Oxygen and Air Pressure
Stove performance at high altitude is impacted by two factors. As you gain altitude, air pressure drops progressively, and as a result oxygen concentration (the amount of oxygen in a given volume of air) also drops. These impact your stove performance in different ways.
- With normal concentrations of oxygen (e.g at sea level) stoves operate extremely efficiently and produce mainly carbon dioxide and water as the main waste products from combustion. Trace amounts of carbon monoxide may also be produced. With reduced oxygen supply at altitude, the combustion produces more carbon monoxide and less carbon dioxide.
- If you are really interested, here is the science.
- The chemical formula of butane is: C4H10
- When butane burns with sufficient oxygen: 2 C4H10 + 13 O2 → 8 CO2 + 10 H2O (butane +oxygen produces carbon dioxide + water vapour)
- When oxygen is limited: 2 C4H10 + 9 O2 → 8 CO + 10 H2O (produces carbon monoxide + water vapour)
- This means that as you gain altitude, your stove will produce more carbon monoxide. Most, but not all, mountaineers are aware of this phenomenon. Every year cases of carbon monoxide poisoning are reported amongst mountaineers who have had to spend long periods of time at altitude waiting out storms in a tent, with a lit stove.
Air Pressure: The Trouton-Hildebrand-Everett rule
According to the Trouton-Hildebrand-Everett rule, for every 1000m / 3000ft gain in altitude, the boiling point of the gas inside the canister drops by 2-3 degrees centigrade . This means that you can use your stove at progressively lower temperatures as you gain altitude.
The graph below shows how this applies to a KOVEA Spider running on low cost butane. At sea level, pure butane struggles to maintain gas pressure a just a few degrees above freezing. At 9,000ft (e.g. on the John Muir Trail), the evapouration point drops to -8.5oC. On the summit of Mont Blanc, you can use butane down to -14oC, and at everest Base Camp, -17oC.
- As you gain altitude, eventually the reduced amount of oxygen in the air will start to impact stove combustion efficiency, resulting in higher carbon monoxide emissions.
For more information how the Trouton-Hildebrand-Everett rule can save you money, see our article: Butane canisters: Low temperature performance improves with altitude.
Carbon Monoxide Danger Increases with Altitude
Carbon monoxide becomes a bigger danger at altitude particularly if you are forced to cook in a small tent due to extreme weather. The reason for this is that the humans in the tent and the stove, are competing for limited oxygen. Without sufficient ventilation, both respiration and combustion will further diminish available oxygen. As a result, you stove will produce more carbon monoxide. The resulting elevated carbon monoxide in the tent, together with reduced oxygen in the blood, is a recipe for disaster. Every year there are cases of mountaineers suffering carbon monoxide poisoning. Some cases prove fatal.
Experienced high altitude mountaineers avoid the life-threatening danger of carbon monoxide poisoning by cooking with tent doors and vents open to ensure that poisonous gases do not accumulate inside the tent.
Water Boiling Point Declines with Altitude
Water also boils at a lower temperatures at higher altitude. But you are unlikely to notice this much below 4000m/ 12,000ft. At altitudes above 5500m, mountaineers use pressure cookers to achieve water boiling point of close to 100oC. Without a pressure cooker, cooking takes longer and uses more gas.
Piezo Igniters Become Less Reliable with Altitude
Piezo igniters can start to misfire at altitude due to the reduced air density. If you are going above 2500m, you might notice piezo igniter performance start to deteriorate. You can counter this by bending the terminal to close the gap which the spark needs to cross. If you are planning on spending time above this altitude it is a good idea to take another form of lighter for your stove (fire steel or flint-based cigarette lighter) just in case.