MRE Heaters

I carry a few of these when I tramp around. They are great for a cup of drink or a warm meal and can be bought cheap. Calcium Carbide has numerous other uses besides cooking but i'll leave that to others. Here's a link to bulk heaters.
Nitro-Pak: MRE Heater Paks - 12 Paks of 12 Heaters - MRE Heaters - MRE's - Products

 

I picked up a few boxes of the heated MRE's when they were 70.00/box. No Inflation my ass..lol.

 

that link is just to the heaters; I don't have much use for the meals.

 

Make your own heaters with Citric acid and powdered iron ( filings), just add water and presto! Heat ( and steam!)

 

What are the proportions of Citric acid, iron filings, and water to be used for the homemade version?