This is a specialised gas stove designed for cold conditions. It has a three-legged burner and a 140 mm (5.5″) flexible hose leading to a separate gas cartridge, and the matching Powermax cartridges are custom to this series of stoves. They are different from the standard screw-thread butane/propane gas cartridges in both shape and connector. The Powermax connection is also different from the French CampingGaz one. While it is a ‘twist-click’ connection similar to the French one, the two have different valve diameters and are not compatible (I’ve tried).
The stove has a pre-heat or ‘generator’ tube just before the jet. Coleman refers to this in some of their literature as their HPX Anti-Flare system, but it is no different from what is used on any petrol or kerosene stove. The liquid fuel goes through this tube before reaching the burner.
The construction appears to be magnesium, brass and stainless steel. I suspect the pre-heat tube (blue line) may be Inconel, a special high-temperature alloy. The stove has three (magnesium) legs, each with a stainless steel wire arm with serrations. These legs rotate together to fold up for packing. Opened out they give a support diameter of 150 mm (6″). The serrations serve to prevent pots from sliding off, and are quite effective. Unlike many other stoves, the tops of the arms do not make a flat surface: it is concave. For small pots this means there is a strong centering force. The bottom ends of the legs have plastic feet which are moderately effective at preventing slipping. The manufacturer advises that the diameter of the pot should not exceed 200 mm (8″) and the weight should not exceed 3.6 kg (8 lb).
The hose between the stove and the control valve or coupling unit seems to consist of heavy rubber sheathed with stainless steel braid, and the ends are protected by heatshrink tubing. The coupling unit seems to be a magnesium casting. There is a single control valve (green line) located on the coupling. This is on the liquid side of the pre-heat tube, so it valves liquid. The control is done by a conventional needle valve in the coupling, and is actuated by the large black knob. There is a wire leg at the coupling which I assume is meant to stabilise the coupling and bottle, although it isn’t necessary in the snow.
A nylon packcloth bag is provided with the stove. The stove legs have to be collapsed together and the hose has to be folded around the body to get the stove into this bag. However, the bag is heavy and I have replaced it with a lighter one as I could see no benefit from the extra weight.
The instructions which came with the stove were in English, French and Spanish. They were fairly comprehensive.
Product Information – Powermax Cartridge
|Measured Weight, gross:||386 g (13.6 oz)||240 g (8.5 oz)|
|Quoted Gas Weight:||300 g (10.6 oz)||170 g (6.0 oz)|
|Measured Diameter:||65 mm (2.6 “)||65 mm (2.6 “)|
|Measured Length:||220 mm (8.7 “)||135 mm (5.3 “)|
|Gas Composition:||60% butane, 40% propane|
|MSRP:||Not found on web site|
Product description – Powermax Cartridge
The cartridge is a rather beautiful slender anodised aluminium tube, with its own ‘push and twist’ connector. One might wonder why Coleman chose to make it different from the other ‘standard’ cartridges on the market: the screw-thread one originally developed by Epigas and the ‘Twist-Click’ one developed by CampingGaz. I believe this was done because the design of the stove really requires these custom Powermax cartridges for proper operation in the snow. That is, the design of the Xtreme uses a liquid feed, and Coleman did not want a user to put an ordinary gas-feed cartridge on it.
So where did Coleman get these beautiful cartridges? The answer becomes apparent when one contemplates large hair spray canisters in the shops. Some of them are identical in shape and size – and also anodised brightly. What’s more, hair spray cartridges are rated to take butane or propane pressures because the propellant in them is, or used to be, butane or propane. In fact, the Lindal valve used in these Powermax cartridges is essentially the same as those used on most pressure pack spray cans around the world.
Coleman has redesigned the feed mechanism inside this cartridge. The Powermax cartridge has a special modification to the conventional Lindal valve. There is a long steel extension tube (green line) with a flexible connection (blue line) to the inside of the valve (purple line), as shown to the right. One can hear this going ‘clank’ when the cartridge is shaken. Fortunately, the end of the steel tube has a little bit of plastic over it to prevent it wearing a hole in the thin aluminium wall. The end of the tube flops to the bottom of the cartridge as it lies on its side, and this draws liquid fuel from the bottom of the cartridge. A very small amount of propane boiling inside the cartridge provides the pressure to drive the fuel to the stove, but the amount is microscopic. The gas in the cartridge does not cool down by evaporation during use, and the mixture or ratio of propane to butane stays essentially constant right to the end. Once the liquid fuel reaches the generator tube it is vaporised (using energy from the flame), mixed with air and burnt. This is absolutely the same process as with a petrol or kerosene stove. Doing this does not subject the stove to any exceptional conditions. Since propane boils at -40 C (-40 F), this means the cartridge will still be pressurised under very cold conditions.
