30.0 29.9 29.8 I 0 0 F E T I 4 5 6 7 9 2 0 8 3 CALIBRATED TO  20,000 FEET ALT Feet in 10,000s pointer Feet in 100s pointer Feet in 1,000s pointer Altitude indication scale Altimeter setting adjustment knob Altimeter setting The barometric pressure can be  changed approximately 10 feet for each .01 "Hg to compensate for changes  in atmospheric pressure. Increasing the  altimeter setting causes the indicated  altitude to increase, while decreasing  the altimeter setting causes the  indicated altitude to decrease. 9 0 1 2 6 5 4 3 Static pressure inlet Display needle Linkages and gearing: Connect the diaphragm capsule to the display needle(s) Aneroid capsule: A sealed copper and beryllium alloy capsule from which the air has been  removed. It is springy and designed to  compress as the pressure around it increases, and expand as it decreases. Enclosure: Airtight except for the static pressure inlet, it has a glass  front through which display needle(s) can be viewed Space 0 feet 1,013 mb 18,000 feet 505 mb 30,000 feet 300 mb The altitude indicated on the altimeter is correct only if the  sea level barometric pressure is standard (29.92 "Hg), the sea  level free air temperature is standard (+15 °C or 59 °F), and  the pressure and temperature decreases at a standard rate with  an increase in altitude. Since atmospheric pressure continually  changes, a means is provided to adjust the altimeter to  compensate for nonstandard conditions. This is accomplished  through a system by which the altimeter setting (local station  barometric pressure reduced to sea level) is set to a barometric  scale located on the face of the altimeter. Only after the  altimeter is set properly will it indicate the correct altitude. If no means were provided for adjusting altimeters to  nonstandard pressure, flight could be hazardous. For example,  if a flight is made from a high-pressure area to a low-pressure  area without adjusting the altimeter, the actual altitude of the  glider is lower than indicated altitude. When flying from a  low-pressure area to a high-pressure area, the actual altitude  4-7 Figure 4-16.  Atmospheric pressure and altitude.  Figure 4-15.  Inside the altimeter. Figure 4-14.  Altimeter. Effect of Nonstandard Pressure and Temperature