The Micro-fuel Cell is a partial pressure device. Its output is proportional to the partial pressure of oxygen in the atmosphere to which it is exposed. In the case of altitude effects on sensors exposed to atmospheric oxygen, keep in mind that the volume percentage of oxygen is constant in the atmosphere as one goes higher in altitude. However, the total atmospheric pressure decreases by 2.5 mm/Hg per every 100 feet of altitude rise. The total pressure of the atmosphere at sea level is 760mm Mercury. At 100 feet of elevation above sea level, the output of the sensor would decrease by a factor of (760 – 2.5) / 760. For example, the new output of sensor at 100 feet elevation = (sensor output at sea level) * (760-2.5) / 760. Further information is available here.