Atmosphere pressure Altitude (ft) Pressure (psi) Sea level 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000 22,000 24,000 26,000 28,000 30,000 32,000 34,000 36,000 38,000 40,000 42,000 44,000 46,000 48,000 50,000 14.69 13.66 12.69 11.77 10.91 10.10 9.34 8.63 7.96 7.34 6.75 6.20 5.69 5.22 4.77 4.36 3.98 3.62 3.29 2.99 2.72 2.47 2.24 2.04 1.85 1.68 0 2 3 1 4 29.9 30.0 29.8 Barometric dial 100 feet scale 1,000 feet scale 10,000 feet scale Aneroids Balance assembly Adjustment knob Out: sets barometrical dial In: sets dial hands Figure 10-31.  The internal arrangement of a sealed diaphragm pressure altimeter. At sea level and standard atmospheric conditions, the  linkage attached to the expandable diaphragm produces an indication of zero. When altitude increases, static pressure on the outside  of the diaphragm decreases and the aneroid expands, producing a positive indication of altitude. When altitude decreases, atmospheric  pressure increases. The static air pressure on the outside of the diaphragm increases and the pointer moves in the opposite direction,  indicating a decrease in altitude.  Figure 10-30.  Air pressure is inversely related to altitude. This  consistent relationship is used to calibrate the pressure altimeter. Many altimeters also contain linkages that rotate a numerical  counter in addition to moving pointers around the dial. This  quick reference window allows the pilot to simply read the  The second-longest point moves more slowly. Each time it  reaches a numeral, it indicates 1,000 feet of altitude. Once  around the dial for this pointer is equal to 10,000 feet. When  the longest pointer travels completely around the dial one  time, the second-longest point moves only the distance  between two numerals—indicating 1,000 feet of altitude  has been attained. If so equipped, a third, shortest or thinnest  pointer registers altitude in 10,000 foot increments. When  this pointer reaches a numeral, 10,000 feet of altitude has  been attained. Sometimes a black-and-white or red-and-white  cross-hatched area is shown on the face on the instrument  until the 10,000 foot level has been reached.  [Figure 10-32] The face, or dial, of an analog altimeter is read similarly to  a clock. As the longest pointer moves around the dial, it is  registering the altitude in hundreds of feet. One complete  revolution of this pointer indicates 1,000 feet of altitude.  zero when this pressure is exerted by the ambient air on the  aneroid. As air pressure is reduced by moving the altimeter  higher in the atmosphere, the aneroid expands and displays  altitude on the instrument by rotating the pointer. As the  altimeter is lowered in the atmosphere, the air pressure around  the aneroid increases and the pointer moves in the opposite  direction.  [Figure 10-31] 10-18