[at-l] of stoves and maps

[gwa at antigravitygear.com (George Andrews)]

Diane,
Here is an email recieved from Russell Trenholme re: stove tests.  The
photos probably will not come thru with this email but it will be posted on
alcoholstove.com as soon as we are finished transfering the website to our
new server.  Hopefully that will be completed this afternoon.
Tin Man
AntiGravityGear.com

Soda Can Stoves: Tests and Observations
Russell Trenholme

In recent years small alcohol stoves made from aluminum soft drink cans have
become popular with ultra-light backpackers. At least two are sold
commercially?one designed and made by George Andrews, available from the
Anti-Gravity Gear website www.antigravitygear.com (called the ?Tin Man?
stove; directions are on the Zen stove site) and one made by ?Brawney? and
?Bookworm? and sold at the Dancing Light Gear website
www.trailquest.net/store.html. Both are made from two 12 oz Pepsi (or
similar)  aluminum cans. The basic construction involves fitting pieces cut
from two can bottoms together by flipping one bottom over to form the top of
the stove and squeezing it into the other bottom (or vice versa).

In three of the stoves being tested a rectangular piece of thin aluminum cut
from the side of one of the cans shaped into a cylinder that is inserted
between the two pieces to form an inner wall. The circular metal inside the
elevated rim of the stove top is removed leaving a well whose sides are the
cylinder just mentioned. Alcohol is poured into well and flows under the
cylinder into the concentric inner chamber bounded by the inner cylinder and
outside of the can bottoms. At the top of this chamber, or along its side,
are 12 to 32 burner holes. When alcohol heats up in the inner chamber, it
vaporizes and streams out through these holes where it oxidizes as a blue
flame. The holes are evenly spaced, according to the design, around the top
rim (accessing the area between the outer and inner wall) or lower down on
the outside of the stove (in the Tin man stove). The basic construction of
these three stoves is described on several websites. The Scott Henderson
design is similar to the Brawny stove with burner holes around the top, but
the Henderson design calls for 24 or 32 smaller (as small as 0.024?) holes
whereas the Brawny stove has 12 much larger holes (about 7/64? in diameter
each).

A related design is Don Johnson?s ?Photon? stove. In this design the inner
wall is formed by the sides of the top. The top is not cut out to form a
well. Instead, a hole in its center provides an opening for pouring in the
alcohol, after which the opening is closed with a screw.


Clockwise from Top Left: Photon, Henderson, Brawny, and Tin Man Stoves

Performance Tests

Many of the stove designers appear to be engineers or craftsmen who provide
precise directions as to how their stoves are to be made and (in some cases)
why they have chosen various design features, including wall height, hole
number, hole size, and hole placement. There are also detailed directions
for making pot stands, reflectors, and priming pans.

I believe that the thermodynamics is so complex that only side-by-side
comparative testing can provide any sort of useful data for evaluating the
relative effectiveness of the different designs. Some of the variables that
affect performance are: (1) definition of ?boil;? amount of water;
temperature of water; type of surface beneath stove; air temperature; wind;
type of pot; distance of pot above flame; type of pot stand; type of
windscreen (if any); method of priming. I decided to compare several of
these stoves under a standard set of conditions. These results cnnot be used
to predict how any particular stove will perform under other conditions?in
fact, I?ve already received complaints that the times-to-boil I recorded are
too long. The most meaningful result is the ranking of the stoves with
respect to boil time. However, I think the results do reliably predict the
relative performance of the stoves under a large range of conditions.

The four soda can designs tested are: (1) Don Johnston?s ?Photon? (homemade)
(2) a Scott Henderson design stove (homemade) (3) the Tin Man stove (4) the
Brawny stove. The first two stoves were made following instructions on the
Wings, Home Made Stove Archive website wings.interfree.it/html/main.html (no
www). Also tested were the well-known Trangia ?Westwind? stove, the
Brasslite Turbo II-D (available from www.brasslite.com), and a canister
stove, the Snow Peak GigaPower.

