View Full Version : Canister Stove Boil Times / Efficiency Question
wcgornto
12-24-2008, 10:10
Canister stoves, Jet Boil, Pocket Rocket, Giga Power, etc., are often compared on relative boil times.
1) Is the fastest boil time for a particular stove achieved with the valve wide open (full flame), half (e.g., to fit the flame to the bottom of the pot) or some other setting (1/4, 3/4, etc.)?
2) Boil time aside, in general, what setting is the most efficient, e.g, most uses before cannister runs out?
I suspect that the answers will vary by stove, pot, etc., but I am interested in getting a general sense.
Overall, I am more interested in maximizing the number of days I can use a canister rather than how fast I can boil water. I would prefer 14 uses, for example, at 4 minutes over 11 uses at 3 minutes.
buckwheat
12-24-2008, 10:28
I'm not a thermodynamist, however I don't think you can look at it like this.
The time it takes to boil water can vary irrespective of the efficiency of the stove you are using. Variables would include:
* amount of water
* temperature of the water to begin with
* altitude
* pressure
What this means is that even though you might get 14 boils from 1 can of fuel at one place and time ... this does not guarantee that you would get 14 boils from 1 can of fuel at a different place if the altitude or temperature was different. (There may even be other variables that could effect the boil time, but I've used these two as an example.)
And so, when comparing one stove to another from a fuel efficiency standpoint, generally, boil times would need to be expressed as the time it takes to boil a given amount of water at room temperature water at sea level. The Jetboil PCS, for example, claims a 2-minute boil time of 2 cups of water (or, 1/2 a litre) under these conditions. I'm quite certain that would be at full blast on the fuel setting.
Lowering the fuel feed setting to 3/4 or 1/2 would just lengthen the boil time, so you'd not gain any boil-time efficiency by "conserving" in this way. (Some foods need to be simmered at lower temperature, so that's why the fuel setting is adjustable.)
The JetBoil PCS uses a 100 gram fuel canister. They claim this will boil 10 litres of water.
You say you want "14 uses at 4 minutes" but you don't state how much water you want to boil per use. So, I'll have to guesstimate that you want 2 cups of water per use (1/2 a litre).
Under those circumstances, the JetBoil PCS, using a standard 100g cannister will give you "20 uses at 2 minutes" (this being enough time to boil 2 cups of water.
Do please recheck my math.
wcgornto
12-24-2008, 11:03
I'm not a thermodynamist, however I don't think you can look at it like this.
The time it takes to boil water can vary irrespective of the efficiency of the stove you are using. Variables would include:
* amount of water
* temperature of the water to begin with
* altitude
* pressure
Thanks. To further clarify my question, all other things being equal (factors noted in your reply above), do canister stoves have an efficiency loss at the full blast setting such that a greater volume of water may be boiled per canister at a lower setting? For example, with some stove and pot combinations, the flame will wick horizontally beyond the base of the pot at the full blast setting, but will be fully concentrated on the bottom of the pot at a lower setting. My sense of common sense tells me that some fuel efficiency may be lost if the flame shoots out beyond the base of the pot such that greater overall efficiency might be achieved by reducing the flame size / fuel consumption to match the flame size with the base of the pot.
buckwheat
12-24-2008, 13:13
... do canister stoves have an efficiency loss at the full blast setting such that a greater volume of water may be boiled per canister at a lower setting?
I don't think anyone has tested this, except perhaps for the engineers who designed the stoves. There is an incentive to design such that the full-blast setting is most efficient (since comparisons of boil times are the standard by which these various stoves are compared).
So I would doubt you'd find efficiencies of the type you are suggesting.
That would not be true, I think, in alcohol or other "hobo" type stoves, which frequently sacrifice efficiency for the gains to be had by ease of construction, weight and availability of materials.
UL-approved stoves, such as Jetboil and some of the other professional stove companies seem to me to be engineered for maximum efficiency given weight and cost considerations.
Something like this http://www.backpackinglight.com/cgi-bin/backpackinglight/lightweight_canister_stoves_test_report.html
might help, but you'll need to buy a membership to view it. They have great links to a bunch of super techy sites regarding backpacking gear.
gearfreak
12-24-2008, 14:12
Not exactly what you want, but you may find this previous post (http://www.whiteblaze.net/forum/showpost.php?p=624451&postcount=6) useful. :cool:
Something like this http://www.backpackinglight.com/cgi-bin/backpackinglight/lightweight_canister_stoves_test_report.html
might help, but you'll need to buy a membership to view it. They have great links to a bunch of super techy sites regarding backpacking gear.here is the chart from BPL
http://cache.backpackinglight.com/backpackinglight/images/backpackinglight-lightweight-canister-stove-test-report-5.jpg
daddytwosticks
12-24-2008, 14:26
Just adjust the valve so the flame stays concentrated on the pot's bottom, and does not lick up the sides too much. I know this isn't gee-wiz technical, but I have found this makes the canister last longer. You just need to be patient...the boil may take longer to achieve.
