The Evolution of a Winter Stove – Part 3

[James Marco]

"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."

Yeah, very small, ignore it.

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

Yeah, that can happen. But this doesn't effect the over-all steady state pressure as you use the stove, well slightly decreasing. You just need to calculate at the new temperature. In your example, once it hits around 32F/0C it should stabilize again, within a minute or two, I would guess. Boiling is simply the explosive release of vapor pressure. In a closed system, this cannot happen, except as you say, by greatly changing temperature/pressure conditions. The valves(lindal and control valve) *limit* the flow of gas and/or liquid. There needs to be positive pressure in the canister for the stoves to work. The pressire also acts on the liquified gas varying the vapor pressure AT the boiling point of the new system. Not above it or below it. (Actually as you burn the gas it is slightly decreasing, drawing more gas out of the liquid state, but not really boiling, vapor pressure is usually enough.) Like boiling water in a closed vessel. Unless you actually flash heat it, no boiling happens till it actually explodes, then ALL the superheated water (at the new lower pressure) will try to boil till it reaches a steady state again.

The data was from: http://www.engineeringtoolbox.com/propane-vapor-pressure-d_1020.html
Your right, I just realized the tables were not all in bars, the F temps are in PSI. Sorry about my mistake. Looks like -13F/-25C will supply about 2 bars. Again, my apologies. I will correct it in the above note.

Anyway, from this it is possible to conclude that the interior surface of the LP gas inside a canister may be more important than we previously guessed. The concave bottoms, once exposed inside, probably cause lower volumes of gas to be available for use…perhaps the last sixth or so of the canister.

[Stuart R]

I'm pleased that we are both right Douglas :-)
My statement "you can expect the stove to work with an inverted canister down to 0F." was beng conservative, an inverted canister can work at lower temperatures depending on your circumstances whch I was unaware of.

You say you were using a Powermax canister, which alters things. I thought that in the US they contained 35% propane, 65% n-butane? (I believe they contained 40% propane in the rest of the world, Roger will correct if I am wrong here) If the first assumption is correct, then the boiling point of this mixture is -15F at sea-level and -31F at 10k feet. So if your ambient was -25F then this would work, just.

[Stuart R]

James, that table is for 100% propane. You need to look at these tables for propane/butane mixtures:
http://www.engineeringtoolbox.com/propane-butane-mix-d_1043.html
Note that this table is refering to the % gas mixture, which is different from the % liquid (weight) mixture shown on canisters: a canister containing 30% propane in the liquid has about 70% propane in the gas mixture, which according to these tables has a positive pressure (Psig) down to around -25F

The concave bottoms, once exposed inside, probably cause lower volumes of gas to be available for use…perhaps the last sixth or so of the canister

You lost me again…

Edit – correct link

[Roger Caffin]

As far as I can tell, the Powermax canisters did not state the fuel composition – and I suspect it changed over time too. But they had the right design (for winter)!

Cheers

[Andy Stow]

"Anyway, from this it is possible to conclude that the interior surface of the LP gas inside a canister may be more important than we previously guessed. The concave bottoms, once exposed inside, probably cause lower volumes of gas to be available for use…perhaps the last sixth or so of the canister."

If you're saying what I think you are, that since in upright use the dome will expose leaving a donut of liquid surrounding a shallow metal dome, with the liquid surface generating vapor pressure and the metal surface generating none… no, pressure does not work that way. The gas volume will stay at the vapor pressure of the remaining liquid until no liquid is left. The wetted surface and its shape are not relevant.

[Roger Caffin]

Time to update on status.

I have shipped a couple of batches, and had nearly completed yet another batch when the X axis on my CNC died. I thought it was mechanical and stripped the machine down and rebuilt the ball screw drive. A word of advice: don't do this yourself!

But the fault reappeared. The power driver for the X axis was telling the world it was seeing an over-current: in excess of 15 A. Normally it draws about 200 – 300 mA. So I sent the power driver off to be checked. A small fault was found and one section was replaced.

