Windscreen airflow calculation?

The scallops ought to work great, you probably need to add a base/heat shield to the bottom. Without it, teh edges can sink into the sand/dirt thereby changining your airflow. Been there / done that.

I was thinking about that as I did the test boil, so I started f***ing around with some JV-level maths, and this is what I discovered: the scalloped notches are 5/16″ tall and just under 3.75″ long, so each one of them can provide more airflow than the stove actually consumes at full blast.  Likewise, the notch for the fuel hose should be able to supply an equivalent amount of air, so – theoretically – I should be able to bury all of the air inlets and still have enough air supply to keep the burner happy.  Theoretically.  Only one way to find out. 😉

Great looking design.

Thank you!  It worked pretty well, thus far.  I’m going to set it up beside an extra-big-ass tomorrow or the next day and see what kind of boil time I get.  If it’s under 6 minutes at a similar throttle level, I’ll be happy.

I find that the 4-6 minunte target works well for me.  That stove output is probably 500-750 wattts.  The Kovea Spider in a wndsceeen (no wind) has consistantly boiled 2 cups of 7p0 F using about 6 grams of fuel.  I am looking forward to seeing your results.

(edited: changed 7p to 70)

a modified version of the Caffin Sandwich
To use that name you MUST include whole-grain mustard.

Cheers
PS: looks good.
PPS: windscreens are good. So are lids.

I find that the 4-6 minunte target works well for me. That stove output is probably 500-750 wattts. The Kovea Spider in a wndsceeen (no wind) has consistantly boiled 2 cups of 7p F using about 6 grams of fuel. I am looking forward to seeing your results.

I’ll do a 500ml boil and weigh the canister both before and after; the scale resolution is a hundredth of a gram, but it’s often thrown off by air currents so the practical accuracy is a tenth or so (unless you’re averaging).  I’ll try to average.

To use that name you MUST include whole-grain mustard.

Okay, it’s official: we can hang out.

PS: looks good.

PPS: windscreens are good. So are lids.

Thank you.  I’m going to work on a lightweight flat lid in order to save half an ounce from the total weight, and to create what may well be the most dangerous pot lid…in the world. 

Hmm.

The figure of 3.7 kW is a bit high compared to the average canister stove, but there are others with a similar rating. The rating does depend somewhat on the design of the burner head. Mind you, running at 3.7 kW is very wasteful of fuel. Dial it back to half that and be happy.

Now, white gas stoves struggle to get to 2.4 kW flat out. There is an old myth that they are more powerful than gas stoves, and MYTH is what that is.

The origin of the myth is however known. The early French Bleuet stoves using those little puncture canisters were underpowered, but they are now antiques. Dangerous antiques, actually. They figure in a lot of burns cases in France. Avoid them.

Cheers

The biggest problem with the Vega is that it’s very hard to tell exactly what power level you’re on until you get some experience with it; the valve is almost entirely non-linear.  It takes about a full turn of the control wire to open it up enough to ignite, and that’s almost exactly 40% open on the valve.  From there, you have another full turn until you hit 80% valve opening, at which point all semblance of fuel efficiency and sanity have fled.  Using the remaining 20% of the throttle is basically like going to Plaid, so it’s both useless and destined for disaster.  Also, my “30% power” guess is just that: a guess.  I don’t really know how to measure it, but I’m also not too interested in doing so: as long as it’s working correctly and heating water as intended in a reasonable amount of time, I’m happy.

Never played with a Bleuet; sounds like I don’t want to.

Bleuet: once punctured, you can NOT remove the stove.

I had one leak in my pack once: there was this strange hissing noise behind my head while I was walking along. The mercaptans made my pack stink for a while.

Cheers

Bleuet: once punctured, you can NOT remove the stove.

Oh, so it’s the stove equivalent of a Renault Vel Satis.  You know, that’s the one that had the “oh dear God, why won’t it slow down!?!?!” problem a few years before Toyota copied it.

