Almost froze my finger!

Was the canister upright or inverted?

Hm…not sure if it was upright or inverted. What are you thinking?

That was my first Q, as well.  I always make and break canister connections with the canister upright so if/when it vents, it will vent gas not liquid.  If it vents gas, the joule-Thomson effect will (for real, not ideal gases), cause cooling of the gas but it’s just a gas with relatively low mass, conductivity, and heat capacity.  But if it’s a liquid, with it’s higher mass, heat capacity and EVAPORATIVE COOLING, then you can freeze flesh pretty quickly.  Usually, I get my skin cancers carved off, but for pre-cancerous lesions like a actinic keratosis, I get them frozen.  The actual dermatologist keeps liquid N2 around, but other docs just use a can of refrigerant.

Were you on the trail to Kendall Katwalk?

Not anywhere near Kendall Catwalk…I was near Esmeralda Basin.

I love Autumn!

And while PV=nRT was going on, Joule-Thomson cooling is actually a non-ideal behavior of most gases (but not Hydrogen, Helium, or Neon).  Hydrogen actually warms during expansion near room temperatures.

Sometimes I hold the stove / hose end in a glove or just a folded bandana to avoid that slight risk of frost-nip to my fingers, but keeping it upright instead of inverted eliminates the risk of major cooling.

Be mentally prepared for that possibility and just keep unscrewing it.  Getting startled and dropping it causes 1) loss of more fuel, 2) rapid cooling of the metal parts that you’ll have to grab again to finish the job, and 3) possibly inversion and loss of liquid fuel and even more rapid cooling.

Some stove/canister combinations seem more prone to this than others.  I assume it is a combination of the last female thread versus pin depth on the stove/hose and the last male thread versus flapper valve position of a particular canister.

And before someone suggests cylinders of hydrogen (it has the highest BTU/pound), the reason we use iso-butane and not propane is that:

One pound of propane is contained in one pound of steel (and 20 pounds in 20 pounds in a BBQ tank), while

One pound of butane is contained in 0.4 pounds of steel, while

One pound of hydrogen is under such high pressure, it takes 50 pounds of steel.  This is one of many issues in moving towards a “hydrogen economy”.  Another is pumping hydrogen through pipelines takes much more energy than pumping natural gas (and a bigger, higher strength pipeline).

Most all stoves do that.  As you unscrew the stove from the canister, there’s a point where fuel will start leaking.  Quickly go past that point.  Only a problem if you’re inverted so liquid comes out – don’t do that.  Unscrew it when it’s upright.

When I transfer butane from one container to another, sometimes liquid will get on my skin.  Yet another reason not to do this : )

While PV=nRT, the biggest impact is probably the Latent Heat of Vaporization.  Unscrewing an inverted canister will cause some of the liquid to come out as it is under pressure.  As the liquid hits your skin, it draw energy out of your skin to transition from a liquid state to a gas state (quite a bit of energy).  Not a fun prospect.  As everyone has said, flip your canister upright before unscrewing and then you will only be dealing with gas and not a liquid.  My 2 cents

My BRS-3000T does that on a full canister even when upright.  My SOTO WindMaster seals and unseals very quickly and I do not have the same problem.

I hope this is on-topic…

One of the funniest things I ever saw (so it obviously didn’t happen to me) was the propane guy pulling the hose off his truck using the valve as a handle.  As he swung the thing down he accidentally opened the valve with the nozzle trained on his derriere.  He stood there about 5 minutes trying to get the feeling back in the affected cheek, slapping and flapping his pants as ice chunks reluctantly broke off.

I think, in retrospect, that I DID hold the fuel canister upside down.

Never again shall I do that!

And I think I will start using a folded-bandana like David mentions THanks!

Thanks for all the comments folks!

My take.  You used the stove for a while.  That causes significant evaporative cooling of the liquid in the canister.  Sounds like to below the freezing point of water (your skin).  Then you unscrewed it upside down and got this sub 32F liquid on your finger which is painful.  I highly doubt any other relevant physics occurred during the unscrewing process other than spilling really cold liquid on your finger really hurts.

Any chemist will tell you that if you get liquid on you from a dry ice/acetone slurry you will be in a world of pain.  Liquid nitrogen/helium are pretty tame in comparison (so long as you aren’t a moron).