In your response to a question in Control magazine concerning Pitot tubes (Ask the Experts,” June, bit.ly/TmgvXq), you implied in the section on Figure 1 that the altitude is derived from the Pitot tube ("…of the speed and altitude of the plane.”) As a commercial pilot, I must correct you. The altimeter is basically a barometer and gets its input via the static port (so your diagram is correct). More modern instruments can use a digital pressure sensor.
Stan Kennedy
[email protected]
Lipták replies:
Thanks a lot, Stan. You're right. My wording was confusing, while the figure is correct because I knew that the static port is separate from the Pitot tube. By the way, which digital pressure sensors did you find to be reliable and accurate?
Béla Lipták
[email protected]
Kennedy replies:
I don’t know the specific companies that make them. However, they’re used in watches and other portable altitude measuring devices. As you may know, the measurement of altitude has to take into account the following:
Z = cT log(Po/P) pressure at sea level and absolute temperature. The constant c must take into account gravity and molar mass of the air. I suspect the chips have these functions included in the chip design. And, of course, GPS has been used for some time now, not as a primary instrument, but as a back-up.
Lipták adds:
Did you know that the Apple 6 mobile phone will, among other things, measure altitude?
This just shows that if people want solutions, they should turn to engineers, and only if they want a mess, should they let things be run by lawyers, Wall Street types and ayatollahs.
