How to combine the leftover gas in Primus cylinders

Risky business! Don't do this!

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! 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!

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. 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.

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 end compared to the giving end. 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 all of the fuel in one go, without having to cool down the receiving end 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.

Antti J. Niskanen <uuki@iki.fi>