Some small and weighty bits-o-data for those that were interested:

Stove: 183.93g
Canister, before connection: 185.03g
Total: 368.96g
Stove + Canister, after connection: 368.39g
Loss during connection: .57g
Stove + Canister, after burn: 360.32g
Loss during ignition/start/burn: 8.07g
Canister, after disconnection: 175.95g
Loss during disconnection: .44g
Total connection loss: 1.02g

I would not have expected to lose that much fuel just popping the canister off and on, despite the rather pronounced explosion of gas that inevitably results: 1.02g is a lot of gas to kill during every cooking event.  I may just waste a canister at some point by repeatedly screwing it on and off of the stove connector and weighing it during those intervals.

Disclaimer: I had a slight problem with ignition; I was trying to start the stove with the valve slightly-too-closed…so it took about 20 seconds to get the stove actually started. I then gave it approximately 20 seconds of warm-up time before dropping the pot on it, prefilled with 500ml of 65° water; that action plus opening the throttle took another 5 seconds or so. Times for events in ambient room airflow are as follows, with all of the warm-up activities listed as a negative number prior to the throttle being opened at 00:00.

Gas valve open: -00:45
Ignition: -00:25
Warmup complete, pot placed on burner: -00:05
Valve opened for main burn: 00:00
First air bubbles forming: 00:53
First air bubbles rise: 01:10
Surface steaming: 01:21
Roiling boil: 02:55
Total time of gas flow: 03:40

It’s definitely a powerful burner, but I wouldn’t have expected 8g of fuel to be used in that short of an interval. Adding in the 1g loss from connect/disconnect, it’s reasonable to assume a 10g usage for every 500ml boil. I would have to test 1000ml times separately, but I’m not sure that it would be a linear increase in the amount of fuel consumed.

So…at these rates, a small 110g canister yields 5 days of hot water at best. I think I’ll do a test with a fan blowing on the stove and see what happens.

The gas loss during connection is bad! Especially in a small space like a tent.

Cheers

I ran a second test and did a 1000ml boil: right at 6 minutes, so the time was linear after all…but my fuel usage wasn’t.  I’ll have to go grab the numbers I scribbled down and do some quick math, but if I’m getting it correct in my head, the usage was about 13g.  The connection loss was somewhere around .8g, I think, and that was me getting the canister off and on very quickly.

You might just connect / dosconnet 10 times to see what the average is.  You have me curious, I am going to do this on a canister topped stove.

Here are the resultss:

Soto WindMaster – 10 connects / disconnects.  Total loss 3.2 grams or 0.32 per sue

Pocket Rocket Deluxe –  10 connects / disconnects.  Total loss 3.5 grams or 0.35 per use

Theese are 2 very differnt stoves.  The Pocket Rocket seals on the top of the lindl valve and the Soto seals in the inside of the drawn cup.  That is the data that I found.

So what is causing the gas loss?
Gas comes out when the brass pin in the bottom of the stove hits and depresses the plug inside the Lindal valve. Of course, this could mean the escape of a lot of gas. This escape is meant to be prevented by the O-ring around the brass pin: it is meant to seal against the top of the canister nipple.

What happens if the pin hits the plug before the O-ring presses on the top of the nipple? You guessed it: the gas escapes, and continues to escape until the stove is screwed down further.

How can you make this worse? (Yes, I mean that.) If you have the canister tilted on its side what escapes my be liquid fuel rather than gas. Liquid fuel is much heavier than gas of course, so you will lose a lot more. Moral of this story: keep the canister dead upright when attaching the stove.

Is there anything else you can do? Yes, there are several things.
* First, you could shorten the brass pin. Tricky to do neatly, and it is non-reversable. Not recommended unless you are really sure.
* Second, you could stick another O-ring in the end, on top of the existing one. This works, but you will need a fairly thin O-ring. A 1 mm cross-section might suit. This can be removed later, so it is reversible. An alternative here is to replace the current O-ring with an even fatter one.
* Third, you could buy a different brand of canister, in the hope that it will be more compatible with your stove. This would need checking, as most canisters use the same Lindal Valve (B188 I think). But I have my doubts about canisters made in China: they may have copies of the valve.
* Fourth, you could return your current stove for a refund. You would need to check first that several other brands of stoves do not have this problem with this canister to be certain that the problem is with this stove.

Needless to say, the second option might be the easiest to try.

