The Evolution of a Winter Stove – Part 3

[Bob Gross]

Roger, is there any necessity for a world wide product recall?

–B.G.–

[Roger Caffin]

Hi Jan

If that photo was taken with the pin removed and the cam valve in place with the retainer plate screwed in properly, I am a totally puzzled by the position of it. It just looks wrong. It should NEVER get there.

Actually, I am really puzzled by the whole photo. Could you send me the original photo without cropping please.

If you wish to post the stove back I will check it out and fix whatever the problem is, no charge. It really is not meant to do that.

Cheers
[email protected]

[Bob Gross]

That couldn't have been the result of a customs inspection, could it?

–B.G.–

[Roger Caffin]

Hi Jan

> I would prefer to have the on and off positions of the lever to be left and
> right (with labels on the sides of the black block) rather than up and down.
Yes, I thought of this, but it failed one test.

7 pm, bad weather & snow & wind, very cold & tired, dim light, incipient hypothermia from pitching the tent. Goes to set up stove. Which way is OFF? Left or right? Can't even read the writing.

Solution: handle out of canister, pin out; handle in towards canister: pin into canister.

Gas is dangerous. I never ever assemble the stove without checking: safety valve off, needle valve gently closed.

Cheers

[Jan Rezac]

Roger, I sent the larger photo via e-mail.

I'm curious whether somebody else can reproduce it. Here are the steps that would lead to this problem if it affects your stove (do not attach the cannister):

1) Set the cam valve to the "off" position (lever points away from the threaded connector)

2) Position the whole thing so that the canister connector end points downward. Gravity pulls the brass pin down, out of the cam. Press the cam valve (the white knob with the lever) inwards. In my stove, it is possible to move it in and out a bit, a little force is needed to overcome the friction of the o-ring inside.

3) Turn the assembly upside down and look into the connector. Turn the cam valve lever – if the brass pin in the connector moves up and down, everything is ok. If it does not (it is stuck in the "on" position), congratulations, you have reproduced the problem. Let us know.

[Roger Caffin]

The photo jan sent me was excellent. I was confused by the small photo shown above.

I may have accidentally included in the pile of white valves for assembly one early unit which was not to spec. Such a unit would cause this problem. I await reports from anyone else who has this problem. In the meantime, the dimensions in the master drawings and CNC programs have been tweaked slightly to prevent this from happening again. It's a beta test after all!

Jan's solution used a bit of tubing, but not everyone will have something suitable. If your stove shows signs of this problem, there is a much simpler cure (which has been tested). Take a short bit of 1 to 1.2 mm copper wire and form the letter C. Round the two ends so they are not scratchy. The OD should be about 7 mm, and must be < 8 mm. It has to fit easily into the 8 mm bore in the block. The two ends of the wire should not touch: it's a C, not an O. Remove the connector (do NOT lose the screws!!!), drop the loop down the hole, jiggle it flat, replace the connector, replace the screws. Caution: do not use hard wire, and do **NOT** scratch the inside of the 8 mm bore! The loop should fall in easily.

Cheers
Roger

[Jan Rezac]

After fixing the problem with the valve, I moved to hacking the stove so that it can be used with lightweight butane canisters (for refilling lighters). The stove with a full canister (50 g of gas) weights less than 160 g!

I created a separate thread for this topic in the MYOG section:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=83643

[Roger Caffin]

> I moved to hacking the stove so that it can be used with lightweight butane canisters
> (for refilling lighters).
Oh My Gawd. My stove has been hacked already!
I suppose I have to see this as a compliment … :-)

(Jan and I have been discussing this by email. Interesting ideas kicked around.)

Cheers

[Anthony Britner]

I'd certainly be interested, mostly in the 116T version without the burner as I already have one. It seems to make sense to use the burner I already have available.

[Roger Caffin]

email me.

Cheers
[email protected]

[Roger Caffin]

Hi all

Yes, I have received some feedback from beta-testers. The resolved issues are:

* Tab at bottom of leg used for opening them could be bigger, for gloves.
Good idea, and I have updated the drawings and the programs. I will be making some more legs soon, with the bigger tabs.