However, the same over-current fault reappeared with the new unit. By now I was getting a bit suspicious. I had installed current meters on the machine to monitor the loads, and faulting at 15 A was not credible. Also, when something fails at 15 A, it does not start working again when you hit the reset button. Things … tend to melt at those currents.

Some electronics diagnostics work followed, plus discussions with the original designer. I began to suspect the fault-detection circuit itself. That is, no actual fault was happening, but a false trigger was being created.

In an effort to reduce the effects of noise on the fault detection, a capacitor had been added as a filter at the last minute. This told me there had been noise problems for a start. Hum. Unfortunately the capacitor had been added in an apparently obvious place (albeit unusual) which was actually very, very wrong. Things get a bit esoteric here, but I had published a research paper on this circuit effect in 1975! It had the potential to worsen the effect of certain low-probability forms of noise.

I have modified the circuit, and started machining again yesterday. We will see how good my diagnostics have been, won't we? Some further tweaks may be needed. My apologies to all the beta-testers who have been waiting.

Cheers

[Roger Caffin]

CNC still running OK.
Legs all made for the next batch. Didn't take long.

All I am waiting on now are the 4 mm SS grub screws & nuts that hold the legs on. I could use plated steel, but that can rust. Not good enough.

The screws were ordered 10 days ago and were 'ex stock for delivery within 24 hours'. But because another ex stock item on the order was not really in stock, the whole order was held up. They were going to wait the remaining 2 weeks to get the last part in to make up the full order. No feedback, no emails asking if I needed the goods soon, …. Scream of rage!

OK, stoves all going into pressure test today, followed by burn test. When the screws arrive the legs will be added. Gotta have legs …

Cheers

[Desert Dweller]

Your persistence and tenacity are to be commended.
Nothing worse when a tool breaks down when doing a project.

[Roger Caffin]

Well, a (not so) quick trip to the Post Office and batch 3 has been shipped.
They were assembled, pressure tested for leaks (no leak is accptable), flow tested (any blockages or unacceptable restrictions) and burn tested. The last verifies that the flame shape is correct and that the heat exchanger system is working.

I did get one stove with a really weird flame shape, way off to one side. Turned out the jet had a small obstruction at one side of the hole so the gas jet came out off-centre. That was corrected by simply cleaning the jet.

OK, back to the workshop.

Cheers

[Roger Caffin]

Hi all

One tester has reported that his stove ran for a short while (inverted) then died completely. This seemed a trifle odd to me, but further clues were forthcoming. He had been using a Primus canister, inverted, when it died. Aha!

Now, I too have used Primus canisters, but there seem to be a couple of different sorts. Some work just fine, but one which was labelled Primus/(campstuff)/(something else) had been made and filled in China. After all, who else would label the canister as having 74.48% butane? In poor English too, if I remember correctly. It was … cheaper.

Anyhow, when I tried to use this Chinese Primus canister my stove died – twice, and quickly. Each time I found fine dust or gunk lining the inside of the stove. I returned the two canisters to the shop and got all my money back. I did wonder later what I had said in the shop, as shop keepers do not like refuding full price for a used thing. Pity – i should have kept one of them as 'evidence'.

So I suggested to the stove owner he clean the valve and jet. He actually stripped the entire stove down and cleaned stuff (black gunk) out of everywhere. Now he reports it runs just fine – on an MSR canister.

My suspicion is that 'they' are filling the canister with unfiltered, undistilled, unpurified gas straight out of the LPG well – or similar. The fine dust may be dirt from the gas well – or corrosion from the plant?

You have been warned!

