suggests, have the system pressure checked by a professional and see that the regulator is doing its job.  We've used our propane down into the low teens with no problems, even when the tank was near empty.

 

[Karl]

hat's a very tiny pressure, so it takes quite a cold night to drop the vapor pressure to near zero.

Yeah, like about -44deg F or -42deg C!

 

[John From Detroit]

That temperture depends on what, exactly, is in the tank.. If he filled it in, oh, say South Texas, he might have a different gas entierly than if he filled it in, oh, say Pinconning, Michigan.

Same for the Fuel tank I might add

 

[blsmith25]

I never tried running either the stove, furnace or water heater after the fridge shut off...
I figured the tank was empty...
Will have to try that next time. I'm good to go now for hopefully some time before I need another fillup.
Everything is new by only a few months. I just got it in August. Would there be problems already???

About my calculations - the first was just an example using round numbers and guessing to make sure I understood the concept.
The last section is my real life stats so far. My calculations are based on Ned's first reply that only 80% is filled. My other assumption is that the hose used to fill it up shuts off automatically when it figures out it has reached the 80% full level. So I'm also assuming that after the propane pump shuts off that I've got 9.6 gallons in the tank as opposed to 12 gallons. I would have to assume this each time I fill up for my calculations to be correct.
If this is not an exact science, then my calcs could be off.

So, how many gallons left would be considered "empty"?

Would I have to bring it back to the dealer to check the regulator? It's a 2 hr drive and likely a day's adventure. Would it be worth it?

 

[Karl]

First off, the fridge going off is not the best indicator of fuel level. What tells you that the fridge has shut off? When you say your fridge stops working, do you get any indicator lights above it? Does it still work on 120VAC? A propane fridge uses so little gas (about as much as a couple of BIC propane cigarette lighters), that it's almost insignificant. A better test would be the water heater or furnace or stove. If any of those are not working properly, THAT would be a better test. As far as testing a regulator, heck, you can buy a new one for about $10-$12 - no need to test; replace. if the regulator is faulty, the other gas appliances would stop working long before the fridge stopped. If it's only the fridge that doesn't work. chances are that the control board, thermocouple, or gas valve is the culprit.

 

[blsmith25]

There is a loud clicking noise when the fridge shuts off. That's when I can tell. Also I was expecting it to go off soon since I knew I should be getting low. No indicator lights. Yes it works on other power sources. It's not that the fridge stops working completely. It's just that it shuts off for lack of propane. I can switch it over to the battery but it wouldn't likely take long to run it down if I'm stopped for long.
But running the fridge all the time on a tank of propane, it can't last forever, right? It's gotta run out at some point doesn't it?
It lasted 58 days since the last time the fridge went off and I filled up again. That's 2 months.
I also use the HWH and the stove maybe once or twice a week (if even that) and I think that's it.
How long do you think my tank of propane should be lasting?