UKH

Interested in the separate little piezo igniter MSR sell. I currently use a flint which is great for my XGK, but scary lighting a gas cooker as your hand gets too close, and in the noisy alpine environment I can't always see if the gas has lit.

The separate piezo would be great if it worked. But there are trillions of reviews online saying it doesn't.

Is it a technique thing, or do they easily fail, and therefore are a waste of money?

It gets poor reviews here.

https://ultralightoutdoorgear.co.uk/piezo-igniter-for-gas-canister-stoves/

Mine has never failed me (edit to add: it sometimes takes a few clicks). However, some of my friends borrowed it and failed to use it on an alpine route and had to eat cold food. I always tell them they just weren't using it properly, but to be fair to them it was at about 4000m and I'm not sure I've used it that high.

I think if altitude was a factor, that'd be the stove rather than the igniter.

No idea what the stove was your friends had, but I learned a tough lesson years ago first time I went >= 4,000 as my stove jet was the wrong type for that and the flame was unstable and kept going out (suboptimal if you need to melt to get water!). If really  unstable it might not even light. So stoves and altitude is very much a thing.

It's just anecdotal, but I find piezo lighters generally unreliable at even moderately higher altitude. Maybe the now more fuel‑rich mixture (less oxygen) is harder to light. Maybe it's Paschen's law and lower breakdown voltage needed for the arcing to occur at lowered air pressure, so it arcs "earlier" with lower strength. Maybe something else...

Any physicist care to step in with a proper explanation?

I’ve found one of these pretty good so far

https://ultralightoutdoorgear.co.uk/plasma-lighter/

Just weighed a full bic minilighter, 11g

Standard practice for a lot of multiday alpine adventures is one lighter each person and some matches in a plastic bag as back up. A lighter can be dried out in an inside pocket and can work even without fuel.

Trying to light even the gashob in a sheltered campervan with a handheld gas domestic gas lighter is fraught  until you realise that neither 100% air or 100% gas is flamable.  Timing is everything otherwise the purish propane just blows out or chokes the lighterflame.

Possible - they were using my stove, MSR Pocket Rocket, which I have used at >4000m fine (I think lit with a lighter). I don't know what the temperature was when they couldn't use it, though certainly not super cold (very much doubt it was lower than -10, probably 0ish). I also don't know what gas they were using.

Used to smoke in my younger days, so a piezo lighter not lighting up at 3000m was way much worse for me than just having to eat cold food and drink sun‑thawed ice runoff (cue in the Monty Python Four Yorkshiremen sketch)

Bic usually works. Not being refillable is a bummer, and keeping it warm in a pocket is another. But a ferrocerium rod never failed me for lighting up any gas stoves. Especially easy with a wide ceramic burner stove like a MSR Reactor – actually easier to light it up that way than with even a gas lighter.

Firesteel or unbranded alternative (ferrocerium rods) plus a bit of sharp metal will never let you down. I use them as my primary source of a spark, very good as a backup to a lighter. 

Rub a thin layer of candle wax on them between trips to stop them oxidising in hot/damp environments.

I've had piezos fail before.

Thanks everyone for the discussion and information.

Yes. I've been using my Firesteel flint for 20 years or so and it is really reliable. That and the XGK worked at all altitudes and any weather.

It's just that I took a little Soto pocket rocket style gas thing yesterday on a high bivvy to try to be more lightweight/compact to save my ageing knees. And found trying to light the gas with the Firesteel awkward and dangerous because I couldn't see or hear the gas flow or ignition with the bright sunlight and roaring snowmelt at the bivvy.

I often suspected it hadn't lit, but couldn't be sure so had to keep turning the valve off and on before trying again.

With the XGK there's a bloody big yellow flame that tells you it lit. If not you could safely try again.

I was thinking an igniter with a longer stalk would allow you to make multiple attempts even if it was already alight without melting your hand.

So I might just need more practise with the gas burner.

But that plasma lighter does look interesting.

for sure - I’m generally on my Tod and usually have at least two lighting options. 
 

I recently got a speedster stove, and was struggling a bit with my sticker to get it lit, so got the plasma thing to try - being able to reach into the burner is handy. 
 

also the rechargeable feature is pretty attractive, and the plasma arc is pretty.

Regarding flame visibility, have you experimented with fixing a short length of thin wire to touch the flame when lit.

Recall from my chemistry at school that copper burnt blue and steel some other colour, 

Edit, just searched online, its called a flame test.

That's an interesting idea. And I guess hits the nail on the head about what my problem really is: how do I know when it's lit if I can't see or hear the gas or the flame.

Maybe school was useful after all.