Cheers

[peter vacco]

possibly the best-ever .. ever , thread on bpl.
gas stratification (it happens. it really really does). eluding to the possible return of dreaded and possibly fatal brownian motion discussions of 1997.
orifice obstructions and flame disruptions (it takes effectively nothing to make this happen). and it's related to another pet subject of mine (ox/acy cutting).
machine tools with perhaps shunt resistance demons. and sketchy demons they be too, with their comings and goings. all now banished from my life with the simple inclusion of a 480vac isolation transformer. (smooth and calm now … like demerol, but for electricity)
electrical issues with capacitors (coming soon to the polar bear fence ! )

all this ! and i don't even OWN a frikk'n "gas stove".

cheers,
v.

[Stuart R]

Yes but "analysis shows that stratification does occur, but it requires a column of static air several kilometers high to have a major impact" i.e. not in your average gas canister.

http://chemdaq.blogspot.co.uk/2012/09/gas-stratification-is-not-relevant-to.html

Go on, get a canister stove, you know you want to…

[Roger Caffin]

Hi All

A photo from one of our beta-testers, Douglas Frick (to whom my thanks).

Douglas tells me it was -30 C (-20 F) when this photo was taken. A shade cool imho. But clearly the stove can be used at that temperature.

Cheers

[Rick M]

del

[Nick Gatel]

"At such -30C extreme cold, does one still follow the standard procedure of starting the stove with the canister upright and then inverting it after a minute or so once the preheat tube gets hot?"

Looks like only one person knows for sure.

With the old Gerry and Mark III stoves (and similar), you had no choice. The were liquid fed, but upright.

[Robert Blean]

Roger,

What do you recommend using for a wind screen with your stove?

[Roger Caffin]

> At such -30C extreme cold, does one still follow the standard procedure of starting
> the stove with the canister upright and then inverting it after a minute or so once
> the preheat tube gets hot?

It's a bit more complex at -30 C. For a start, the gas canister will not be warm enough to push out ANY fuel if it is at ambient, and this goes for ANY canister stove. So you need to get the canister a bit warmer than -24 C to get any joy (ie fuel) at all.

For ANY inverted canister stove at very low temperatures, some skill is needed. You can start with the canister upright and hope to get some gas out – it will be mainly propane with just a little butane. But running the stove for some time with the canister upright could be said to be storing up problems for the future, as the %propane will be falling rather fast. See http://www.backpackinglight.com/cgi-bin/backpackinglight/effect_of_cold_on_gas_canisters.html for an in-depth discussion of what happens inside the canister at very cold temperatures.

You might like to note there is a Coleman Xtreme behind the lit stove, to the left. The canister for this stove is the Coleman Powermax, and this always give a liquid feed. You cannot get a gas feed out of it. So the standard method for lighting an Xtreme is to first warm the stove in your hands (which happens as you set the stove up), then to light it at very low power. There may be a few burbles of orange flame right at the start – no matter. As the flames heat the preheat tube, the stove starts to function in the normal inverted canister mode.

You can do exactly the same with my stove and any canister. That is, you can start with the canister inverted if you wish. Just run it kinda slow for a little while as the heat shunt warms up. Gradually increase the power when the stove body feels warm.

One has to be flexible …

Cheers

[Roger Caffin]

> What do you recommend using for a wind screen with your stove?

Have a look at When Things Go Wrong, about half way down. The pic is labeled 'Cooking dinner in the vestibule of the tent'. That is a Trail Designs windshield – one of the plain unvented ones. I leave a gap where the stove control valve handle sticks ou, may 20% of the circle. I leave a gap of about 15 – 20 mm between the pot and the windshield. OK, it's probably a bit more in this photo – it isn't really critical. And I have the windshield come about 1/2 way up the side of the pot, or maybe 3/4 of the way.

But any similar bit of aluminium shim or heavy foil will do just fine.

Cheers

[Douglas Frick]

Sorry for not responding sooner, I was out camping this weekend.

>At such -30C extreme cold, does one still follow the standard procedure of starting the stove with the canister upright and then inverting it after a minute or so once the preheat tube gets hot?