Three of the soda can stoves share the same basic construction and weigh the
same, 0.4 oz. The only functional difference is in the size and placement of
the burner holes. The homemade Scottt Henderson stove has 32 small (.024?)
burner holes around the top (other instructions specify fewer and larger
holes); the Brawny stove has 12 larger holes (7/64? is my best guess) also
around the top, and the Tin Man stove has 24 medium holes (made by a #57
drill bit, .045? in diameter) distributed around the outside wall of the
stove 1? above the bottom (5/8? below the top). These three stoves have an
open well at the top which faciliates fueling, since a line may be scored on
the wall indicating 1 ounce of alcohol or whatever amount is normally used.
The Photon stove design is different; the top is covered except for a small
hole which is closed with a screw that is removed to put the alcohol into
the stove.

Test Conditions

 The tests were performed in my kitchen at a mild (70 degree) temperature.
(Some follow-up tests were performed later on a day when the temperature was
in the high 80s. This resulted in faster boil times.) 16 oz. of tap water
were placed in a  .9 liter Evernew Titanium pot (handles removed) which was
cooled after each test. No top was placed on the pot in order to observe the
time of boil without disturbing the process. Each stove (with the exception
of the Brasslite which has an integral bottom rim that can serve as a
priming ring)  was placed on a small piece of foil cut from the bottom of a
disposible muffin cup (that weighs less than 0.05 oz); this was placed on a
wooden cutting board. (Aaron Rosenbloom of Brasslite pointed out that an
insulating surface should be under the stoves because the flat bottom of the
Brasslite will transmit more heat than the curved bottoms of the soda can
stoves thus making the Brasslite slower to heat up.) No wind screens or
reflectors were used. A open structure, homemade pot support (made from two
pieces of 1/8? bent steel rod with one end of each inserted into a short
piece of aluminum tubing) was used on all tests as a pot support, except
with the Brasslite which has a built-in wire support, and with the Tin Man
design which serves as its own pot support. Since the arms may be spread to
different distances this support accomodates a large range of pot diameters.
The homemade support height above the burner holes of the Brawny, Henderson,
and Photon stoves (1 1/8?) was identical to that of the support sold with
the Brawny stove. (I did not want to bias the test of the Brawny stove by
using the pot support supplied with it. This is addressed later.). The pot
support used was cooled between tests.


The Homemade Pot Support

One ounce of denatured alcohol, measured carefully into a small graduated
cup, was used for each test. All the stoves will bring two cups of water to
a full boil with a little less alcohol, but one ounce was a safe standard.
Priming was done with 1.5 ml of alcohol (a little more than ? tsp.),
measured by graduated eye-dropper and dribbled in a the foil base piece
described above. This is not the priming method recommended for all the
stoves, but it provided a uniform start for all and appeared to be perfectly
safe. In the case of the Brasslite, 1/2 tsp. of alcohol was put around the
built-in primer base in accord with the winter start procedure; in the case
of the Photon, half the priming alcohol was placed around the top ring as
per Johnston?s directions and the other half dribbled around the base.)

Boil time is not the only important variable in a timed test. It took
approximately 30 seconds to open the alcohol container, measure out the
alcohol, pour it in a stove, load the eyedropper, dribble the priming
alcohol around the base, and light the stove. Under field conditions things
would go faster since precision would not be important; moreover, in the
case of open well designs like the Brawny, Tin Man, and Henderson stoves
alcohol can be poured from the bottle into the stove using a mark on the
inner wall as a guide as to the amount. In the case of the Photon stove, an
additional 25 seconds was required to remove the screw, pour the alcohol in
little by little (waiting for it to drain through the small center hole),
then replace the screw. Even though with practice this could be done more
quickly, it adds at least 15 seconds.

Boil Times

Boil time was recorded when the suface of the water broke into a rolling
boil. Undoubtedly, much faster results will be reported if bubbling from the
bottom is taken as boiling, or active bubbling short of a rolling top
surface. I chose a full boil because it represents a stable end point, easy
to recognize in each case. Even then I think subjective judgement could
affect the result by up to 10 seconds either way. I then recorded the total
time from ignition to burnout. This gives some indication of how much the 1
ounce of alcohol might have been reduced and still achieve boil.

