Larry Curcio
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I can't provide an answer to the question regarding the behavior of BMP180 below 300mBar, but the reason why the Perfectflite altimeters are specified to 100kft is that they actually use a different sensor. The pictures show what appears to be an MS5607 or MS5611 from Measurement Specialties (formerly Intersema). Both the old plastic version as well as the more recent plastic version can be seen. Those also match the description by Perfectflite (10mBar ~ 100kft, 24bit ADC). The BMP180 has a noticeable different package and can therefore be ruled out, unless the altimeters have since been redesigned.
Generally, every altimeter manufacturer that specifies its products to work at 100kft (PerfectFlite, Altusmetrum, Featherweight *), Black Magic Missileworks *), Entacore **), Altimax) appears to be using Meas Spec sensors.
*) Different sensor in different package, either MS5540 or MS5803
**) Speculative; the pictures of the AIM Extra that I found, don't show the underside with the sensor
Reinhard
There are different altitude/pressure formulas in different atmospheric regimes. For example, in the troposphere, one assumes a constant temperature lapse (decline) rate with altitude. For much of the stratosphere, the temperature doesn't decrease with altitude - that's another formula. Then it actually rises with altitude. (The one formula in the chip spec is the tropospheric formula.)