Cheers

Jon, I’m definitely going to check the connect/disconnect losses tomorrow when I get back to the shop and can use the accurate scale, and do some averaging.  I lost 1.07g during the c/dc cycle on the 1L boil, but I’m pretty sure that I was tilting the canister unintentionally.  To be honest, I’ve never thought that losing so much gas during that action was possible, so I’ve never formed good habits.  That’ll change.

Roger: I think it’s mostly bad habit that’s causing the loss.  I’m sure that I let some liquid out, and that’s just due to the ergonomics of the system.  I don’t think I’ll modify the brass pin, yet; I think some habit-reforming is in order first.  I’ll also try a different canister; I’m using a Snow Peak right now (Korean manufacture) but I have a fresh MSR laying around that I can check.

I guess I should put that info in a different thread; we’re way off- topic at this point. 😉

Interesting thread.

I’ve been using a MYOG windscreen for the same pot for years, made of aluminium flashing. It’s 70mm tall, dictated by what will fit vertically in the pot. Weighs 29.3g. Sits around a Fire Maple 117T remote canister stove.

I’m surprised by how little combustion air yours needs. I find I need to use rocks to raise mine 1-2″ off the ground, else the stove chokes. Perhaps this is because there isn’t a lot of clearance for exhaust gases? But raising it reduces the gap between pot and shield, though perhaps exhaust is now venting down.

No measurements, but this set up does a great job of protecting from the wind. Tempted to add 3 riveted legs that swing down to prop it up, but finding rocks or similar is usually easy.

Biased opinions:

Those little holes are not very good at low pressure. You would be better off with a small number of larger holes, and much better with a 1″ vertical gap in the windscreen.

Even more important is the gap between the windscreen and the pot. What the photos show is far too small/narrow. The last photo looks terrible to me. I would expect the stove to ‘choke’ a bit with that small a gap.

Suggestion: keep the current windshield but undo the join and leave it about 1″ wide. You can put the hose through it. Have a look here:

Any gaps smaller than shown here are, imho, too small. That is for both the inlet gap and for the exit clearance around the outside of the port.

My 2c
Cheers

Even more important is the gap between the windscreen and the pot. What the photos show is far too small/narrow. The last photo looks terrible to me. I would expect the stove to ‘choke’ a bit with that small a gap.

I agree with Roger that the annualar gap is too narrow.  The exhaust gassses are probably choking off the overall airflow (pressure build up).   Lifting the windscreen drops the pressure by allowing the exhaust gasses to also escape out the bottom.  I am less concerned about the inlet holes in the bottom.  My 2 cents.

Might try a V2 with ½” gap around pot, no holes but 1″ swing-down legs. See how it goes enclosed vs open.

To emphasise the point: if a windscreen with a fixed join fits inside the pot, it is too small.

Cheers

I think mine works so well with such small gaps at the bottom because of the large hose port and the wider exhaust gap at the top.  I’ve been playing with it and it’s surprisingly wind-resistant…and it does a good job with helping heat transfer.

Why, Roger? If it’s raised an inch above the ground, there’ll be as much or more air intake than shown in your picture; and if there’s an inch around the pot, plenty of exhaust space. Might not block strong winds as well, but can block windward gap pretty easily if needed.

It isn’t a fixed joint. Actually, it looks OK for low to medium power applications. ie up to about 2000-2500BTU (0.5861421 kWh-0.7326777 kWh.) I usually run the exhaust a bit smaller (about a quarter inch but on low power) with just a air inlet slot for the stove handle and a 2in/5.1cm notch for the hose. Caffin v1 with a aluminum pot/lid gets 4-6g/half liter per 7min consistently as long as temps remain above 32F/0C. The stove should be low enough that the wind doesn’t effect the airflow much, due to ground friction. A few more air holes on the inlet side will insure a clean burn.
I think you are good, just don’t turn the stove up beyond low/medium unless you open the wind screen. There is really no set number for dimensions of the input/output. It is variable depending on stove power/wind/ambient temp…more art than science or should I say “black magic.” A slot will not restrict outflow, but still slow it down good for good heating. One improvement would be to make it about 1.5-2″ taller to cover the lower 25%-50% of the pot. This does more to use the exhaust as a buffer against winds (0-5mph) than for extra heating of the sides.