Friday, April 13, 2012

Gas in Cold Weather: The Myth of "Fractioning"

I'd like to dispel a myth that's out there: the myth of "fractioning."

Supposedly, if you've got a typical canister of gas (i.e. containing propane and some form of butane), in cold weather (temps below the vaporization point of all but the propane), only that fraction of the fuel that is propane will burn. Supposedly.

Here's an example of the myth: You have a canister that is 70/30 butane/propane. Propane vaporizes at -44F/-42C; butane at +31F/-0.5C. Now, say it's 20F/-7C out. According to the myth, only 30% (that fraction of the mix that is propane) will burn off.

That's the myth.  What really happens?  Well, take a 70/30 canister out in 20F/-7C weather and burn it until it's empty. Now weight it. You'll find the weight is lower than if only 30% of the fuel had burned off.

Why? Well, when you mix liquid hydrocarbons, they combine to form a blend. The blend has properties that lie "between" the properties of the constituent fuels. For example, the boiling point of a 70/30 blend of propane and butane will have a boiling point between -44F/-42C and +31F/-0.5C. The math is non-trivial, but if you know the formulae, you can calculate the resultant boiling point. In this case, a 70/30 blend has a boiling point of about 0F at standard pressure (1 atmosphere/1013mBar).

Now there is a little bit of a catch to all this. This "blend" of liquid hydrocarbons in a canister of gas (recall that there's so much pressure inside a cansiter that our gasses liquefy) still has two distinct molecules in it: Propane molecules (C3H8) and butane molecules (C4H10). The propane molecules have a higher vapor pressure than the butane molecules. In other words, the propane molecules tend to vaporize more than the butane molecules in the blend. How does a canister typically work? Well, you draw vapor off the top, and then more of the liquid portion of the fuel vaporizes to replace the drawn off vapor. If the propane is what most readily vaporizes, then the propane will be removed from the blend more rapidly than the butane. This is called "preferential" vaporization. Because the propane is vaporizing faster than the butane, your blend will change over time as the canister empties.  As the blend changes, so will the vaporization point.  Toward the end of the life of the cansiter, all the propane will be gone and only your secondary fuel will remain.

Uh, isn't this "preferential" vaporization the same as "fractioning?"

No, not at all. Remember that the myth of fractioning calls for only the propane to burn leaving behind all of the butane. What happens in reality is that the blend vaporizes some butane and some propane throughout the life of the canister until the propane is gone. Yes, you will reach a point where all of the propane is gone and only butane remains, but the fact of the matter is that you will be burning off far more fuel than if only propane alone were burning.

With fractioning (if it were true), you could only burn of 30% of the fuel in a 70/30 canister. With preferential burning, you burn more like 75% of your fuel before you wind up with nothing but butane left (the actual amount burned varies with altitude and temperature of course).

This brings up two important points:
1. Don't use canisters with regular butane in cold weather. Eventually all your propane will burn off, leaving behind whatever else is in your canister. If you've got isobutane (vaporizes at +11F/-12C) as your secondary fuel you've got a 20F (10C) degree advantage over a canister that has regular butane (vaporizes at +31F/-0.5C) as its secondary fuel.

2. Stoves that can handle running with the canister upside down (inverted canister stoves) draw fuel off the bottom of the canister. If you're drawing fuel off the bottom, you're only drawing liquid. If you're not drawing vapor, then it doesn't matter which fuel vaporizes at what rate. Inverted canister stoves start and end with the same fuel blend. If you start with 70/30, you end with 70/30, and your vaporization point stays low, allowing you to run your stove in much colder weather than a conventional upright (screws onto the top of the canister) gas stove. With an inverted canister stove, you can burn all of your fuel in cold weather (assuming that your blend's vaporization point is sufficiently high for the weather).

So, there you have it: What's really going on in that canister of yours in cold weather. I hope this puts the myth of "fractioning" to bed.

I thank you all for joining me on another Adventure in Stoving,


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Monday, April 9, 2012

Wood Cooking Clean Up

Ever find yourself holding back on using a particular pot or pan for wood cooking?  I mean they do get pretty darned black if you use them on a wood fire.  Sometimes I hesitate to use a pot I usually use with gas or alcohol on a wood stove because I don't want to get the darned thing all black.

Well, hold back no longer.  There's a simple trick out there that can get your pot clean again after cooking on a wood fire.  The trick?  Dish soap.
Putting a thin layer of dish soap on a pot before cooking with wood helps tremendously with clean up
Yep, that's right, just simple old dish soap.  Smear on a thin layer before you head out.  A little dish soap makes clean up go so much easier.

