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Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re Stuart's comments on 11/21/2013 02:51:02 MST Print View

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
(Otter) - MLife

Locale: Wyoming
Re: Re: Temperature limits? on 11/21/2013 05:47:01 MST Print View

Oh well, back to school...

Jeff McWilliams
(jjmcwill) - M
Temperature Limit on 11/21/2013 06:09:24 MST Print View

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
(Scunnered) - F

Locale: Scotland
Re: Re: Re: Temperature limits? on 11/21/2013 06:25:25 MST Print View

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
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Temperature limits? on 11/21/2013 08:11:48 MST Print View

"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
(Scunnered) - F

Locale: Scotland
Re: Re: Re: Temperature limits? on 11/21/2013 08:49:50 MST Print View

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
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Re: Re: Re: Temperature limits? on 11/21/2013 09:07:03 MST Print View

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.

Edited by jamesdmarco on 11/21/2013 14:04:24 MST.

Stuart R
(Scunnered) - F

Locale: Scotland
Re: Re: Re: Re: Re: Temperature limits? on 11/21/2013 09:32:55 MST Print View

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
(Otter) - MLife

Locale: Wyoming
Re: Re: Temperature limits? on 11/21/2013 11:56:26 MST Print View

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.

Edited by Otter on 11/21/2013 13:34:51 MST.

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Re: Re: Re: Re: Re: Temperature limits? on 11/21/2013 14:00:01 MST Print View

"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
(Scunnered) - F

Locale: Scotland
Re: Re: Re: Temperature limits? on 11/21/2013 14:11:25 MST Print View

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.

Edited by Scunnered on 11/21/2013 14:14:55 MST.

Stuart R
(Scunnered) - F

Locale: Scotland
Re: Re: Re: Re: Re: Re: Re: Temperature limits? on 11/21/2013 14:25:04 MST Print View

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

Edited by Scunnered on 11/21/2013 14:41:51 MST.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Powermax on 11/21/2013 14:36:04 MST Print View

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
(AndyS) - F - M

Locale: Midwest USA
Re: Re: Re: Re: Re: Re: Re: Temperature limits? on 11/21/2013 14:51:55 MST Print View

"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
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Manufacturing Update on 11/21/2013 14:53:31 MST Print View

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
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Manufacturing Update on 11/24/2013 16:45:20 MST Print View

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
(Drusilla)

Locale: Wild Wild West
Evolution of..... on 11/24/2013 16:59:03 MST Print View

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

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Batch 3 shipped on 12/02/2013 17:01:42 MST Print View

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
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Beware Chinese canisters! on 12/04/2013 04:08:10 MST Print View

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
(fluff@inreach.com) - M

Locale: no. california
Re: Beware Chinese canisters! on 12/04/2013 07:34:25 MST Print View

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.