Results: Tin Man 4:58 (burn-out at 7:20); Photon 5:07 (burn-out at 7:55);
Brawny 5:20 (burn-out at 7:45); Henderson design 6:27 (burn-out at 8:43);
Brasslite Turbo II-D 6:37 (burn-out at 10:10); Trangia Westwind 8:35
(burn-out at 14:40). For comparison, the Snow Peak brought the water to a
boil at 2:20.

The tests were repeated for the Brawny stove and for the Tin Man stove on a
hot (85 degree day) and the boil times were faster: 4:25 for the Tin Man
stove and 5:12 for the Brawny stove. (The large burner holes in the Brawny
stove appear to result in a slighter cooler (more yellow) flame than the
smaller holes in the other stoves.)


The Tin Man Stove (modified holes) in Action

One-Cup Test

Since these small soda can stoves will find an application as a light
day-hike or single overnight source of hot water, I tested the Brawny stove
and the Tin Man stove with a small Snow Peak Titanium cup containing a
single cup (8 oz.) of water. This time I reduced the alcohol in the stoves
to 20 ml, about 2/3 oz. The results were much faster, but consistent with
the two-cup results: Tin Man stove with pot directly on top of stove: boil
2:52 (burn-out 4:15); Brawny stove with the pot on wire stand: boil 3:13
(burn-out 5:01).


The Small Snow Peak Pot and the .9 Liter Everlite Pot
Pot Supports

The Tin Man stove requires no pot support?at least not for pots the size of
the Everlite pot (5 ??) or even larger depending on wind and the surface
under the stove. Since the upper rim that serves as a pot support is only 1
15/16? in diameter, a simple calculation shows that over 86% of the pot
bottom is exposed to the flames. I tried to improve on the Tin Man stove by
constructing a stove with outer walls that extended up to a little above the
upper rim of the central opening, thinking that this would provide a larger
diameter base (2 9/16?) for supporting a pot. However, boil times were
considerably longer. This is probably because over 24% of the pot bottom was
cut off from the flames versus only 13.5% in the original Tin Man design.

I had feared that the pot support sold with the Brawny stove might bias the
boil time in its favor since it is described as a combination pot support
and reflector. It consists of a 2 7/16? high section of steel can 3 7/8? in
diameter with 12 7/16? diameter holes centered ?? up from the bottom, and
two large (3? wide) wedge-shaped holes cut out on opposite sides (presumably
for air flow). Surprisingly, the pot support slowed the boil time
considerably, and the stove was just approaching full boil at 7:45 (first
test) and 7:34 (second test) when it went out. The test was repeated because
the result was so unexpected. It appears that the large areas of can left
intact at the top (about half the original top of the can) prevented the
burner flames from reaching large areas of the bottom pot surface . Instead,
half the flames from the stove seemed to be heating the support rather than
the stove, with the flames spilling through the cut-out wedges doing most of
the water heating. The homemade open design pot holder used in the test
performed much better, as seen in the above result for the Brawny stove.
(The test with the Brawny pot support was repeated on the 85 degree day with
a boil time of 5:56 and burnout at 6:20?still considerably slower than the
5:12 time recorded with the open structure wire stand.)


The Homemade Support and the Brawny Support
[Insert picture of two pot supports]

I haven?t tested hardware cloth pot holders (other than the built-in one on
the Brasslite stove) but they should perform better than the can support,
although perhaps not as well as the open design used in the tests. In
addition, I did not experiment with different pot elevations above the
burner holes. The Brawny support and my open design support were 2 3/8? in
total height thereby elevating the pot 1 1/8? above the burner; Don Johnston
recommended a hardware wire stand 2 ?? high for the Photon Stove, and the
Tin Man stove only supports the pot bottom 5/8? above the burner holes.