I had three inverted-canister stoves set up: the beta Caffin stove, a Coleman Xtreme, and an MSR Whisperlite Universal. All had set out overnight (low of -34C / -29F) with their canisters attached (so they wouldn't warm by handling), exposed to the air, and insulated from the snow with silicone pads. The Caffin stove had a new Snow Peak Giga Power 110g canister. I lit it with the canister upright (small flame) and _immediately_ inverted the canister, without waiting for the heat shunt to warm up. As expected, it flared up; I dropped my pot on the burner; and after about 20 seconds, the heat shunt warmed and it settled down to a nice flame. As Roger mentions in his post above, it was burning mostly propane at the start, which is why I immediately inverted it to save the propane for pressure. The wind had dropped so I didn't need a windscreen, and the heat from the burner warmed the canister a bit more, which kept the stove going without intervention.

The Coleman Xtreme had a Powermax canister that was about 1/3 full. It wouldn't start until I poured some (precious body-temp) water over the canister. It ran after that, but the flame didn't really get roaring until I heated more water to pour over the canister. If I had set the canister in a basin before pouring, I would have wasted a lot less water.

The MSR Whisperlite Universal had a 3/4-full Snow Peak canister, and it wouldn't start. I closed the valve, inverted the canister, and poured warm water into the concave base. I let it sit for a minute, so the heat from the water could work its way down the side of the canister to the liquid fuel, then lit it in fireball mode. It flared a while until the preheat tube warmed up, then settled down nicely. The water in the bottom of the canister soon froze solid, but it had done its job of producing sufficient gas pressure.

So two of the three stoves were non-starters (insufficient gas pressure in the partially-used canister to force either gas or liquid down the hose to the burner), and probably the only reason the Caffin stove started was that it had a brand-new canister. However, by simply applying a bit of warm water to their canisters, both non-starting stoves soon ran fine. I intentionally tried starting the stoves with chilled canisters, and inverted them cold, to see if/how they would work under these conditions. If I had either slept with the canisters or set them in water baths before attempting to start the stoves, the stoves would have had no problems starting and properly heating the preheat shunt/tubes, and thus I could have avoided the flare-ups from burning liquid-state fuel. I should also mention that I slept under a tarp, so the flare-ups weren't a problem; there's no way I would invert the canister without a hot preheat shunt/tube in the vestibule of a tent!

I also tested an upright-canister stove, but I'll post about that in a separate message.

[Bob Gross]

I might point out one more trick. When you get up on a cold morning and you think you will need some warmth to start these canisters… just drain the dragon, so to speak. The fluid is plenty warm, and it is going to go to waste anyway. Just make sure that you don't get frostbite in this process.

–B.G.–

[Douglas Frick]

I also wanted to test an upright canister-top stove under challenging conditions.

That is a stock Fire Maple FMS-300t stove with a nearly-empty canister (approximately 3 grams / 0.1 ounce of fuel left), which is about the worst case for cold weather. At room temperature, I had shaken the canister and couldn't hear any liquid slosh. With the canister/stove combo having sat out overnight and exposed to ambient air at -30C / -20F, I opened the valve and held a lighter to it: no hiss, no flame.

I then set the canister in a shallow cold-water bath. (I used a lid from a jar of protein powder; 1.0 oz / 28g.) After waiting one minute, the stove lit easily and ran without flare-up, just as if it was warm out. The flame guttered and died after about three minutes–canister empty. Was it just the propane that burned off? Nope. Afterwards, I let the canister/stove combo warm up to room temperature. I lit it, and it held a very small flame for 10 seconds, then went out. Clearly, it had burned the remaining isobutane/propane mix, not just the propane.

I prefer and trust an inverted-canister stove when it's cold, especially for longer burns, melting snow, stabilizing larger pots, with a close windscreen, etc. But upright-canister stoves also work just fine at -30C with proper technique. I think if people knew this trick there would be a lot less trepidation about using canister stoves in the cold.

[Roger Caffin]

Well, this is batch 4, all 19 of them. 8 have been sold already and will be
posted in the next couple of days. The rest are available ex stock. We have
moved from many weeks of lead time to on-the-shelf!

