Barometer/altimeter

EE 20 November 1986 BAROMETER/ ALTIMETER Entirely in tune with the general idea of this month"s front cover photograph, this article deals with a portable, easy to use instrument which combines the functions of altimeter and barometer to delight amateur aviators, mountaineers, and meteorologists The proposed portable instrument is in essence a true aneroid barometer featuring a 31/2-digit read-out in- dicating millibars (mb, barometer function) or relative height (m, altimeter function) as selected with a switch. The maximum parameter values are about 1200 mb and 1999 metres, respectively, which would appear to be suitable for most practi- cal applications. Standard atmosphere It is generally known that the at- mospheric pressure at sea level dif- fers from that at a certain altitude; in fact, pressure falls with increasing height. It is only when atmospheric pressure changes suddenly that we notice the inverse relation with altitude, for instance in a fast lift, or a ride in moun...