Why bother you say?  It's just going to get dirty again?  Well, and that is one way to handle it.  You could just pack your blackened pot in a sealed plastic bag so soot doesn't get all over.  But I use my pots with multiple fuels (gas, liquid, alcohol, wood, and hexamine).  A coating of soot will insulate your pot from the flame, and you'll have to use more fuel to heat things up.  So, for me, when I'm done with some wood cooking and want to switch to another fuel, I clean my pot.

How well does it work?  Let's have a look.

I'm just going to use an old scrubber type sponge here.  You probably could use steel wool on a steel or titanium pot.  A word of warning:  Whatever you use to clean, you're going to get it very dirty.  You'll probably want to throw the scrubber sponge away afterwards so it won't get other things dirty.  Either that or you'll have to have a designated scrubber sponge for really dirty clean ups.

So, here's my pot.  Pretty black.  Just waiting to get all over the inside of my pack, and pretty inefficient since heat is going to have to penetrate that coating before it heats the pot.
A rather sooty pot that has been used on a wood fire
Let's try a quick scrub with some soap and hot water.
A few scrubs, and bare metal starts to appear
Say, that's not too shabby for a few quick scrubs with an old scrubber sponge.  Makes quite a difference.

Let's try to clean the worst spot, the bottom of the pot.
Quite a marked contrast between the cleaned and the uncleaned sections of the pot
Hey, now that's quite a difference between the upper half of the pot that I've cleaned and the lower half that I have not.

Now, I'd be a liar if I said that it came off like magic.  Nope; it just doesn't work that way.  It takes some real elbow grease.  And, even when I finished cleaning the pot, there were still some little spots of black in the cracks and such.  But try cleaning a pot that's been used for wood fired cooking without first putting some dish soap on it.  The soot is not going to come off anywhere near as easily.

So there's your wood cooking clean up tip:  Smear a little dish soap on the exterior of your pot before you start cooking.  You'll be glad you did come clean up time.

Thanks for joining me on another Adventure In Stoving,


Sunday, April 8, 2012

Blog Status, 8 Apr 2012

Well, I've found a job.  After about a quarter of a year's worth of unemployment, I'm quite grateful.   There's just one catch:  The work is in the next county over, and I'll be commuting, at least for the short term, a couple of hours each way.  Necessarily, my hiking and therefore my blogging is going to be greatly reduced.  Gone will be the mid-week evening hikes to test stoves.  Also, chores and such that I could do on a weeknight will perforce be pushed to the weekend, reducing my time there.  A pity, but work is necessary if I'm to feed and house my family and all that.

So, just an update.  I intend to keep blogging, just at a less frequent rate.

I thank you for joining me on these Adventures in Stoving,


Friday, April 6, 2012

How Much White Gas Do I Need?

How much white gas should you bring on your trip?  That's actually going to depend a lot on your style of cooking, your stove, the conditions, etc, but let's take a look and see what we can come up with.

Don't use white gas?  No worries.  For canister gas, check out my post How Much Canister Gas Do I Need?

The white gas stove I've used the most is my MSR Whisperlite.  I'll be using my experience with my Whisperlite to generate the numbers that follow.
My MSR Whisperlite
A word about style:  On most of my backpacking trips, I keep cooking fairly simple.  Most of the time, I'm boiling water for re-hydrating foods and making hot drinks.  I almost always have tea in the morning and cocoa in the evening.  I normally boil at least two cups and more typically three cups per boil.  Simmering for the most part is kept to a minimum, but I do simmer sometimes to re-hydrate things at higher altitudes.  I don't typically have a hot lunch, but I almost always have a hot breakfast and supper.

Over time, I've noticed that I use something on the order of 1.5 fl oz (44ml) of white gas on a solo trip per day.  For two people, I find my usage is something on the order of 2.25 fl oz (67ml) per day.  If I'm melting snow, I figure on roughly doubling those amounts.  These amounts include fuel used for priming.  These amounts are the amounts I expect to use.  Particularly in winter, you should plan for the unexpected.  I typically bring an extra day or two's worth of fuel on winter trips.

So what does that equate to in terms of fuel bottles?  There are a lot of different sized fuel bottles out there.  I'll list some common sizes, and you can hopefully adjust from there.

A note on fuel bottles:  MSR, Sigg, Primus, Optimus, Snow Peak, and Brunton fuel bottles all have the same threads and are generally interchangeable.  However, you should always test your particular stove with the particular fuel bottle you intend to use before your trip.  My Primus pump fits in my MSR fuel bottle, but the opposite is not true:  My MSR pump does not fit in my Primus bottle.  The threads on the Primus bottle are compatible, but the threads start down too low in the neck of the bottle for my MSR pump to engage.

