A variability of 10 millibars in pressure what is the associatedon randon error in my altimeter?

Answer 1

For barometric altitude measurements done "near" sea-level, 1 millibar is roughly equivalent to 8 meters of altitude. So a 10 millibar error would be about 80 meters of altitude, or about 250 feet if you are more comfortable with the English system. The "near sea-level" generalization should hold for the measurements that will typically be made. If you are the pilot of an aircraft, or you are climbing very high mountains (4000 meters are higher?) this approximation will degrade, but shouldn't vary by a large amount (less than an order of magnitude). Many/most barometric altimeters allow you to calibrate the device if you can find a known altitude. When the time domain of your altitude measurements is small (a day or 2), the overall atmospheric pressure will likely change little (although it can change dramatically if a weather front moves in). By calibrating in this manner, an altitude measurement of an accuracy significantly better than 250 feet should be achievable. Over time the certainty of this measurement will degrade. Large temperature variations can affect the accuracy as well. But obviously the ultimate accuracy will depend on the device in question. Hope that helps.