Going from a one-off MYOG hack for myself to a small production process
safe for others has been 'interesting' – especially for an academic. My
respect for all those cottage industries out there has gone up
considerably. The number of production problems encountered was more than I
had expected, but probably par for the course.

My CNC had a recurring 'fault' in the electronics which kept shutting the
machine down. The system said it was getting a 15 Amp over-current problem
on one of the drive motors – which sounded bad as things start to melt at
those current levels. It turned out to be a design fault in the
fault-detection circuitry. I modified the circuit so it would work
properly.

There were delays in getting some parts – with the supplier in one case not
bothering to tell me that there would be a 3 week delay for an order quoted
on their web site as 'ex stock'. We 'discussed' the matter (politely!), and
the critical parts were supplied Express (overnight).

One of the supplied components changed source during production, with the
new part being 'less than satisfactory'. I think the new part came from
China, but can't be sure. I had to find a completely different part to
replace it and redesign the stove body to handle that. Fortunately that was
not hard. You might not even notice the difference.

A couple of early beta testers found some tiny hiccups, which were
individually fixed, but nothing major. One tester had a problem with a
Powermax canister valve which I don't understand (it would not open
properly), but I do know that the crimping of the valves on those canisters
was 'variable' as the supplier was changed. A slightly longer pin solved
that for him.

A couple of almost invisible cosmetic changes were implemented when I had
the chance. Here's the burner:

More user photos of the stove in use in the field (especially in the snow) would be appreciated.

Cheers

[Ethan A.]

Roger, best wishes for the New Year.

That's a beautiful sight…

Could you post a picture of the 300T and 116T burner stoves compactly folded up? Or just the 300T if that's the only one you'll be making going forward.

[Roger Caffin]

Hi EJ

OK, herewith a photo of the stove folded up inside an MSR Titan 1.5 L pot – our normal dinner pot for most walks. I usually get a lot of other stuff in with the stove. In addition, my folding windshield is inside the pot along with some rather solid Ti mini-stakes for holding the stove down. I think the stakes are a bit of an overkill, but the weight is negligible.

Then I thought 'how about a stove comparison'? So herewith for that too.

On the left we have an MSR XGK with a small (white gas) fuel bottle, 429 g / 15.1 oz with no fuel in bottle

In the middle we have a Primus Eta Power with integral windshield and optional pot, with a standard 230 g screw-thread canister, 375 g / 13.2 oz plus canister but without the pot shown here

On the right at the back we have an MSR Reactor stove plus pot, with a standard 230 g screw-thread canister, 571 g / 20.1 oz with special pot, plus canister extra

Finally, front right hand corner, we have my stove with a small Powermax canister, 85 g (3.0 oz) plus about 68 g for the empty canister.

Cheers

[Billy Ray]

I have a Roger Beta Stove.

So I've read the directions and played around with it at about 20F.

I have a question. Not about the stove itself, but about what happens inside the canister when using an inverted canister stove.

First, Roger's directions recommend using the canister upright for 30 seconds or a minute to prime the stove… to get the warming chamber warm enough so that the liquid fuel easily vaporizes when the canister is inverted. Question: If the typical canister contains only 20% Propane, will this priming cause you to run out of propane (and thus priming ability) before running out of butane??? Obviously, if you only primed one or twice or thrice during the life of the canister (cooking for long times inverted) this would not likely be a problem. But I'm only boiling 8 oz of water in the morning and maybe 12 oz of water in the evening. That's a high proportion of prime time to cook time (even if you cook while priming) so I fear I will run out of propane and thus the canister will be useless at cold temps before it is empty.

Second Question:
After the canister is primed and inverted, does the stove burn equal amounts or equal proportions of butane and propane? I'm thinking if one of the two is heavier/denser than the other it will settle to the bottom and more of that will burn.

Please enlighten me.

thanks,

Billy

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