* Brass pin in canister connector not retracting.
This was probably partly due to a bad clash of tolerances, but also to the white cam valve being pushed a long way into the canister connector while the brass pin was fully out. The user solved that problem by pulling the white valve back out by a millimetre or two using the Ti wire handle. Should any other user have this problem, please contact me. The cure in the field is to pull the valve back out a millimetre or two – it won't come right out as the retainer plate holds it in. But I do have an extremely simple 'fix' for the problem as well.

* Tiny flame
User cleaned out the needle valve, removing two little bits of aluminium swarf. My fault, apologies, more care with cleaning needed.

* Filler cord jammed and broke when user tried to extract it from hose
This required a detailed examination of the hose, which was returned to me. It turned out that the clearances I had been using were a shade small for comfort. I had to buy some special drill bits to deal with this: 1.25 mm and 1.35 mm. They exist, they just are not common. However, with those drill bits the problem was eliminated. All future stoves will have the improved clearances. Anyone with an existing stove who want the upgrade should contact me. You will need to remove the hose and post it back to me.

There is one more bug which has yet to be fixed: I am waiting on the parts being returned.

Meanwhile, I have repaired my CNC machine and started machining again. Happy days!

Cheers

[Jeff McWilliams]

On about 10/09, jerry adams stated he felt the stove would be good down to about 12F.

Is that widely regarded as an accurate estimate of its lower range of usability?

I'm planning a trip to the Adirondacks at the end of the year. Last year's temps in the area averaged 15F. Accounting for elevation change of 2000', that's about 11F in the proposed camp area. However, several days saw lows of 0F to -3F, which could be almost -9F at elevation. If an inverted canister is useless at that altitude, I guess I need to take a white gas stove regardless of the "cool factor" of Roger's stove.

I have the 116T burner version, and was sort of hoping to take it with me, but not at risk of being unable to melt snow and boil water.

[Douglas Frick]

> On about 10/09, jerry adams stated he felt the stove would be good down to about 12F.
> Is that widely regarded as an accurate estimate of its lower range of usability?

It's possible Jerry was referring to the Winter Stove running from a pure-butane canister, or perhaps the original Fire Maple canister-top stove, without any form of warmth being transferred to the canister. (+12F is the approximate boiling point of iso-butane.) The context of his post wasn't quite clear.

However, +12F is NOT a lower limit for an inverted-canister iso-butane/propane stove, such as this Winter Stove. That, in fact, is exactly why and how Roger's design differs from the original Fire Maple stove. As long as the propane in the canister can vaporize (the approximate boiling point of propane is -40F), then the pressure from the propane gas at the 'top' of the canister will force the iso-butane/propane liquid out of the (inverted) canister valve and down the fuel line.

I wouldn't expect the stove to run quite as low as -40F without some help, however. My Coleman Exponent stove gets a bit balky below -30F under worst-case conditions: canister sitting out all night, exposed to the air and sky and insulated from the ground, then allowed to cool even more as the stove runs. However, simply putting the canister in a shallow pan of cold water (or even packed in relatively-warm ice or snow) should allow the Winter Stove to run when the air temperature is below -40F.

[Jerry Adams]

According to wikipedia, the boiling point of isobutane is 8 to 16 F.

If you have propane and isobutane, and operate at the boiling point of isobutane, the propane will evaporate more than the isobutane so after a while, you will be left with mostly isobutane. What do they have, 15% propane? You should be able to use that much.

Then, with an inverted stove, it will only work above the boiling point of isobutane, which I said might be 12 F. Maybe you could go down as low as 8 F.

This is theortical, I haven't used an inverted stove. Maybe the actual boiling point of your canister is different. It also depends on altitude I believe.

You can always use one of the techniques to warm canister, like with body heat. An inverted stove would work much better because it has very little evaporative cooling as happens with upright.

[Douglas Frick]

Jerry, the problem you describe applies to canister-top stoves (canister sits upright, with burner on top) operating in the +12F range, where the propane alone boils and burns off, leaving 3/4 canister of (useless) liquid iso-butane. However, the key to the liquid-fed Winter Stove working at well below the boiling point of iso-butane is that the canister is physically _inverted_ after the stove warms up: the "top" of the canister (with the valve) is pointing down, and the "bottom" (concave) part of the canister is facing up.

