Hi Blogs,
>I didn't get the last part about conversions with respect to sea level pressure. When do I need to do this (just when comparing to ata?) and when don't I?
It depends on the boost indicator.
If the airplane is parked with the engine off at sea level in a standard atmosphere and the boost indicator reads zero, you have to add sea level pressure to get true pressure. British instruments work like that. I think Japanese do, too.
American, Russian and German instruments usually provide absolute pressure (the second "a" in "ata") and indicate correct ambient pressure when sitting on the ground with engine off.
With standard sea level pressure being P0 and indicator reading Pi, you'd have the following conversion to SI units:
For psi:
P = Pi *C(psi) + P0
For Japanese instruments and mm Hg:
P = Pi * C(mm Hg) + P0
For Soviet instruments and mm Hg:
P = Pi * C(mm Hg)
It can be tricky at times to find out whether adding P0 is required or not if you don't have good documentation on the aircraft.
The German practice of using ata for absolute and atü for relative pressure provided good clarity there - one of the few cases in which I can say something good about non-SI units :-)
Regards,
Henning (HoHun)
