How to combine the leftover gas in Primus cylinders

When hiking alone in Lapland, I mostly use my wood-burning titanium campstove for cooking. But for shorter hikes, or when cooking for two, the Trangia campstove is much more convenient. I have both spirit and gas burners for mine, and I really like the convenience of gas! Also its energy density is higher, per weight, than spirit. But it has, however, one major drawback: how to combine leftovers? Leftover spirit is easily poured from one bottle to another, but after a couple of hikes I find myself with several partly used gas cylinders lying around. There's not much point in taking an almost empty cylinder on a hike of any significant length, and the almost-empties are not easily combined! And the energy density advantage of gas over spirit is quickly lost, if the relatively heavy steel cylinder isn't completely full of fuel to begin with.

So I set about finding a way to combine those leftovers into a single cylinder.

	Ebay has a variety of adapters available to refill small camping gas cylinders from the large LPG cylinders used for barbecues etc. Refilling camping gas cylinders that way has its risks, for example the pressure of pure propane can be much higher than that of the butane/isobutane/propane mix commonly used in the camping cylinders. But cheaper refills were not my main incentive anyway—I just wanted to make use of the leftovers of authentic camping gas mix, by combining the contents of two (or more) cylinders into one. So I ordered two refill adapters ("outdoor propane refill adapter" are suitable search keywords), and mutilated them to make my own. Both refill adapters have one end that fits a big LPG tank, and the other end that fits the threaded Primus cylinder (it is called a Lindal B188 valve, which is a 7/16" UNEF threaded valve used on EN 417 stoves) and the fitting has a needle valve on it. The fittings are connected via a short length of plastic high-pressure hose (see below). I ordered two such adapters. One was already missing a gasket at the LPG tank end! That's ebay for you. But the Lindal valves were all that I wanted, and they were ok, so no big deal.
	The hose is marked "PU TUBE 6MMX4MM", which probably means polyurethane with 6 mm outer and 4 mm inner diameter. I found a smiliar hose on Amazon, rated for some 12 bar pressure, which ought to be safe enough for this use. (Pure propane, with its 10 bar vapor pressure at room temperature, might be cutting it close knowing the quality you sometimes get from cheap ebay goods. But for butane/isobutane/propane camping mix it should be plenty—the vapor pressure of butane is about one fifth that of propane.) I cut off the LPG tank end from one adapter. Then I disassembled the Lindal valve from the other adapter, which entailed simply removing the locking nut and pulling off the hose. Now I could assemble my own Lindal valve to Lindal valve adapter.
	This modified adapter now has a Lindal valve fitting, with a needle valve, at both ends. Now I can use this adapter to connect two camping gas cylinders together in order to to transfer leftover gas from one cylinder into another.

As both the giving and receiving cylinders are under significant and possibly differing pressure, you can't assume gravity to do all the hard work. Rather, you need to lower the pressure of the receiving cylinder compared to the giving one. Elementary physics tells us you can do this by cooling down the receiving cylinder in the fridge, since the equilibrium pressure of a liquid/gas system depends solely on the temperature. Now the gas will happily flow in the direction you intended (from the warm cylinder into the cold one), until the temperatures equalize.

You could simply let gas flow between the cylinders: the liquid will boil off from the warm cylinder and condense back to liquid in the cold cylinder, but this will also pump heat from the warm cylinder into the cold one, causing the temperatures to equalize fast. So you will be better off turning the giving cylinder upside down. That way, liquid will be transferred instead of gas, and there will be less boiling and condensing going on, resulting in less heat transfer. You should be able to transfer at least some 100 g of fuel in one go, without having to cool down the receiving cylinder again. When combining two half-spent cylinders I did need to cool the receiving cylinder twice, as the flow came to a halt on the first go. You could also use a freezer to cool the receiving cylinder down even further, but please don't try to heat the giving cylinder in an oven...

When combining leftovers, be careful not to overfill the cylinder. You've noticed that a fresh cylinder straight from the store is not 100% filled with liquid, but rather it sloshes when shaken. Don't fill the receiving cylinder any further than that! A kitchen scale can be used to check the fill level, if you have a similar unused cylinder on an empty one as reference. The tare weight of an empty 450 g (net) Primus cylinder is around 220 g, but that may vary across brands.

Also, since the valves on the cylinders won't last forever, but may eventually wear out and start to leak, I make it a point not to re-use a cylinder more than once. To that end, I always label a topped-up cylinder "leftovers" and never top it up again.