Most manufacturers recommend that you use only their fuel bottles with their stove.  That's fine, and you can't go wrong with that, but that recommendation is more about legal liability than it is about the technical requirements of running a stove.  Generally, any fuel bottle designed by a reputable stove company for use with a pump should be fine, provided that it has compatible threads.
An Optimus Nova stove in use with an MSR fuel bottle.  Works just fine.
I would not use "no name" fuel bottles or drink bottles as a fuel bottle for a pressure stove.  Note: Coleman and Soto fuel bottles have proprietary threads and are not interchangeable with any other brands.

Now, fuel bottles:  I'm going to list MSR's bottle simply because that's what I have a lot of.  MSR has three bottles:
10 fl oz "working" capacity, 11 fl oz total capacity (300ml, 325ml)
20 fl oz working capacity, 22 fl oz total capacity (600ml, 650ml)
30 fl oz working capacity, 33 fl oz total capacity (900ml, 975ml)

Note that there is a "working" capacity and a total capacity.  Why two different capacities?  Well, when you run a pressure stove, you need some air space in the bottle for it to work right.  MSR always marks their bottles with a fill line.  Do not fill past the fill line.  The capacity up to the fill line is the working capacity.  The capacity up to the physical top of the bottle is the total capacity.  For running a stove, use the working capacity.  For storage and transport, you can fill the bottle to the total capacity.

Examples (assuming 1.5 fl oz/47ml per day)
10 fl oz (~300ml) = 6.5 days
20 fl oz (~600ml) = 13 days
30 fl oz (~900ml) = 20 days

For almost all of the trips that I've done, a 20 fl oz/600ml bottle has been plenty.
A 20 fl oz/600ml MSR bottle in use with an MSR XGK II stove
The only time I've really wanted a 30 fl oz/900ml bottle was when I was doing a lot of snow melting.  At home, I use 30 fl oz/900ml bottles for storage.  If I were going to buy just one bottle, I'd probably buy a 20 fl oz/600ml sized bottle.  For short trips, I'd carry a bottle with a lot of empty space in it.  for longer trips, I would fill the bottle.  If all you take is short or weekend trips, you might consider a 10 fl oz/300ml size.  If I did a lot of winter  trips, or wanted bottles for storage, the 30 fl oz/900ml size make a lot of sense.

Now, these are my numbers.  The only way you're going to know how much fuel you're going to use is to get out there and do some trips.  These numbers are somewhat conservative by design, but you might want to carry a little extra fuel until you get it dialed in.  Be aware that in windy and cold conditions, your fuel usage may go up.  Hopefully these numbers will give you some rough idea of how much fuel you might want to bring along.

I thank you for joining me on another Adventure in Stoving,


Wednesday, April 4, 2012

Canister Stoves 101: Thread Care

Don't screw on your canister stove too tight.

Why not?

Well, take a look at the threads on a canister some time.
Notice how the threads do NOT go all the way out and form a sharp line.  These threads are rounded off.  That means that the only part of your stove's threads that are going to grip the canister is the very tip (very outer edge) of your stove's threads.

Whoa!  Wait a minute.  Do you mean to tell me that the only thing holding the stove on to a canister is the edge of the threads?

Yes, that's what I'm saying.  And that's a fairly thin bit of metal.  Too much pressure will put excessive wear on your stove's threads.  Heavy users have reported threads so worn that stoves have literally fallen off the canister.  Once your stove's threads are so worn that they no longer grip the canister, you're pretty much out of luck; you have to replace the stove.

So, Canister Stove Thread Care 101:
  • Don't over tighten.  If you're getting good gas flow and no leaking, that's tight enough.
  • Use the little cap on your canister.  Sure, it's a hassle to keep track of the little plastic cap, but dirt, sand, etc on the canister's threads could screw up that nice  stove you've got.  
  • Keep the threads clean on your stove.  Keep it wrapped in a bandana or something to keep crud out of the threads.  I normally keep my canister stove wrapped up and in my cook pot when it's in my pack.  Some stoves come with a little pouch or case which may be a good alternative if you don't store your stove in your pot.  Don't wrap your stove in something that sheds a lot.
  • Inspect the threads on your canister.  If there's damage on the canister threads, get a new canister (if available).  It's not worth it to use a damaged canister which might screw up your stove.
  • Do NOT cross thread.  Make sure you thread the canister on correctly and that it turns smoothly.  If it feels "funny," stop and check.  Cross threading is a great way to damage your stove.
I generally don't recommend lubricating the canister threads because crud might be more prone to stick to the lubricant, but I do know some people who lubricate the canister threads with silicon grease to prevent binding.  Perhaps there may be some advantage to this approach.

The main thing to remember with canister stoves is that a little care goes a long way.  Keep things clean, don't over tighten, and be careful how you screw on the canister.

I thank you for accompanying me on another Adventure in Stoving.


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