I have a question for some of the more technical folk here.

I've been playing with the numbers for stove output etc. and wondering if I have this right. I've calculated that it takes about 317 BTU (from memory as I don't have the paper in front of me) to raise 1L of 20°C (68°F) water to boiling. If a stove puts out (typically) 10,000 BTU, presuming per hour, it would take 1.9 minutes to boil the water assuming 100% heat transfer. If a stove took 4 minutes to boil the water it would only be 47.5% efficient? I've noticed that most stoves are rated at just about 10,000BTU, even the large Coleman camp stove/grill I have is rated at 10K BTU for the burner and the grill assembly. I'm curious as to why the 10K ceiling/limit; does this have to do with the output properties of the hydrocarbon fuels?

I'm trying to wrap my head around this because of my recent stove purchase, as how an old Coleman 440 being rated at (digging deep into the recesses of my brain) 10,000 BTU and brand new Brunton Vapor AF at 10,000BTU could vary in boil times by such a great amount. Coleman 440 around 4 minutes and a Vapor AF at 12+. Something just isn't adding up in my mind as to how stove could vary so much in efficiency when most are designed very similar. If someone could go through the technical aspects of this I'd appreciate it. Thanks.

I can't tackle your question head-on, but can at least add more confusion!

As you doubtless already know, maximum heat output seems far less relevant to cooking time and ease than the overall cooking system. Also, maximum output in the field seems limited to liquid fuel stoves.

For example, I've never used a cartridge stove that allows me to use full flame with a fresh cartridge--the flame is far too high and wraps far around the pot edges. They can only be cranked when the fuel's nearly used. But I'll bet the makers measure output WFO with a new cartridge, making the BTU number worthless (and certainly unsustainable for an hour).

White gas and kerosene stoves do tend to be used on high, at least when boiling water on a wide pot. (Nearly all alcohol stoves have no flame control.) If we could ascertain that all makers used the same measurement technique then we could conceivably compare them, but I'd again question whether a good fraction of them can hold that flame for an extended period. OTOH stoves with large fuel bottles, such as the MSR, Brunton and Primus models, can burn steadily for an hour or longer, but need a lot of fiddling with the pump and valve(s) to do so. Do maker tests include fiddling?

On your efficiency numbers, the big variables are pot size, shape and material, and windscreen. All have to be matched to each stove's burner head design (flame pattern) and pot support height. I believe this process is ignored by most stove buyers, who presume there's no difference. One thing's certain: a wasteful high-output setup heats the pot contents more slowly than an efficient (well-matched) system with a lower output burner.

All that said, I don't know why your Brunton is acting up. I'd try different pot and windscreen combinations to see if you can get that time to drop. Also, buy brand new fuel if you haven't already. I'd suggest controlled tests, carefully weighing each stove's fuel use to see whether their output is truly comperable. You might even try fiddling with pot height above the supports to see whether it's properly spaced on this stove.

Generally, wide pots seem more efficient than narrow ones. It could be that simply switching the pot and fitting a good windscreen will really improve performance.

I'll be curious to hear what you figure out! Winter's good for these sort of investigations

Perhaps the claimed output of the Brunton is inaccurate.

Crunch numbers all you want but there is no substitute for real world experience. I think Rick D is spot on. You have to consider the entire system and not just the stove.

I like this quote: "The difference between theory and practice is far greater in practice than it is in theory".

That explains my non-technical answer to a technical question.

Nightforce
quote"
Coleman 440 around 4 minutes and a Vapor AF at 12+. Something just isn't adding up in my mind as to how stove could vary so much in efficiency when most are designed very similar."
_____________________________________________________________

Where did the 12+ come from? Was that your experience with your new stove? If so you are doing something drastically wrong? Are you using a tiny pan with no windscreen? Does the stove appear to be working correctly? If you ran an experiment, exactly how did you set it up. Was the stove pumped up adequately?

RickD did an excellent job describing the whys and wherefors. As Rick says - there is a lot of room for errorin your set-up.
Jim

The heat output of a stove is a balance between the amount of fuel that can be delivered (white gas can be higher than propane or butane because there are more BTUs per unit of fuel and the fuel doesn't rely on heat from the environment for vaporization), the efficiency of the carburation (how much air can be drawn into the combustion process), and the surface area of the pot being heated (big pots allow bigger burners and more btus). I'm not an expert, but if there is a 10k BTU limit, it's probably due to the size of the fuel cannister.

For instance, they sell this adapter that permits you to "save" your BBQ event by attaching a disposable propane cannister if your refillable one goes empty. The only problem is that the BBQ doesn't get as hot because the cannister rapidly becomes quite frosty and loses pressure. The cannister doesn't have enough surface area to absorb the heat needed for the fuel to vaporize and produce the volume of gas required for the full rated output of the BBQ. I suspect that the amount of vapor that can be delivered on a typical 70-degree day is about 10,000 BTUs worth.

