How much camping gas does it take to melt snow into a litre of water?
me
Jeez, surely that’s easy to work out? But it’s Internet mayhem. If you find an error, please email me!
TL>DR
It’s lighter to carry gas and melt snow – by a ratio of 3:1 – compared than carrying water.
ALSO ME
On a snow trip, one 227g canister will last two days.
That’s for one person
A 227g canister will boil 7kg of water, or melt 7kg of ice.
you guessed it
Assumptions
<fancy voice>First let us assume:</fancy voice>
- Ice is at 0°C
- Boiling means raising temperature by 100K
- Propane gas (50,000kJ/kg energy released)
- Ice->Water (334kJ/kg energy required to melt)
- Water->Boil (420kJ/kg since 100K at 4.2kJ/kg/K)
- Efficiency is considered only as a ‘system’1
At 100% efficiency
At 100% efficiency (lol) this sciences to (per 1kg of propane) :
- 150kg of ice melted
- 120kg of water boiled
Best efficiency is 50%
Great. Unfortunately efficiency is really low. Kitchen stove efficiency is 50%. But a hiking stove is totally different? MSR says 28g boils 2kg, or 14g boils 1kg water or 71kg of water->water boiling per kg propane. (They assume 20°C to 100°C, so adjust to 57kg water/kg propone).
That’s a thermal efficiency of 50%. Wow! Impressive: that’s must be under “ideal camping conditions”. Tell them they’re dreaming, mate.
Use 25% efficiency in practice
Let’s get real and use an efficiency of 25% for melting snow for drinking water. I think that’s reasonable2.
Based on 25% efficiency, 1kg of propane will:
- 37kg of ice melted
- 30kg of water boiled
Using little canisters of 227g, we get:
- 8kg of ice melted
- 7kg of water boiled
Round that to 7kg for each. So 3.5kg of ice melted and boiled per canister.
In the snow, assuming 2L water per day, some of that boiled, that’s 1 canister per 2 days for each person.
Ultralight?
Depending on the canister, roughly half the weight is packaging (i.e. steel). So now 1kg of gas will melt only 4kg of water.
So it’s better than carrying water directly. But it excludes the weight of stove, pot, lighter! Assume 250g for that and we only have 750g of gas, so it’s 3kg of water. Hence a ratio of 3:1 in favour of carrying gas, not water. But higher annoyance quotient, obvs.