How to estimate of the amount of gas that you need for backpacking with the Primus Eta Spider?

fuel consumption formula

The variables involved are:

• `p` = number of people
• `d` = number of days
• `f_p` = the amount of fuel needed to heat water for purification tablets (grams of fuel / liter of water)
• `f_b` = the amount of fuel needed to boil water, usually for cooking (grams of fuel / liter of water)
• `w_d` = the amount of water each person needs for drinking, each day (liters)
• `w_c` = the amount of water each person needs for cooking, each day (liters)

The amount of fuel you need is: $$ p \cdot d (f_p w_d + f_b w_c) $$

Some notes:

• `f_p` and `f_b` will vary based on your stove, as well as whether you're melting snow or not
• `w_d` varies a lot based on the person.
• The `f_p w_d` term is only needed if the water is very cold / frozen to start with. If it's warm enough, you can ignore this part.
• you may need to cook one fewer day than you need water for (because you usually can eat breakfast before leaving, and eat dinner after returning)

articles about gas canister consumption

• If you burn large gas canister (230g) for an hour a day, it will last 5 days.
• an 8 ounce (~220g) canister will burn for ~3 hours
• more articles: [1] [2] [3]
• more threads: [1]

Canisters come in 100g (3.5 ounces) and 230g (8.1 ounces) sizes.

more precise estimates of grams of gas / liter of water

• gas calculator
  • 0.25 ounces fuel / 1 pint water
  • 1 ounce / pint = 28 grams / 0.47 liters = 59.91 grams / liter
  • 0.25 ounces / pint = 15 grams gas / liter water
• boiling 2 cups of water (starting at room temperature) consumes 0.25 ounces (7g) of gas
  • this agrees exactly with the figure from above, so this is also 15 grams gas / liter water

drinking water

• 4 liters a day

28 Dec 2014 trip

• 3 cups per person per day for cooking
• drinking water
  • assume we hike 10 hours a day
  • so that's 56 cups a day