The boil time differences between stoves most likely are due to the methodology of the test. Unless identical conditions are present, boil times can vary quite a bit. What is the initial water temperature? What is the bottom surface area of the pot? How much water? What's the altitude? The ambient temperature? The humidity? Are they measuring to a simmer or a full boil? Is the water in the pot being stirred? Does the water have to reach 212F to be considered "boiling"?

Why not just be happy with the stove you bought? If it boils your water in a reasonable amount of time and doesn't weigh a ton, isn't that what really counts?

I suspect most members here do not consider BTU output or boil time in their stove selection. The stove needs to be compatible with your style. For example, I hang my hammock while the water is heating. I do not want a fast boil time.

If I wanted to make a cuppa during a race then boil time is very important.

As a Boy Scout my patrol competed in a timed event where you had to start a fire with 2 matches and only a 18" length of 2"X4" for fuel. Time stopped when you flipped a pancake over a 6' high string. We could generally finish in less than 10 minutes. Technique is more important than gear.

Jim, test 'procedure' was as follows:

• tests were conducted in an approx. 68-70°F garage, free of air movement
• stove was setup, pumped up 30 times instead of book recommended 20
• alcohol applied to the priming pad and ignited, then stove turned on and 'warmed up' until the output was smooth with no sputters.
• test pot has been a flat bottom ~2.4L popcorn tin with wire bail; I've also done tests using my REI 1.5L Al pot with similar results/times. All tests are done within reasonable time from each other, i.e. boil, dump out water, let stove and pot cool down back to room temp., fill with fresh water, set up next stove and repeat test. Tests were stopped when water was boiling, as in 212°F measured with a calibrated Omega HH318 (IIRC) thermocouple.

I've also performed the tests at night so I can turn off the lights to see the flame pattern. Generally, only a very small amount of flames emerge from the bottom of the pot; I can also hold my hand fairly close to the sides and bottom of the stove for a good amount of time, indicating that a large portion of heat is being absorbed by the vessel rather than escaping. I was burning 15 year old Coleman fuel in the stove and tried new fuel to see if results changed, which they didn't.

I know it sounds like I'm making a mountain out of a mole hill here, but what really ticks me off is the fact that a company can advertise like they do, without, at minimum, publishing their test procedure...or make it available to a consumer, of which I have requested. I don't absolutely require water to boil in 3½ minutes (if I did, I'd use a Reil burner that I've, made putting out over 100K BTU :grunt: more power), but it would be nice if the stove I purchased actually gave me the option as advertised.

INFO NOTE:
Consequently, a #78 drill bit has made a pleasant difference. Boil times are now down to around 6-6½ minutes with no adverse affect to the stove. To test, I boiled 1L of water, about 6 minutes, then continued to boil the water at a full rolling boil until reduced by half, then turned the stove down to a very low simmer to test vaporization stability at low settings and continued to boil the water until reduced to about half again, then went back to maximum until only about 100ml was remaining. Total time was close to an hour. I bought the stove for the features (honestly) rather than flat out boil time and now that it's been modified, I like the stove. For those of you reading this post for information on the Vapor AF, I'd say purchase one, drill out the jet, and enjoy the stove. I've done tests with a good selection of fuels. I've burned 15 year old Coleman fuel, new Coleman fuel, kerosene; best to use alcohol to prime and let it burn for a while before opening the valve, watch out tho, unless you to like the little mushroom cloud. I've also burned low odor mineral spirits and denatured alcohol! To burn the denatured alcohol turn the bowl to the gas setting to get the right fuel air mixture. It doesn't have the output like the other fuels, but it does burn. Of course, canister fuels work good and I'm sure with an adapter it would burn propane from a grill tank or 16.4 pound disposable cylinder. That's all I've tried for now, but I feel confident that this stove will in fact burn most vaporizable fuels. I'll try some common household oils, like vegetable, corn, 3 in 1, etc., at a later date to see how this stove would stand up to an emergency or international situation.


Sidebar: Is there something that we consumers can do so that a consistent test be performed by manufacturers so that consumers aren't misled? It sure would be nice to have a standard so apples could be compared to apples.

Exactly. A welding torch has a lot of BTUs but is not good for cooking. BTUs and a non standard labratory 'boil" time are not relevant measures of performance.

Stoves need to be stable -- I have dumped a number of meals.

Stoves need to handle wind well.

Stoves need to be reliable -- the fewer the moving parts the better.

A winter stove needs to work in subfreezing weather.

Look for stoves that advertise relevant features.

I seriously doubt stove manufacturers are required to measure the heat output of a burner - they just calculate it. And since BTU numbers are important to the unenlightened (those who don't use alcohol stoves), I suspect that manufacturers put the most positive spin on them.

Obviously you can burn a lot of fuel under the right conditions, but will you encounter those conditions on the trail?

The older Colemans were jetted such that they would blow themselves out if you pumped it too much. And I know that they wouldn't boil a quart in 4 minutes in a typical saucepan when it was cold out. But they got the job done and lasted forever.