Thursday, February 21, 2013

Fixing the Fire Maple FMS-118 Inverted Canister Sputter


The Fire Maple Fms-118 is a nice looking and cheap stove which, on first look, seems to be a good option for winter use with an inverted canister. My testing revealed a sputter issue when inverting the canister for cold weather use, which the I posted about earlier, but I think i've found a fix.

Following some advice from user Stuart R in this thread on backpackinglight.com I worked a bit of wire into the preheat tube to reduce it's ratio of internal volume to surface area. This proved to be a non trivial operation thanks to the sharp bends in the tube, but I found something that worked and the supplies I used and steps I took are below.

I haven't had a chance to test this extensively in cold ambient temps but the mod eliminates the sputter when tested with a canister fresh from our freezer. The Fire Maple now seems almost as sputter resistant as the MSR WindPro II, though the Windpro stove still gets my vote in the "Designers Clearly Knew Their Stuff" Category.

I'll do some more testing with the Firemaple next time we get temps bellow 20 F at the house but the Windpro is the stove we will take into the mountains when we expect it to get that cold. 

Supplies


  • A selection of stranded copper wire to make sure you find a size strand that works.
  • Wire strippers. 
  • Small pair of long nose pliers. I used the one from this Crafstman set from amazon.
  • Yellow Thread Seal Tape (yellow is for use with gas lines).
  • Small crescent wrench.
  • Wire cutters and scissors. 

Steps

  1. Strip six inches of wire and cut off one six inch long strand. Save the other strands incase you mess up and bend the first one.
  2. Remove the fuel line fitting from the burner assembly. I actually disassembled the entire stove but this is unnecessary.
  3. Begin feeding the strand into the pre heat tube. It will be stopped by the bend almost immediately.
  4. Using the long nose pliers grasp the wire as far inside of the burner assembly as you can and push it into the preheat tube. Be methodical and only feed in about 1/8-1/4" at time. Any more and the wire might bend instead of pushing into the tube. Once a wire bends it is difficult to keep feeding it.
  5. Feed wire until you can't any more. I fed enough wire into the preheat tube to reach just past the upper bend before I bent the wire and was unable to continue. 
  6. Cut off the excess wire leaving enough to pull the wire out if necessary but not enough to reach past the metal fitting on the end of the fuel line as the wire could get hot enough to melt the plastic inside the fuel tube.
  7. Cut a narrow strip of thread seal tape and wrap the threaded fitting on the end of the fuel line.
  8. Slide the fuel line fitting over the wire and reinstall it into the burner assembly.


4 comments:

Hikin' Jim said...

A nice mod to be sure. Still, I think you're wise to take the Windpro along (as you have decided to do). I wouldn't want to be out in seriously cold weather -- the kind of cold where I'd want an inverted canister stove -- with a stove I was a little hesitant about.

HJ

Ryan Bressler said...

Thanks Jim. We will probably use the FMS for shoulder season stuff and I might try making some new legs for it to drop the weight but I definitely trust the MSR more. It also doesn't hurt that, as a former Seattleite, they're the home team when it comes to stoves.

Jim Sweeney said...

Is the copper wire functioning as a heat conductor (to improve pre-heat)? If so, would it also work to wrap around the metal tubing? (Which would add highly conductive therml mass).

Ryan Bressler said...

I tried using conductors and reflectors to make the tube heat up more before I tried adding the wire outside the tube with poor results.

As I understand what is going on, the wire is more to reduce the internal volume of the tube and act as an internal baffle.

The tube (and even the base of the jet) get hot enough to vaporize all the fuel you need but apparently does so in a turbulent/non uniform fashion where it is possible for a "bubble" of liquid surrounded by gas to make it through the tube to the jet.

The wire breaks up such bubbles and forces all the liquid fuel into contact with the tube making sure it is well vaporized.

On a side note the behavior of vaporizing gas is very complex/chaotic and I think that the sharp bends in the fms tube (vs the smooth ark of the msr) may be the fault in the design and are causing enough turbulence to create these bubbles.

While the simplified ideal gas laws would lead one to believe sharp bends and edges should have no effect the actual math is much more complex. It would require a super computer to do an analysis but watching some wind tunnel videos of turbulence gives one an idea of how much of an effect a sharp transition can have.