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Old 03-24-2016, 07:42 PM   #1
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Proper way to fill the onboard propane?

Just had my propane tank filled for the first time by me and when he said it was full the gauge on the tank only showed 3/4. He just hooked the fill hose and ran it until the pressure sensor shut it off. Isn't there a procedure to open the vent while filling to get it truly full? Can't find anything on google or YouTube.

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Old 03-24-2016, 07:43 PM   #2
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LP tanks should never be more than 80% full.............

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Old 03-24-2016, 10:12 PM   #3
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When I get propane, mostly from the same place, they connect hose, start pump, opens the vent with screwdriver, closes vent when vapor comes out, shuts off pump and disconnects hose. Gage reads full and have never had an LP issue. What % full is it? I do not know and don't much care. If using a different filler, I watch and ask questions just so I feel it is being done correctly.

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Old 03-25-2016, 10:33 PM   #4
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Proper way to fill the onboard propane?

Yes, you have to open the vent to fill but they probably did. The reason the fill stopped is because the overpressurization (OP) valve kicked in. Once that hits, you are as full as you can get.

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Old 03-26-2016, 06:49 AM   #5
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The collective knowledge of this group is amazing. This is something I won't have to worry about now when it comes time for our first refill. Thanks to OP for bringing this up.
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Old 03-26-2016, 01:25 PM   #6
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They do usually vent it, but the 3/4 is correct. Have to leave room for the liquid gas to vaporize at the top of the tank.
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Old 03-26-2016, 02:12 PM   #7
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Pressure in an LP container (as long as there is both liquid and vapor present) is a function of the liquid's temperature only. Two identical containers, at the same temperature, one 80% full, one 20% full will both have the same pressure.
The 20% vapor space is so that the liquid can expand if it's temperature goes up - not for vapor to expand into. Propane in the liquid state expands as it gets warmer. If the liquid expands to completely fill the vapor space, hydrostatic pressure caused by the liquid acting on the inside walls of the tank will be far higher than vapor pressure at the same temperature. The tank can then rupture - or in the best case, the overpressure protection device operates.
OP is an overfill protection device - it automatically shuts the inlet valve when the tank's liquid level reaches 80%. It's a relatively recent requirement (on your cylinder, it's presence is identified by a triangular valve handle). It responds to liquid level, not pressure (it's not an overpressure valve). The small vent plug, part of the tank valve, has been on tanks since ??? ages ago it allows the tank to be purged by dealers, and can be opened to let the tank fill more quickly. Older tanks did not have automatic overfill protection devices and could be filled to over 80% by an inattentive operator who did not manually open this vent or walks away while the tank was filling.

Overpressure protection is a different device altogether - it's a fusible plug or disk type relief device. It is designed to open in case of exceptionally high pressure caused by such things as heating by fire. It will release tank pressure in a "controlled" manner and direction.

The absolutely correct way to fill a propane cylinder is by weight but most operators simply fill 'till the overfill protection stops the flow, or the vent starts spewing liquid. They then read the gallons pumped - avoiding the tare weight, Water capacity, specific gravity calculations needed for a proper "fill by weight" procedure.
Obviously, propane tanks permanently bolted in a motorcoach cannot be removed for weighing, so they should be installed with a liquid-level indicator. They also have overfill protection.
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Old 03-29-2016, 04:22 PM   #8
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On our 2000 Four Winds, 3/4 full on the gauge was full. On our new Sunseeker, it reads full. However, if you examine the physical gauge, you'll notice there's not much distance between 3/4 and full, then the distance expands for the 3/4 to empty range.

I fully understand the need for the 20% expansion room. Therefore, when the tank is 80% full, technically it is 100% full - as full as it is designed to be filled. I would expect the designers to engineer the gauge so it shows this 80% range as 0 to 100%. After all, when you fill your gas tank, your dash gauge doesn't read 3/4, right? Seems to me someone fell short in designing the gauge on these tanks.
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Old 03-29-2016, 04:31 PM   #9
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When mine is totally empty, 9 gal. fills it. Class C RV.

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