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Old 05-21-2016, 12:50 PM   #1
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How much propane left?

My 18 gal propane tank meter says 1/4 left. My display panel says 1/3.

Full is 14 gals, I believe.

How much do I really have? Can I last a couple of night furnace on for 10 mins about 10 times each night.
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Old 05-21-2016, 01:12 PM   #2
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I'd suggest looking for a gauge like this:
Accu-Level Removable Propane Tank Gauge

Often available at Walmart, Home Depot, etc. Magnet attaches it to the tank, pour hot water on it and the level of the liquid will be revealed. That way you'll have an idea how much propane is left when the mechanical and electronic gauges don't agree.
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Old 05-21-2016, 01:42 PM   #3
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I would think the gauge on the tank would be the more accurate, but even that would likely not be highly accurate.

Too many variables to guess of how long that will last you.
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Old 05-21-2016, 01:57 PM   #4
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Originally Posted by BFlinn181 View Post
I'd suggest looking for a gauge like this:
Accu-Level Removable Propane Tank Gauge

Often available at Walmart, Home Depot, etc. Magnet attaches it to the tank, pour hot water on it and the level of the liquid will be revealed. That way you'll have an idea how much propane is left when the mechanical and electronic gauges don't agree.
That may be good for a vertical tank but horizontal tanks hold progressively less as you go below the halfway point.

Which gets me wondering, is the float gauge calibrated for that.
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Old 05-21-2016, 02:04 PM   #5
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If you have no plan B in case you do run out, I'd assume you're going to if your luck runs anything like mine. I say fill it....
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Old 05-21-2016, 02:10 PM   #6
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You might go ahead and fill it and make a note of how much it took.. Then you will know the next time it gets to the 1/4 mark how much you have left.. Do you have a small electic space heater you can safely use if you run out?
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Old 05-21-2016, 02:42 PM   #7
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That may be good for a vertical tank but horizontal tanks hold progressively less as you go below the halfway point.

Which gets me wondering, is the float gauge calibrated for that.
The gauge I suggested was not to get a reading of # of gallons or pounds left, but it will give an accurate reading of the level of liquid in the tank. When full, the tank is only 80% liquid. If it were me and it was cold enough to need the furnace and I had a 1/4 level reading, I'd go ahead and fill it to be sure, thus knowing what a 1/4 reading represented in real gallons/pounds.
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Old 05-21-2016, 07:23 PM   #8
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You might go ahead and fill it and make a note of how much it took.. Then you will know the next time it gets to the 1/4 mark how much you have left.. Do you have a small electic space heater you can safely use if you run out?
No but we were able to get some propane today at a KOA. Unfortunately their pump was not working right, so they could only give us 3 gallons so we still don't know exactly what that quarter mark means.
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Old 05-21-2016, 08:18 PM   #9
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I found out that our 30000 BTU Suburban furnace use uses 42 ounces of propane every hour. A third of a gallon.

So I calculated that on these cold nights we've been burning about a half a gallon all night so we could have gone six evenings on what we had in there. Probably less though because cooking on the stove and the refrigerator using some when on house battery power
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Old 05-22-2016, 07:47 AM   #10
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Everybody is right about the accuracy, but you should be fine. Taking the most conservative figure, 25% of 14 gallons is still 3.67 gallons, about the usable amount in a 20 lb portable bottle (BBQ tank). That will run the furnace the amount you estimated. But what about water heater? Fridge on propane? That might add to the consumption.

Your LP system runs off vapor, not liquid, and the vapor pressure in the tank drops as the liquid level decreases and the rate of vapor flow goes down. On a very cold night, it drops further. If the tank gets really low, e.g. under 10%, that might cause some intermittent problems with the furnace. It gulps fuel, so needs a strong volume of flow as well as adequate pressure. An insulating blanket around the LP tank helps. Or maybe a light bulb adjacent to it to warm the tank walls a bit.
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Old 05-22-2016, 08:46 AM   #11
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Your LP system runs off vapor, not liquid, and the vapor pressure in the tank drops as the liquid level decreases and the rate of vapor flow goes down. On a very cold night, it drops further. If the tank gets really low, e.g. under 10%, that might cause some intermittent problems with the furnace. It gulps fuel, so needs a strong volume of flow as well as adequate pressure. An insulating blanket around the LP tank helps. Or maybe a light bulb adjacent to it to warm the tank walls a bit.
Sorry to correct - but the vapor pressure is only dependent on the temperature not the amount of liquid in the tank (with the caveat that for relatively fast flows the remaining liquid in the tank supplies the energy to vaporize the molecules leaving as vapor: less liquid = cools off more and vapor pressure goes down). The energy to warm the liquid back up has to come from outside the tank, so a blanket would not help (it would just keep the tank colder). A light bulb to warm the tank would help . But would one really want to put a light bulb right next to a potential flammable gas source???
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Old 05-22-2016, 08:47 AM   #12
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When we sold a motorhome at our dealership, It always was delivered with a full tank of LP gas. I have probably filled a thousand tanks or more. I can say for sure that inside gauges are all over the place. The gauge in the tank is mechanical. Actually it is mechanical float with a magnetic gauge on the outside of the tank. It is by far the most reliable. (I did say most, not totally) Of course the motorhome must be level.
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Old 05-22-2016, 08:48 AM   #13
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Originally Posted by Gary RVRoamer View Post
Everybody is right about the accuracy, but you should be fine. Taking the most conservative figure, 25% of 14 gallons is still 3.67 gallons, about the usable amount in a 20 lb portable bottle (BBQ tank). That will run the furnace the amount you estimated. But what about water heater? Fridge on propane? That might add to the consumption.

Your LP system runs off vapor, not liquid, and the vapor pressure in the tank drops as the liquid level decreases and the rate of vapor flow goes down. On a very cold night, it drops further. If the tank gets really low, e.g. under 10%, that might cause some intermittent problems with the furnace. It gulps fuel, so needs a strong volume of flow as well as adequate pressure. An insulating blanket around the LP tank helps. Or maybe a light bulb adjacent to it to warm the tank walls a bit.
If you have shore power for a light to warm the tank, why not reduce LP gas use by plugging in a ceramic heater? I'd have no problem with a lightbulb next to the tank, it's not an open flame. Finding LP gas isn't hard, there are even apps to help locate them, and why drop to 1/4 a tank or less? I choose not to tempt fate by running down a tank in cold weather, perhaps causing pressure issues.
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Old 05-23-2016, 12:25 PM   #14
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Can I last a couple of night furnace on for 10 mins about 10 times each night.

Yes.
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