I dunno what the prob was, I suspect something is different this time as the coke stove is the same one. It was the best performing one last week so I used the same design, sizes and all except for the number of jets. The only variable I wanted to play with was the jets.

The coke stove last week gave a fierce rolling boil at 9 min 18 sec and contiued to burn untill 11 min 20 secs, but using the same stove under the same conditions and water, fuel amount etc this week it couldnt even achieve a boil and was burning out at about 8 min 30 secs??

The stoves had no internal wal they were just simple two halfs pressed together the same height.

Im dayshift for the next wee while so I cant really do so many tests at work in peace, but it will give me time to have a wee think about this......oh yes and get out on the hills more often

> When I added the fuel bottle the total was 44gms (1 1/2 oz) for a nice little daywalkers cuppa kit.

Yes having made my 'just a bit of fun' comment last night, I thought along the same lines as you last night, and tried heating 250ml of water in a mini trangia pan.  I overfilled the Fresh Box Stove, so that it burnt liquid fuel for quite a while, 'coughing' fuel from the jet ports every now and then, until it finally started burning properly.  It never got to a boil, though, but got up to about 80C.

I think a slightly bigger burner might make a nice cuppa kit, too.  I even thought about making a combined burner support/cup, along the lines of Ray's oil burner thing a bit further up the thread.

I've just been given a supply of salbutamol cartridges from colleagues, having put out a request on our bulletin board...  Two of them have little counters one them, and an interlock when they reach zero.  What a terrible waste...

On the meths fuel thread, Graham Coe questioned the use of the conic wall in my red bull burners, so I'll discuss this a bit.

I made a burner from little Lilt cans, years ago, but it was a side burner type. It wasn't very successful, but I pondered using the small cans to make a Trangia-style, open-topped burner. Compared with 'normal' drinks cans (330ml, 500ml), the pressed ring in the base is very close to the outer wall, so, if you put an inner wall between the upper and lower rings (as is usual with drinks can burners), the inner and outer walls will be very close together.

It struck me that this might not be a great idea, as the gap between the walls is where vapour is created for the burner jets; what I termed the 'vapour chamber'. I thought that a conic inner wall would produce a bigger vapour chamber, and that this might be good.

So I did the trigonometry to generate the design for the conic section, and tried it out. It seemed to work.

However, since the bottom of the cone sits on the dimple, the fuel ports at the bottom of the wall (which allow liquid fuel to enter the reservoir in the vapour chamber) are some way from the bottom of the can. The Trangia-style burner relies on the presence of fuel above these ports to maintain pressure in the vapour chamber, and thus maintain a good jet pressure and burn. Once the fuel level in the reservoir falls below the fuel port, the pressure drops, and the jets fail, and the burner reverts to a simple open-pot burner, with a reduced output power.

So, the higher the fuel ports are, the sooner this happens. It's not such a terrible thing; in fact, if you use the right amount of fuel, it can even be a good thing; the pan reaches a boil, and then goes on to simmer as the burner power falls.

With a simple parallel-walled burner, the fuel ports are right at the bottom of the burner, so the burner continues to jet almost until the fuel has completely run out.

In practice, I haven't found too much difference between the performance of the conic-walled burners and a parallel-walled burners. Or the closed-top, low-pressure burners.

So, if you're after an easy-to-build burner, go for the closed-top, low-pressure burner, as it needs no inner wall, and doesn't need the top dimple cutting out, both of which require more skill with tools.

I tried a Frumalike burner last night. Not having any tealights in the house, I used the bottom of a red bull can, and made the T-piece heat exchanger. With my standard test setup, the Frumalike burner got the water making noises in 5m15s, but went out after 5m30s. So, whilst it burns and heats faster, it's less efficient with the fuel. This is fairly obvious, since the flame ring is very big, and licks up the side of the pan (uselessly).

I tried a couple of versions; one where I made a crude heat exchanger, with an overlap, and one with a more carefully made heat exchanger, that butted the ends of the inner ring, forcing the inner wall flush against the wall of the reservoir can. In the former case, the flame was much bigger, with a lot of flame coming out between the inner and outer walls. I didn't try a measured burn with this one. In the latter case, since there was very little gap between the inner and outer walls, the flame was more controlled, but still overwhelmed the pan support and pan. This is the one I did a timed burn on.

Whilst the redbull can is a bigger diameter than a tealight, so I might expect a tealight to produce a slightly smaller flame, I still expect it will produce an inefficient burn, with flames licking up the side of the pan.