Windscreens

There are a number of designs for windscreens and field testing would be
required to determine which are most effective. The Tin Man stove came with
a windscreen formed from thin aluminum flashing metal. It is a rectangular
piece of aluminum 2 ?? wide by about 23? long with a ?? fold on each end;
when assembled it is rolled into a cylinder 7? in diameter (held by
interlacing the folds on the ends). The cylinder is placed around the stove
and pot with a clearance of about 1? on all sides. This was tested on the 85
degree day and was found to reduce the performance of the stove considerably
with burn out at 7:42 just as the stove came to a full boil (compare 4:25 to
boil with no windscreen under the same conditions). It seems clear that the
Tin Man windscreen was starving the stove of oxygen. In fact, similar
designs provide small ports along the bottom of the windscreen to eliminate
this problem and it would seem that some such modification is needed.
However, because windscreens require field testing, I did not perform
further tests.

I did make a cylindrical windscreen of this type, although considerably
taller (3 ??) to cover much of the side of the Evernew pot. I found it is
was necessary to cut large notches in the bottom to allow sufficent air
flow. I then mounted thin brass rods through holes at 90 degrees to form a
cross pattern as pot support. This would seem to be the simplest and
lightest solution for a combination windscreen and pot holder for stoves
like the Brawny, Henderson, and Photon stove that require pot supports.


Left Homemade Reflector with Pot Support Rods. Right Tin Man Reflector.


Simmering

The Anti-Gravity site sells a variety of insulated cozys as a fuel-efficient
alternative to simmering and an ultralight hiker will probably go this route
to minimize fuel consumption. However, when simmering is desired, it is
necessary to use some kind of control to reduce the flame. The Trangia has
an inexpensive simmer control and the Brasslite Turbo II-D has a built-in
simmer control. There are instructions for making various simmer controls on
the websites for soda can stoves. The Dancing Light website that sells the
Brawny stove has directions for making a very simple, light-weight simmer
device out of two disposible muffin tins. The construction takes about two
minutes. With this simmer cover in place, I got two oz. of alcohol to last
over 1 ? hours; their website claims over 2 ? hours on 3 oz. of alcohol.

Under normal conditions, the simmer device is only placed after the water
has come to a boil. This will reduce these extremely long burn times
exhibited by use of a simmer device from the time a stove is lit. Still,
even when the simmer device is put on after the stove brings the water to a
boil, 2 oz. of alcohol should produce a long enough simmer time for any
camping recipes. I constructed a simple simmer device for the Tin Man stove.
Since the pot sits on the stove, the muffin cup design can?t be used. I cut
a strip of aluminum 9 ?? X 1 ?? from the bottom of a disposable baking dish,
then cut slots ?? from each end, one from the top and the other from the
bottom. These slots interlace to create a cylinder a little larger than the
stove. The point where the ends overlap is secured with a paper clip whose
top is bent over at 90 degrees to provide a little handle for dropping the
device over the burning stove after the water has come to a boil. It was
found necessary to punch holes along the base of the cylinder to provide
enough air to keep the stove from going out. The device was tested by
bringing two cups of water to a boil as before (time 4:52). Then the pot was
removed from the stove, the simmer device was dropped over the burning
stove, and the pot replaced. The water continued to simmer for over 35
minutes longer, and the stove finally went out at 43:11.


Left Homemade Simmer Device around Tin Man Stove. Right Muffin Tin Device
Over Brawny Stove

These simmer devices (especially the one I made), need field testing under
conditions of wind and cold, but they do suggest that soda can stoves stoves
are quite capable of simmering with the right design simmer device.

Some Comments on Stove Construction

The designers of these stoves take great pride in their work. To those of us
who are not engineers or craftsmen/craftswomen, their directions may seem
fussy and intimidating. My tests suggest that a wide variety of designs will
function well. Moreover, much of the precision seems unnecessary. For
example, I punched the burner holds holes by eye using the following quick
and dirty procedure. I spotted the bottom center of the well by eye, then
made two dots at opposite side of the rim. This can be checked by laying the
edge of a piece of paper or other straight edge across the top to make sure
that the center spot and the two rim spots line up. Then I eyeballed a point
half-way between the first two dots, marked it, and repeated on the opposite
side, again using the center spot at the bottom of the well as a guide. Next
four intermediate points were spotted on the rim to produce eight dots
evenly spaced rim; minor corrections can be made by eye. In the case of the
Tin Man side-burner design, these dots were brought down by eye to mark dots
along a circle drawn around the stove at the desired height. Again a visual
check insures that these eight marks are equally spaced. At this point I
punched eight holes through the dots with a small drill bit held in a pin
vise. (Only leave about 3/32? of the bit protruding.) I then wiped out all
the marks with alcohol, and evenly placed either one dot between each pair
of holes (for a 16 hole design) or two dots (for a 24 hole design). The new
marks are verified by eye, then punched out. This procedure has proved
accurate and I have yet to create a stove that produces an asymmetrical
flame pattern. A drill bit of the desired final size in then put in the pin
vise and the holes are enlarged.