Start a journal entry and/or read the register if camping near a shelter.
Before you know it, your meal is done. Works for any stove. ;)
skinewmexico
12-24-2008, 16:23
I don't know if it's true, but the Primus Eta I just bought on SAC claims to use less fuel due to the cheat exchanger bottom, and cozy/carrying case. Who knows? Pretty heavy though. I've seen a lot of tests concerning fuel consumption on BPL though.
wcgornto
12-24-2008, 19:02
Thanks for all the replies.
Fiddleback
12-25-2008, 11:53
As pointed out, there are lots of variables regarding boil time. Add to those above; ambient temperature and wind speed.
IMO, 'boil time' is a near-useless measure. While it'd be great to compare stoves' boil times there is no standardization for such measurement and no way to accurately compare stoves' relative performance.
Still...it's what we've got and, until there's an information clearing house which lists test results from stadardized tests with 'constant variables' (;)), it's what we're stuck with...
What we need are comparisons using fixed amounts of the same fuel, heating the same amount of water starting at the same temp, in the same surrounding temp, wind, and atmospheric pressure (which calls into question the instruments used to measure all this). I imagine there are some other variables that should be included and controlled but there is a bigger concern... What is boiling and how do you define it? Bubbles forming on the sides and bottom of the (standarized) pot? A "rolling boil"? How much roll?
Perhaps attainment of a lower, specific temp be a better goal/comparison point?:-? Especially since those rolling bubbles occur at different temps depending upon pressure.
FB
My inner engineer is going to get loose here for a few minutes.
Don't confuse time to boil with efficiency.
Efficiency is boiling the water at minimum fuel use.
Minimum time to boil is a power thang.
The biggest honking flame you can get the stove to make will get the water boiled in minimum time. You will waste a bunch of fuel doing it.
Setting up the stove to put as much heat into the pot and not heating up anything else would lead to most efficient use of fuel.
Jet boil has the fins on the bottom to get more heat out of the hot gases and into the pot. That works for efficiency.
If you just have a plain old pot. You want a flame that is concentrated on the bottom of the pot. Excess flame licking up the sides will waste fuel.
Make sure you have a lid on the pot and a good close fitting windscreen to keep the breeze from blowing heat away from the pot.
Okay I think I got the engineer put away now.
Whew!
skinewmexico
12-25-2008, 17:44
My inner engineer only learned thermo long enough to pass two test, then looked for a career that didn't involve that.
Skidsteer
12-25-2008, 17:59
My inner engineer burns his fingers a lot.
buckwheat
12-25-2008, 18:31
Don't confuse time to boil with efficiency.
Yes, exactly. That's why that chart a few posts above lists how many grams of fuel it took to bring the water to a boil. In the JetBoil's case, it was 10g of fuel, owing (according to my inner engineer) to the efficiency of the flux capacitor.
If you also then compare boil times, you'll find that the JetBoil beats most competitors by a minimum factor of 2. Less fuel, quicker boil = most efficient in my book, and thus worth the extra pack weight.
I was told there would be no math here, so I can't say for sure, but I would hazard a guess that the efficiency of the JetBoil PCS means less total ounces per hiked mile when fuel weight is added to the equation.
Mathematician anyone?
Skidsteer
12-25-2008, 19:10
...but I would hazard a guess that the efficiency of the JetBoil PCS means less total ounces per hiked mile when fuel weight is added to the equation.
Than what?
That is really the question.
buckwheat
12-26-2008, 16:59
Than what?
That is really the question.
Than any competing stove/fuel combination (look at the chart above). By orders of magnitude.
Skidsteer
12-26-2008, 18:02
Than any competing stove/fuel combination (look at the chart above). By orders of magnitude.
Here's another chart to ponder:
http://hikinghq.net/stoves/weight_time_compare.html
After looking at the chart with a measure of bewilderment, I realized that the Cat stove in the test is not the "Supercat" stove but the original "Cat" double-walled stove originally made from steel tuna cans and later made from catfood cans - lighter but no more efficient.
Roy Robinson - "Cat" stove. http://royrobinson.homestead.com/Cat_Stove.html
Jim Woods - "Supercat" stove.
Completely different animals.
Chart is from 2001.
The heck with all the charts. I do remember seeing a test that showed it was more efficient to keep the flame at somewhat less than full bore blast. The way I see it (in other words, my observation) is that any flames licking up the sides of the pot are heating the handle and the great outdoors more than they're heating the contents of the pot.
I throttle my stove down a bit to try to get what "appears" to be most efficient - I eyeball it - for the conditions. It works for me - I seem to be able to go forever on a canister.
YMMV.