Two other points should be made about the Powermax cartridges. The first is that the 60/40 gas mix used has the highest percentage of propane of all the cartridges I know, and that makes it extremely well adapted to the cold. Propane boils at -40 C (-40 F); Coleman claim the system will work well down to -20 C (-4 F). With a little care I would be confident in using the system in an ambient down to -30 C (-22 F).
The other point is that Coleman provides a little hard metal spike with the stove, which is designed for puncturing the empty cartridges to make them safe for recycling. They call this a ‘Green key’. Many screw-thread cartridge makers warn against any such puncturing, making disposal of the cartridges difficult.
Since I use this stove in the snow I should explain how and why. I will start with a description of the problem faced by ordinary upright gas stoves in the cold to illustrate the problem this stove solves. The initial problem is that butane boils at -0.5 C (31 F), and the process of boiling extracts heat from the liquid. This means that butane in the snow would rapidly drop below its boiling point and a ‘pure butane’ stove would stop working. (The puncture cartridges are usually pure butane.) So the manufacturers add propane in the screw-thread cartridges, and this boils at -40 C (-40 F). Now there is some gas which can boil off even at very low temperatures. A typical screw-thread cartridge contains 70% butane and 30% propane, although there are variations. However, this does not solve the whole problem. On a cold day – say around -10 C (14 F), the propane will boil off but the butane (below its boiling point) will sit there in the bottom of the cartridge doing relatively little. While the gas is evaporating the remaining liquid is getting steadily colder as the energy or heat required for the boiling comes out of the liquid. Carried too far, the liquid can easily drop to 10 – 15 degrees C (20 – 25 F) below ambient. As a result the propane can get all used up and the cartridge is left with lots of liquid butane but not giving any gas out. This is why some people think gas stoves don’t work in the snow: they simply haven’t managed their stove properly. They haven’t kept the cartridge warm. (Actually, the physics is a shade more complex, but this explanation will do here.)
This stove has two design features for use in the snow. The first is the non-evaporating liquid feed from the bottom of the cartridge; the second is the really quite conventional pre-heat tube on the burner. In effect, this is a liquid-feed gas stove. The analogy goes further: butane and propane are just lighter fractions from the same refining process which gives petrol and kerosene. There are all long-chain hydrocarbons.
As mentioned above, with ordinary upright gas stoves one has to be careful to keep the cartridge slightly warm in the snow, otherwise the propane will be all used up before the butane. This does not happen with this design of stove, so the question arises as to whether the cartridge needs insulation and warmth. My experience is that it does not, at least down to -20 C (-4 F). In fact, I usually leave it lying on the snow.
The biggest problem with this stove is attaching the gas cartridge. The valve fitting has to be pushed firmly into the coupling and given a one-eighth turn to make the connection. This actually takes a bit of practice before it happens easily. It has happened that I have thought I had done it correctly, only to find that the connection was loose. Fortunately, the design is such that the Lindal valve inside the cartridge is not activated (depressed) until the connection has been properly made. I have never found gas leaking out from this connection. In some ways this is actually better than the standard screw-thread connection, as I have found some conventional stoves which can leak gas slightly from the join if they are not screwed down properly. Anyhow, once I had done it a few times I got the knack and it does not worry me at all. It is not hard to recognise a poor connection: opening the valve does not let any gas out.
The second biggest problem with this stove is the rigidity of the hose. It is flexible, but it is not floppy. For safety reasons the strength of the hose is a good thing of course. But the stove itself is very light, and it is possible for the hose to make the stove stand on only two legs. This did happen a bit when I first received the stove, and this made it less than stable. However, I found that it is possible to just slightly slacken off the lock nut (purple line, first picture) which anchors the rigid part of the hose to the stove so the stove can be rotated with respect to the hose and cartridge. This let me adjust the alignment so the stove did sit square on all three legs. I then carefully tightened up the connection. It would not do to leave this connection loose. Since doing this the stove has sat squarely on all three legs, once I open the hose out properly. Again, once adjusted this ceased to be a problem.
