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Old 12-12-2016, 10:37 PM   #1
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Filling Butane to 80%

After reading about the filling the butane tank I want to make sure I understand this correctly.

So when you go to say, a Flying J to fill your butane tank, the tank itself has an internal sensor/valve that will only allow the tank to be filled to 80%.

When the butane gauge points to full, it's actually just 80%.

Are these statements true?

Are there any other things I should know here or special precautions to take when filling the tank?
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Old 12-12-2016, 10:58 PM   #2
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Yes, the biggest thing to know is it's Propane not butane. And yes to your questions. The tank needs room to expand when it gets hot, that's why it won't fill to the maximum

https://www.reference.com/home-garde...17473470876c30
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Old 12-12-2016, 10:59 PM   #3
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I assume you are asking about the propane tank. If so, my understanding is the relief valve is opened while filling and once the liquid reaches the valve they stop filling. This is the 80% fill. Flying J will require all appliances be off and the coach be evacuated while filling.
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Old 12-12-2016, 11:07 PM   #4
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Quote:
Originally Posted by marjoa View Post
After reading about the filling the butane tank I want to make sure I understand this correctly. You are probably talking about your propane tank.

So when you go to say, a Flying J to fill your butane tank, the tank itself has an internal sensor/valve that will only allow the tank to be filled to 80%. That is correct, the internal valve will shut off incoming flow of propane at 80%.

When the butane gauge points to full, it's actually just 80%. My tank gauge shows 3/4 after filling.

Are these statements true?

Are there any other things I should know here or special precautions to take when filling the tank?
Remove all ignition sources, like operating refrigerator. Some fill stations want everyone out of the RV when filling. Getting a propane fill is easy, as a properly trained attendant should do everything.
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Old 12-13-2016, 01:13 AM   #5
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This picture may help visualize it...



.
.
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Old 12-13-2016, 04:56 AM   #6
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Originally Posted by vettenuts View Post
I assume you are asking about the propane tank. If so, my understanding is the relief valve is opened while filling and once the liquid reaches the valve they stop filling. This is the 80% fill. Flying J will require all appliances be off and the coach be evacuated while filling.
Yeah I meant to say propane. It was late when I posted this.
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Old 12-13-2016, 05:37 AM   #7
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Worthy of mention certain dealers fraudulently substitute butane for propane. Something I had never heard of until reading it here on iRV2.

Of note also, sometimes in the bitter cold with smaller tanks you need to fill less than 80% to allow for more room to create vapor. The colder it is the longer it takes to boil off into a vapor.
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Old 12-13-2016, 06:25 AM   #8
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On my tank the OPD, overfill protection device, stops the flow of liquid propane at 80 %. The gauge reads around 3/4 at that point.

The OPD is like a toilet float valve. Once the liquid lifts it to a set point, it stops the flow.

The bleeder is also at the 80% line. Once liquid propane reachs that, some will start to spit out as a strong vapor.

The space above the liquid, is the gas that is used in your appliances. As you use that, more liquid boils off to fill that space.
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Old 12-13-2016, 07:09 AM   #9
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I always fill my motorhome propane tank at Tractor Supply Store. They have the best prices I have found. Anyway the other day I was there to get the propane filled and this young lady with a Tractor Supply vest on came out and as she was dragging the hose over she looked at the motorhome and said "are you all alone?" I couldn't pass that one up so I said "Just what do you mean by that?" where as she suddenly got all flustered and turned as red as Santa Clause. Then she stammered "I mean is everyone out, nobody sleeping inside...oh darn, you know what I mean". I let her off the hook and said "there is no living thing still inside, it is safe to fill the tank."
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Old 12-13-2016, 07:33 AM   #10
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Quote:
Originally Posted by LJowdy View Post
Yes, the biggest thing to know is it's Propane not butane. And yes to your questions. The tank needs room to expand when it gets hot, that's why it won't fill to the maximum

https://www.reference.com/home-garde...17473470876c30

It does not sound quite right to me that the 80% fill is to allow room for expansion (liquids do not expand that much). More likely (as noted in a later post) that the 80% fill is to allow headspace for vapor generation and to avoid liquid propane from getting in to the external hoses (Bad Thing). Anyone know for certain??
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Old 12-13-2016, 07:52 AM   #11
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I got a 500 gallon tank at the shop. I had it filled to 80% this summer when the temperature was 90 degrees. I opened the tank valve and turned the heat to in the shop last week. The ambient temperature was 38 degrees. The gauge read 72%.
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Old 12-13-2016, 07:57 AM   #12
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Vapor space

The 20% is the space for the propane to change from a liquid to a vapor state. You can't use propane as a liquid. Another concern is the propane (property) exchange rate changes with tank tempature, which mean in very cold outside tempatures your tank may not produce sufficient gaseous propane for the heater needs.

Now that is another story!
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Old 12-13-2016, 08:19 AM   #13
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ScottyBye -

Thanks for the cut-away pictures. They are very helpful!
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Old 12-13-2016, 08:23 AM   #14
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Quote:
Originally Posted by MN_Traveler View Post
It does not sound quite right to me that the 80% fill is to allow room for expansion (liquids do not expand that much). More likely (as noted in a later post) that the 80% fill is to allow headspace for vapor generation and to avoid liquid propane from getting in to the external hoses (Bad Thing). Anyone know for certain??
It is possible to out run the tanks ability to replace the vapor. It depends on the rate of use, square inches of surface area on top of liquid, headroom, temperature outside etc.

Yes you do not want to suck up liquid into the pipes.
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