Testing in the laboratory and in the field
It is important to remember that in the Aprovecho lab, the 3 Stone Fire has used less wood and made less pollution than cooking fires and high mass stoves operated by cooks in the field. The fires in the lab tests were carefully made using dry and uniform sticks of Douglas fir fed into the fire in a controlled way to optimize the performance. Well-constructed 3 Stone Fires protected from wind and tended with care scored between 20% and 30% thermal efficiency.
Open fires made with moister wood and operated with less protection from the wind can score as low as 5%. The operator and the conditions of use largely determine the effectiveness of operation. Stoves have to be tested with careful repetition in order to achieve statistical confidence in the results. Because there are so many differences between laboratory and field results, it is difficult to use the results of laboratory testing to predict exactly how stoves will perform in the real world.
However, side-by-side comparisons can be used to generally estimate performance. An automobile that gets 40 miles per gallon on a dynamometer is more likely to use less gas driving down the highway than a car that only gets 20 miles per gallon in the same test. A cooking stove that used less fuel or made less pollution in a standardized test will, one hopes, translate into reductions in the field, but field surveys are needed to establish the actual performance. Field tests are essential to also learn lots of important things, such as if the cook likes the stove, whether the stove product will be successful in the market, and how much PM2.5 and CO is inhaled by members of the household.
In our opinion, no lab test can replace going into the field and learning from reality. And, being taught about stoves from cooks is one of the most fun parts of this job.