I have not seen any research specifically looking at this topic. For the penetrometer itself, there should be no effect on its ability to measure force at different altitudes.

While the penetrometer is often called a pressure tester, and the measurement units can be calculated in terms of force per unit area (like pressure), it is important to remember that the traditional Magness-Taylor style destructive firmness measurement using a penetrometer is the maximum force when the tissue fails.

My intuition would be that non-destructive firmness measurements of the elastic properties of the tissue would be more likely to be affected by the change in altitude (and possible effects on turgor pressure) than the tissue failure strength type measurement of a penetrometer.  However, this is only speculation.

There may also be biological differences related to whether this is an apple grown and tested at high elevation or if it is an apple grown at low elevation, then shipped to high elevation and then tested.

                                              --David Slaughter