There are other short cuts. For example, it is not necessary to make a jig
to scribe the sides before cutting the top and bottom pieces. Just stack up
magazines or thin books so that a standard utility knife can be held flat
against the top of the stack. Score score the side of the can at the correct
height by rotating the can while firmly holding the knife. This produces a
perfect circle at the desired height which is then cut out with ordinary
scissors. A similar procedure is used with a fine tipped marker in place of
the knife to draw the circle used to locate the burner holes (in the Tin Man
design).

George Andrews describes how to enlarge the diameter of one half of his
stove so the other half can slide inside it. But it?s easier to just cut
(again by eye) eight slits in the inner piece, and carefully push this piece
into the uncut half. The slits will be hidden in the final stove in any
case.

Nor have I found it necessary to use glue to secure the cylinder that forms
the inner wall; I have just used a piece of scotch tape which will later
burn away. There is no seal between the outer chamber and the central well
in any case and the cylinder seems to close well without glue.

I am not urging sloppy construction but simply wish to reassure
non-craftsmen that making these stoves is simpler than most directions
suggest and that rough and ready products will function just as well as
crafted and engineered products.

The one part of the construction that I have found difficult is cutting out
the top opening of the central well. I have had to score the circle over and
over with a new blade and still end up with a certain amount of scissors
work. (George Andrews advises using a drill press?not an option for most of
us.) Of course a sloppy cut, provided it does not distort the rim or cut
into it, will work just as well, and is safe enough if any sharp edges are
filed down.

Because of the good results with the Tin Man stove, I obtained two
additional stoves from George Andrews, but without holes. I proceeded to
make 16 .026? holes (using a small drill bit in a pin vice) in one of these
at the same height as the holes in the Tin Man stove and 24 such holes in
the other. I then enlarged the holes step by step to try to determine the
optimal number and size of holes. My best result was with 24 holes of .038?
diameter (made with a #62 drill bit)?boil time was 4:36 and the stove went
out at 7:01. I repeated this test on the 86 degree day and got a slightly
faster boil time of 4:26?but this was no better than the results for
original Tin Man stove tested on the same day. It seems that 24 holes of
either  .038? or  .045? diameter produce similar results.

It?s fun to make one of these stoves, but since both the Tin Man and Brawny
stoves sell for $12, it is hardly necessary.

Notes on How the Stoves Work

Various designers have given theories about the operation of the stoves. For
example, closing the well on the Photon stove with a screw is supposed to
provide a pressure advantage, but the boil times indicate that open well
designs can work just as well. At one point, I tried closing the open well
on the Brawny stove with a disk, but the boil time was the same. Nor does it
appear that air intake openings are needed (either by means of an open well
or by means of the small holes used in the Photon stove). The Tin Man stove
works well without any holes and with a heavy flat bottomed pot firmly
closing off the top of the well.

It appears that the key to the functioning of these stoves is the
heat-induced vaporization of alcohol within the walled outer chamber. As the
alcohol vaporizes it expands and jets out through the burner holes,
oxidizing in the heated air surrounding the stove. The burning of the
alcohol vapor heats the outer walls of the stove (and of the chamber)
thereby  vaporizing more alcohol which continues to flow in from the center
well. If a partial vacuum were created in the center well of the Tin Man
stove because of the pot bottom sealing its well it would be off-set by
vapor expansion of the heated alcohol. This analysis is at least consistent
with the test results that demonstrate the irrelevance of air intake ports
(or an open well) for performance. (I did make small air intake ports in one
of the Tin Man stoves, but they worsened the stove?s performance.)