I also tried a simple open-pot burner, by turning one of the red bull burners upside down, and filling the dimple. Unfortunately, you can't get much fuel in the dimple (about 7ml?), so I couldn't do my usual timed burn, but I did notice that it produced a nice flame ring, and burnt in a very controlled manner, with a blue flame.

captain paranoia wrote (see)

Whilst the redbull can is a bigger diameter than a tealight, so I might expect a tealight to produce a slightly smaller flame, I still expect it will produce an inefficient burn, with flames licking up the side of the pan.

True of the Frumlight, unless you use a very wide pan, but not true of the 'naked' tealight, which is a Frumlight with the supercharger taken out.  This will happily, but slowly, heat water in a 740ml Heineken can without any flame up the sides of the pan. 

Frum, I'd say the hardest thing to determine is the 'time to boil'.  What do we define as boiling?  First cavitation noises? First bubbles?  Rolling boil?  I'd say that will cause more uncertainty in measurements than anything else, especially compared with the 5% difference between 472 and 500ml...

I wanted to standardise on a delta temp measurement, say, between 30C and 95C.  It would require a decent electronic thermometer with a quick response, but at least it would eliminate the uncertainty.

One other method might be the measure to the first point at which steam is seen escaping between pan and lid, but this would depend on the tightness of fit and seal between pan and lid.

And then there are the pan differences which also influence the heat transfer, and the distance from burner to pan, and the starting temperature of the water, and the ground temperature the burner is in contact with, and the air temperature and pressure...

I've been using 500ml for years, measured using the tare function on my digital scales.  Given all the variables that we'd have to try to control to allow true comparative tests, I think I'll just stick with my own testing.  After all, if I see a stove that seems to perform well, I'll just build one and try it out in my 'standard environment'...

I finally got around to playing with foil windshield ideas last night, too.  I got emailed a while back by a UKC member (and former OM member), asking where to get foil.  And I suggested BBQ trays, which I'd been thinking about for a while.  So I went on a foray and found some in Sainbury's and Poundland.

5 trays for £1.29 from Sainbury's.  They're bigger than the Poundland ones, and the holes are bigger and better formed, being folded back.

The Poundland ones are 10 for £1, obviously...

I cut the shorter ends off, and cut across the tray to make two sections.  I unrolled the edge of the trays, and used the excess metal to form a seam, by re-rolling the foil.

You can just about see the way the foil is rolled up to make the joint.

I also went around and opened up the slots with a narrow blade, by folding back the roughly punched solts.  I hope these will provide enough airflow for the burner.

The corrugations make the windshield a bit more sturdy than simple foil, but make it harder to pack away.  I think it would be best rolled up inside the pan.

Made a burner yet, Trev?  Go on, try the red bull penny stove; you can't get much simpler than that.  And it's fairly efficient and effective, too.

Take two cans, cut off the bottoms using the scribe & tear method (about 30-35mm tall).  Cut yourself a shim from the body of the can.  Punch a hole in the centre of one of the dimples.  Use the shim to mate the two halves, and press together until the rim of one meets the shoulder of the other.  Then take a thumb tack and carefully prick 12 holes around the perimeter of the dimple.  Open up the central filling hole to a few mm.  Pour in some meths, stick a penny (or the lighter € cent) over the hole, spill a few drops around the base of the can, and light it.  That's it...

Just realised that Frum's 2 US cups at 474ml didn't make sense to me.  I thought that the US pint was 16 fl oz, where an ounce if 1/16 of a pound, which is 454g.  Since 1ml of water weighs 1g, I'd expected 16 fl oz to weigh one pound, or 454g, so being 454 ml.

However, wiki begs to differ... and agrees with Frum's figure.

Which must mean that the US fluid ounce weighs more than a UK fluid ounce, which, assuming it's based on water, means that a US pound must weigh more than an imperial pound.

And yet, it's article on the pound shows that the US adopted the international pound at 453.59243g in 1958.

So what is their fluid ounce, and how does it relate to weight measurements, and why isn't it the same...?  Oh, here we are... 1 U.S. fluid ounce = 1.040842731 avoirdupois pounds.

My head hurts...

At least an imperial fluid ounce weighs one avoirdupois ounce, which kind of makes sense to me...

> Stop chatting and start stove making Trevor.  You'll enjoy it once you start.

Seconded...

> You would not want holes in a windshield would you?

Yes, you would.  Ideally, you want a windshield to have small holes in it to allow air in to the burner in a controlled manner.  The air is drawn in by the combustion process encouraging a chimney effect.  The main body of the windshield stops the wind blowing the flame about, and it's a matter of fluid dynamics that determines how the holes behave in stopping wind.