To fire up the stove, I first lay out my stove base: a piece of 3-ply about 200 mm (8″) square. Smaller would suffice. This gives me a stable base for the stove; I do not like having my stove on a wobbly footing. Next I open out the two rotatable legs to the ‘click-stops’ and sit the stove on the base. I then remove the plastic cover from the valve on the gas cartridge, insert the neck of the cartridge into the coupling, press firmly and rotate until I hear a bit of a click. (The plastic cover on the gas cartridge is invaluable for keeping dirt out of the connection.) Then I put a windshield around the stove, both for safety and for fuel efficiency. Finally, I turn the valve (the black knob) on gently and light the gas. If nothing happens I turn the valve off and repeat the assembly steps. Once the flame is burning I turn the liquid flow up: this avoids any chance of an initial flare-up. I would emphasise here that I do normally start my stove inside the tent: done at a low setting there is no risk of a flare up in my experience.
To alter the burn rate I turn the valve slowly, and not too much at a time. The reason for the caution is that the hose is usually partly full of liquid fuel, and changes do seem to take just a little while to propagate through the system. Well, a few seconds delay is usually enough. The stove can go from a very low simmer to quite a fierce roar. I have not found any problems with flame lift-off, as reported in my Review of the Trekka Stove. At medium to high settings the flame does make a little ‘propane’ noise: a sort of faint roar. It’s very comforting as it means dinner will soon be ready.
The pot stand diameter of 150 mm (6″) is quite large enough in practice. I use a Trangia kettle for boiling water for tea and coffee, and sometimes to help with dinner, and this is 150 mm (6″) in diameter. I cook dinner in a light stainless steel pot for the two of us (myself and my wife) and this has an outside diameter of 180 mm (7″). The stove supports both the kettle and the pot very well; neither gets anywhere near the load limit!
I was given this stove by Coleman while writing a stove review for a local (Australian) walking magazine. It is hard to say whether I would have bought it without prior use as it is much heavier than my much-loved Snow Peak GS(T)-100stove. However, having since studied the problems associated with gas stoves in the snow, I find I am much more confident using this one when it is very cold as the design eliminates the normal problems. It is now my preferred stove for winter ski touring. I have to report that the cartridges, while very light, are not so widely available (in Australia), so I carry a couple in stock at home. I don’t use it in the summer; I use my GST-100 instead.
I carry the Xtreme in a light nylon bag. Sometimes I just pack it in with other soft things in my pack; other times I carry it inside my cooking pot. It seems to survive quite well – but I do take good care of it. Actually, when folded up it makes a rather awkward 3-D object due to the stiff hose. However, this is a minor quibble.
A close look at the neck of the cartridge in the first picture will show the number 374 written on it. This was the last measured weight of the cartridge. By subtracting the known empty weight of the cartridge (86 g), I know how much gas is still there. I also know I use about 45 g per day for my wife and myself in the snow, so I know that this cartridge will last for another 6 nights. I do however always carry extra in case I have to melt snow. But at least I do know how much gas I have and use.
The special precaution to be taken with any stove with a hose is to treat the hose with much care. If it is damaged and leaks, a rather hot problem ensues! Cleanliness at the coupling between stove and fuel supply is important. With some simpler gas stoves I remove the controlling needle valve occasionally and grease it; I have not done so with this stove yet as the control unit looks a bit more complex to reassemble – and so far it hasn’t needed it either. Finally, just a little care is needed during set-up to make sure the stove is stable on its three legs.
Because I am paranoid I carry spare small O-rings for the connection. They are threaded on the wire stand on the valve housing. However, I have never needed to use them.
|Cold weather reliability||Increased weight|
|Ease of assembly||Cost|
|Large pot stand||Stiff hose (mild)|
Would I buy another? Quite possibly, as it is just so reliable in the snow. However, there are several other stoves which can use screw-thread cartridges tipped upside down which might also serve, such as the SnowPeak GS-200 and a Primus one.