The goal of an optimal design is to avoid wasting alcohol through excess
flaring or through incomplete combustion due too low a heat or too little
oxygen. A steady blue flame indicates good combustion. The small hole
designs (including the 32 hole Henderson design) were efficient enough, but
did not produce sufficiently large flames to boil water as quickly as other
soda can designs.

Conclusions

1. Denatured alcohol is inexpensive and readily available. A full 8 oz
plastic bottle of alcohol weights 8.2 oz versus 7 oz for the small (110 g)
Snow Peak cylinder. But since the Snow Peak Gigapower stove weights 4 oz
verus 0.4 oz for the soda can stoves, a soda can stove outfit will be both
smaller and lighter.
2. The Trangia Westwind is relatively heavy (2.4 oz. without the simmer
device, stand, or screw top) and slow to boil water.
3. The Brasslite Turbo II-D is in a different class altogether. It is a
proven candidate for serious conditions where the fragility of the soda can
stoves is an issue. However, it is both larger and heavier (2.7 oz.).
4. The Photon stove, despite its clever design, seems to present no
advantage over other soda can designs which are quicker to fill and as fast
to boil.
5. The Brawny stove (0.4 oz) performed well and has a simmer device (0.3 oz)
that has been field tested. It would probably perform a little better with
more than 12 holes of a smaller diameter. The Dancing Light pot support is
not recommended because it adversely affects performance, but it is easy
enough to make a support that does not raise the boil time.
6. The Tin Man stove (0.4 oz) appears to be the best choice for ultra-light
hikers since it requires no pot stand and requires less pack space than any
alternative with the possible exception of Esbit tablets. Its principal
limitation is the possible instability of larger diameter pots, as well as
the fact that no simmer device has been field tested with it yet. Where
cozies are used, the latter is not an issue. The windscreen supplied with it
should be modified by adding air holes at the base (this is a matter for
experimentation).
1. The optimal size and number of burner holes has yet to be determined. The
Photon with very small (0.024?) holes was no faster than the Tin Man stove
with 0.045? holes, and the Henderson stove with 32 small holes was slower
than the Brawny stove with 12 large (7/64??) holes.

Please send any questions to me at rstrenholme@cox.net




----- Original Message ----- 
From: "Diane Thornton" <dianet@midsouth.rr.com>
To: <at-l@backcountry.net>
Sent: Friday, July 09, 2004 8:13 AM
Subject: [at-l] of stoves and maps


>
> Dear at-lers,
>
> I've built various sized soda can stoves, even one with a Foster's Lager
> can!  Who here knows or can explain the physics involved in the height
> of the stove/can and the diameter.  I'm trying to figure out if it's
> better to have one or the other to produce a greater surface flame.
> Since I'm cooking for me and 2 hiker trash kids I need a longer cook
> time and something tells me just adding more alcohol to the little soda
> stove isn't the best option.  I have a boil time of about 12 mins. For 4
> cups of water at about 56 degrees starting.  Should I build a short
> Fosters (1/2 inch but a large diameter) or a taller soda can (1 1/2
> inches, but a smaller diameter) or a tall fosters or put more fuel in
> the soda can!  Options options.  It's how I entertain myself when I'm
> not on the trail.
>
> Speaking of trails.  It's July and in the south this is NOT hiking
> season, but I can barely stand it.  I think I'll drag the kids out in a
> couple of weeks for a 3-4 day trip depending on temps, snakes and bugs.
> Thinking of the Natchez Trace area around Nashville or Buffalo River
> area near the AR/TN border.  I've never been to either Natchez Trace or
> Buffalo River area and wondered if any of you have (hiked those areas)
> and found you actually needed a map.  I'm debating spending the $ for a
> topo versus using the park's maps (which are usually just about
> useless).  If the trails are well blazed we often don't consult maps
> except for general directions at intersections.  Opinions?
>
> Weasel 2 and hiker trash kids
> P.S. I LOL reading someone's account of hiking on a closed trail.  I've
> done that, but won't admit to where!  Let's just say I'm grateful it was
> shoulder season for hunting!
>
> ---
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