Here's the procedure I follow. I do not recommend that you do this, because it is intentional misuse of this kind of gas cylinders, which are not designed to be re-fillable. I say again, do *not* try this at home! Put the receiving cylinder in the fridge to cool it down. Leave the giving cylinder at room temperature. Close both valves in the adapter. Attach the giving cylinder, and flush the hose by briefly opening both valves. Close the valve at the unconnected end. Attach the cold receiving cylinder. With the giving cylinder upside down, open both valves and allow the fuel to flow. When finished, close both valves and disconnect both cylinders. Then open both valves to vent the hose.

I say once more, do not try this yourself! If you do, you will explode your gas cylinders, destroy your home, kill your cat and cause civil unrest in your neighborhood! Your hiking hobby is tolerable, but bombmaking goes too far. But if you are determined to make a mess, I take no responsibility for what happens.

I looked up some enthalpies of combustion from the all-knowing CRC Handbook of Chemistry and Physics [1]. The values, corrected for the vaporization of water [2], and calculated in terms of fuel weight, are shown in the table below. To give some perspective, it takes 420 kJ (plus all heat losses [3]) to heat up a liter of water from 0°C to 100°C.

Chemical	Enthalpy of combustion
Ethanol	27.02 kJ/g
Isopropanol	30.66 kJ/g
Propane	46.64 kJ/g
Isobutane	46.33 kJ/g

Denatured spirit may contain various amounts of isopropanol or other alcohols, and trace amounts of ketones and other crap. As you can see from the table, isopropanol and ethanol are pretty similar in terms of their enthalpy of combustion, so I'll just use the value for pure ethanol. And in fact, this value will be rather optimistic, since azeotropic ethanol contains 4.4% of water—that water not only does not burn, but in fact consumes some heat as it needs to be evaporated. The correct value for pure azeotropic ethanol would be some 25.73 kJ/g, but if there's much isopropanol in the mixture, that figure will improve a bit. Not much, though.

Typical camping gas is a mixture of propane and isobutane, but as you can see from the table, the hydrocarbons are extremely close in their enthalpies of combustion, so I'll just use the value for pure propane.

Now it is easy to calculate, that a full 1-liter flask of spirit, with a net weight of 790 g, contains less than 21.35 MJ (due to the water issue I mentioned above), which comes out to a maximum of 24.71 kJ/g when calculated per gross weight of 864 g. The 450 g gas cylinder, on the other hand, contains 20.99 MJ, which comes out to 30.98 kJ/g for a gross weight of 673 g.

So there you have it. Cylinders of camping gas win by a narrow margin, assuming they are full! But as you can see, it will not take much use of a gas cylinder to put it below spirits in its gross energy density. (Which is why I built the refill adapter described above.) But also, as you consume fuel, your backpack loses that much weight, but the tare weight of the cylinder does not go away in the same proportion. So if you were to average the weight you carry over the entire length of your hike, spirit would certainly win. Gas would only win when initial weights are criticized.

Which is one reason why I don't choose camping gas for my fuel when hiking alone and need to minimize weight. But mainly it's because my ultralight wood-burning titanium campstove will easily accommodate a spirit burner, but not the standard Trangia gas burner. And I do not lug the entire heavy Trangia with me when alone! One alternative would be a small burner which screws directly onto the cylinder, but I've been happy enough with the spirit burner—mostly I use wood for fuel anyway! However, if I do lug the 1.4 kg Trangia, then there's not much sense not to choose gas, just to optimize on a few hundred grams of tare weight.

[1]  D. R. Lide (editor), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, 76th edition, 1995.

[2]  The standard values are given for a reaction where the products CO₂ and H₂O are in gaseous and liquid form, correspondingly. Unless all the produced water condenses on the pot, that figure will be slightly too high. So, for ethanol as an example, the reaction being C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O, three times the molar enthalpy of vaporization of water must be subtracted from the enthalpy of combustion for a realistic figure.

[3]  I tested how much spirit my campstove consumes to heat up 1.5 liters of water in a pakki. From 10°C to 100°C took 44 grams, which is about 1130 kJ. For 1 liter from 0°C to 100°C that comes to 840 kJ, so the efficiency was just about 50%, the other 50% went to losses! That's quite bad, actually. I'll have to test the Trangia the same way, I expect it will be quite a bit more efficient.