Mh propane capacity discrepency

Rule #1: As others have mentioned, the tank's listed capacity is not the actual capacity that you will be able to fill that tank with propane at a refilling station.

Rule #2: The gauges in the RV and on the tank are not calibrated so they could be off. Use those gauges as a general indication of the capacity level but never assume they are 100% accurate, so don't base what you think it must hold on those gauges.

Rule #3: You need to know how your tank operates, where things are located and what all the valves and opening do. You never want to find yourself at a fillings station being given some bad advice because you're not familiar with the tank. Likewise, you don't want to assume something is or isn't being done properly because you're not familiar with your tank or it's operation.

Rule #4: If a refilling station isn't following proper refilling procedures (like ensuring the overfill mechanism is being used) and you keep running into the same problem over and over again where ever you go, either you really need to stop refilling the RV from the guy doing it at 2 am in the morning from the back of his mini-van behind the closed K-mart or maybe the proper procedures are happening and you're just not familiar with them on your RV tank.

Rule 5: If the propane filling station is not following proper refilling procedures, STOP them and go elsewhere. You should never accept anyone messing with your propane tank who isn't doing it properly. If you're not sure what they should be doing, learn! If your not sure they have done something, ask!

Don't know about you but I certainly don't want to be on the evening news or on some Youtube RV Propane Fail video.

down home wrote:
Clerk who filled it this am suggested letting it run out and opening petcock somewhere on the tank to let moisture out. I think he must have meant lube. Neither seems reasonable.

You are correct!

IMO, it is NEVER a good idea to intentionally allow a refillable propane/butane tank to go empty and remain at at/open to atmospheric pressure for any length of time. That's what gives moisture (and other contaminants) an opportunity to enter the tank. (unless you suspect that the tank is already contaminated) If so, the tank should be professionally inspected before filling/use.

Chum lee

Both Butane (R-600a) and Propane (R-290) make excellent refrigerants. They have a zero factor of ozone depletion and are very good at returning lubricating oil back to the compressor that gets past the rings. Of course the BIG problem is their flammability.

At 0 psig Propane boils at -42C degrees (-44F) while Butane boils at -0.4C degrees (31F). Isobutane boils at -11.75C degrees (11F). So you can see, propane is much better for colder climates. It's possible to get the tank cold enough for it go into a vacuum - very undesirable!

Butane vs Propane vs Isobutane - What is Butane?

Here's a chart showing the pressure/temperature of mixtures of butane and propane.
Propane Butane Mixtures - Evaporation Pressures

I have noticed the attendants have not open the bleeder screw any of the times it has been filled here in Az and I can't remember if they have other times elsewhere.
The tank/pump stopped at exactly 24 gallons which seemed odd also.
The last time e had it filled there we didn't think it wa right and went to Flyin J, which is a $1.50 per gallon higher, to have it topped off but their pump wouldn't add any more propane either. However at an indicated app o1/4 tank n the control panel and on the, at the same Flyin J the time before that it too28. something gallons.

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Quite often you are getting a blend of propane and butane. Butane while richer in energy (BTU's) boils at a lower temperature therefore the pressure at any given temperature will be lower than propane. If you fill at a place very far south in the wintertime you may have a butane rich gas that will not have enough vapor pressure if you drive far north to use it. This has happened to people who fill in Mexico and then drive to high altitude in January.

dougrainer wrote:
Explain this. It is the standard Float Level unit for ASME tanks. It is what I have seen and repaired for 40 years. While it does use Magnets, it IS Mechanical. Doug

What I'm saying is there is no "direct mechanical linkage" between the float and the gauge needle itself. They are both attached to magnets separated by a layer of brass. Just as if you were to place magnet under a piece of glass and lay another on top, moving the bottom one will cause the top one to move. The movement will not be completely smooth all the time. It moves in spurts only when the pull of the lower magnet overcomes the drag is overcome of the one on top of the glass. It's not as if the float was directly directly turning the same shaft the gauge needle is connected to - they are separated by a layer of brass machined under the gauge dial in threaded plug housing which makes the gauge leak proof.



When the tank is being filled and the purge valve is open - that's not "air" that's being purged, it's propane in its gaseous form. All the appliances use propane in this form. The liquid propane is boiled off before it leaves the tank.

You can use this pressure - temperature chart to see the pressure in the tank at various pressures in the tank and viceversa: HC-290 (PROPANE) PRESSURE - TEMPERATURE CHART

On a correctly purged LP tank WHEN NEW, there should be NO AIR in any LP tank DOT or ASME after that. But, air can be introduced when the tank has been serviced or repaired. IF that happens you tell the LP station and they will PURGE the tank to get rid of any possible air. BUT, if a tank has never been "messed" with there is no real way to introduce AIR into the tank. The filling nozzles are connected and only then is LP liquid installed. The gauges are NOT that accurate. You can still have a few gallons even when the tank is showing empty. Doug

DFord wrote:
There is no mechanical linkage between the float and the gauge. It's done with magnets. There's a lot of slop there. Don't expect to achieve a lot of repeatability. It's only designed to give you a rough estimate of the level of liquid propane left in your tank. When filled to 80%, your tank is full and you shouldn't put any more than that in it. If the gauge was completely mechanical, it would be venerable to leaking and that can't be tolerated because of the nature of propane.

Explain this. It is the standard Float Level unit for ASME tanks. It is what I have seen and repaired for 40 years. While it does use Magnets, it IS Mechanical. Doug

https://rochestergauges.com/product/6200/

The OP is using 1/4 etc, suggesting he is using the LP monitor on his range hood and not the actual gauge on the tank. As noted above, "full" and "half" are not the same on the two.

The lowest light on the monitor indicates from when it first turns to only that light and stays that way until all the LP is gone, so how much it takes to refill varies. Also ISTR that amount varies "as corrected" for temperature.