As long as the temperature of the canister is above -40F (boiling point of propane), then as the pressure in the canister drops, some of the propane will boil out of the liquid iso-butane/propane mix. The propane gas floats upwards, and so occupies the space between the liquid mix and the uppermost part of the canister (its concave bottom). There's no way out for the propane gas, so the pressure forces the liquid iso-butane/propane _mix_ out of the valve (which is the lowest point), along the fuel line to the stove, where heat from the lit stove (transferred through the heat shunt or a vaporizer loop) converts the liquid mix into vapor, which burns.

So, the stove should work down to the boiling point of propane. Once all the liquid iso-butane/propane mix has been pushed out of the canister's valve, its last gasp will be mostly pure propane gas, leaving an empty canister.

[Stuart R]

Douglas, you need to read about the effect of cold on gas canisters

The gas above the liquid mix does contain propane, but it also contains butane (and/or iso-butane, depending on the contants of the canister). As a rough analogy, think about air – it contains water vapour at temperatures well below the boiling point of water. The proportion of propane depends on the constituents of the liquid mix, but is typically 60-70% propane. It does not depend much on temperature – the percent propane gas will be almost the same at -40F as at +40F. This is not the same as the air/water vapour analogy, but then the boiling point of water is vastly different from oxygen and nitrogen.

"So, the stove should work down to the boiling point of propane."

Definitely not.
The minimum operating temperature will depend on the pressure generated by the aforementioned propane/(iso-)butane gas mixture. For canisters such as MRS IsoPro Red (20/80% propane/iso-butane), Coleman (30/70% propane/n-butane) and Primus Power Gas (25/25/50% propane/iso-butane/n-butane) you can expect the stove to work with an inverted canister down to 0F (-18C).

[Roger Caffin]

I'm with Stuart here.

One of the most common mistakes is to think that the propane in a butane/propane mixture will behave the same way as 100% propane. It won't. Read the article Stuart referenced – yeah, he and I wrote it!

Cheers

[Douglas Frick]

Oh well, back to school…

[Jeff McWilliams]

So, if I understood the article that Stuart and Roger wrote, it says,

"When the canister is used upside-down (inverted) with a stove which has been designed for this mode of operation, the effect of decreasing % propane described above is avoided."

Does that mean I could look at Figure 4: Threshold Temp(C) vs Gas Remaining at given Pressure, and assume 100%?

At 3000', that would give 20/80 a threshold temp of about -26.5C, and 30/70 a threshold temp of -30C.

Is that correct?

I like the extra "oomph" of Kovea's 30/70 compared to 20/80, which is what MSR IsoPro Red and JetBoil (I think) contains. Unfortunately, I've not seen Kovea canister fuel for sale anywhere in my area.

[Stuart R]

Learning is good. To be fair, most of the referenced article is about upright stoves, as that is where there are interesting effects that need explanation. However there is a paragraph about inverted canisters. In essence, using a canister inverted preserves the propane % content and so the boiling point of the mixture remains at the "100% remaining" point shown on the graphs. In other words, the minimum operating temperature is preserved throughout the life of the canister, unlike the upright canister situation.

[Jerry Adams]

"Jerry, the problem you describe applies to canister-top stoves (canister sits upright, with burner on top) operating in the +12F range, where the propane alone boils and burns off, leaving 3/4 canister of (useless) liquid iso-butane"

Oh… Good point. Another reason for using an inverted stove : )

Having 20% propane for an upright doesn't make that much difference, although if you just had one or maybe two really cold nights it would be pretty uesful.

[Stuart R]

Iso-butane is not useless. In fact, if you are using an upright stove you want the highest proportion of iso-butane that you can get: MSR IsoPro or Jetboil are good candidates. It's regular n-butane that is useless in an upright stove in winter.

On the other hand, with an inverted canister, butane vs iso-butane doesn't matter much, it's a high propane content you want.

Reason: with an upright stove, any propane is quickly used up, so it's what's left (butane or iso-butane) that matters. With an inverted canister, the propane is preserved (in % terms), and the more propane the better for low temperature use.

