I posted earlier about whether using stoves inside the tent is safe or not. After a five day ski trip to Kilpisjärvi Wilderness Area, I have some data that shows that, well, it depends.
I used a standard indoors carbon monoxide alarm with a screen showing the particles per million (ppm) concentration of carbon monoxide in the air to see how much carbon monoxide is generated when two using gasoline burning stoves inside a tent (Bergans Helium Dome III).
To start the experiment, we almost closed off our ventilation to make the air flow minimal. The pots used were Eta power pots that have a heat exchanger, which makes them much more efficient but also much more likely to generate carbon monoxide.
Two stoves without any pots on: totally safe
At first we ran the stoves to generate heat without having any pots on to melt snow or boil water. At no point did the meter measure any carbon monoxide.
Conclusion: using stoves for heat is totally safe. The risk for carbon monoxide from just burning gasoline is minimal, at least with the two types of MSR multifuel stoves we tested.
Two stoves with a pot melting snow: generating carbon monoxide
We then filled one pot with snow and some seed water to keep the pot from overheating and going crooked. So the contents were a very cold sludge of snow and water.
We kept the meter close to the burner and set about doing other chores. At some point I glanced at the meter and saw a reading of 136 particles per million of carbon monoxide. This had not set off the alarm.
One hour at 200ppm of carbon monoxide will generate a stinging head ache so this was already a concentration that you might want to start worrying about.
Conclusion: using a single heat exchanger pot with cold contents will produce level of carbon monoxide that may be cause for concern if proper air exchange is not taken care of.
Two stoves with two pots of melting snow: carbon monoxide levels become alarming
When we put on another pot with snow-water sludge, the carbon monoxide levels quickly rose to a point where the alarm started beeping. The highest level stored in the memory was 187ppm, still the below 200ppm mentioned but close enough.
At this point we were satisfied with our results and wanted to cut the level of carbon monoxide in the air. We opened the doors wide enough to generate a noticeable flow of air through the tent. Quite quickly the level of carbon monoxide started to dissipate.
Conclusion: Using gasoline stoves with two heat exchanger pots will generate enough carbon monoxide to be very alarmed.
Although the results can seem alarming, I think they pretty much confirm what I was thinking all along. First of all, having good ventilation is essential with a single pot and with two heat exchanger pots it becomes crucial. Although I feel we had very little ventilation with a single pot, I believe the level of CO in the air with a single pot depended very much on how much managed to dissipate vs. how much stuck around inside the tent and then the second stove pushed the CO concentration over the alarming level.
It could just as easily be a single stove in a tent with worse ventilation or weather conditions that disturb the ventilation that push the carbon monoxide concentration to dangerous levels.
My take from this is that if you are in a hurry to melt a lot of snow and want to use two pots, then sacrifice some heat and comfort and keep a good airflow going. If you have enough gasoline and time (i.e. not skiing to south or north pole, or crossing Greenland) then maybe only melt water with one pot and keep the other burning only for comfort and you should be that much safer.
Disclaimer: I do not advice anyone to use their stove inside the tent. Many do it relatively safely, but you should have the proper equipment and practice to do it.