[James Marco]

Propane is a slightly smaller molecule than butane, so it tends to concentrate slightly near the bottom. So it isn't quite 100%, but clse enough not to worry. Gravitional effects are very slight compared with solutes and usually ignored.

Pressure changes (as in a closed canister or one running at constant low power) will dictate that canister temperture will change for "toppers." The liquified gas never really boils in a pressure vessel. Rather, the vapor pressure/gas pressure is maintained at an equivalency, at the temperture it is currently at. (This is why we don't ever hear a canister burping and boiling inside.) By drawing from the bottom, in liquid form, we reduce the effect of cooling on the container, since the liquid has 1/200-1/300 the volume of the gas(given some delta-T.) The heat normally lost through drawing the gas off a topper, is supplied by the heat shunt on Rogers stove. As long as there is enough pressure to force the liquid out of the cannister, through the tubing, valve and and connectors, the stove WILL operate. Though, there will be a lot of fluttering to start with till it reaches a balance. How much is needed? Probably not much. I would guess you could blow water through the system. So maybe 3 atmospheres? This would easily take the stove down to -40 or so, since there is some vapor pressure evan at -40F/-40C(yup, same number…darn COLD) as long as there is some propane in the cannister. Likely closer to -50F(about 4-5 bar in a canister,) since the propane doesn't actually have to boil to hit these pressures, though the stove may not light too easily since the mixed liquid would not gasify too well.

EDIT: Sorry, the tables I looked up had both Bars and PSI. The -40F/C temps were based on that. My mistake, it should be about -25C.

[Stuart R]

As usual, I don't understand a word you write James.

Propane is a slightly smaller molecule than butane, so it tends to concentrate slightly near the bottom.

Really? At the bottom of what? I rather think the thermal energy of the molecules will ensure a pretty even distribution in something a small as a canister on planet earth.

The liquified gas never really boils in a pressure vessel. This is why we don't ever hear a canister burping and boiling inside.

Take a cold canister, put it in some warm water and listen – you WILL hear it fizzing inside.

there is some vapor pressure evan at -40F/-40C(yup, same number…darn COLD) as long as there is some propane in the cannister

Sure there is some vapour pressure due to the propane, there is even some due to the butane at -40F/C BUT the combined vapour pressure is LESS than atmospheric pressure (unless you are on the summit of Everest:-) so when you open the valve, air will go INTO the canister: gas will not come out.

[Douglas Frick]

Jerry, you might want to ignore the details in my explanation, after I got schooled on it. :-)

An (important) correction: I checked my stove log, and found that the coldest I'd operated the Coleman Exponent stove with Powermax fuel UNDER THE CONDITIONS I STATED (fuel actually at ambient air temperature) was -25F, not -30F. I've used the stove at -30F and lower, but on those trips I always did something to keep the canister warmer than ambient.

So, I've been trying to reconcile my experience with the operational limit of my Coleman Exponent stove being -25F, with Stuart's comment "you can expect the stove to work with an inverted canister down to 0F." However, after looking at the charts, it seems both of our statements can be correct (after mine was corrected ;-). The left chart of Fig. 4, typical canister mix at sea level limit is -23C, +5 degrees to get some working pressure, so -18C = 0F. The right chart, Powermax fuel (30% propane, 70% iso) at 10K altitude (where I use the stove) limit is -36C, + 5, so -31C = -24F. Assuming I haven't screwed up again, that explains why my stove operated (poorly) at -25F.

Thanks, Stuart and Roger, for reminding me about your excellent article. I forgot just how much difference mix and altitude make; I'll try to remember that when I'm outside Wyoming. Also, thanks for clarifying the mixed-gas boiling science.

To the original question, perhaps the most relevant point is that, while all canister stoves stop operating at some low temperature if you don't do anything to keep the fuel warm, you can run these stoves regardless of the air temp if you simply keep the canister at, say, +32F. As mentioned in this thread (and in the article), that can be done with a pan of liquid water. The bottom cover for my original JetBoil is has a diameter just slightly larger than a small fuel canister, so I set the assembled stove in the cover and fill it with water. After a while a skim of ice forms on the water, but it runs just fine. For my Coleman stove, I usually work-pack the canister in snow to keep it 'warm' and prevent it from chilling in the overnight air.

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