Gas Models
At high altitudes (above 4000 ft.), fuel supply is decreased in same proportion as air density. Normally, control pressure is regulated by a diaphragm and a spring. To achieve altitude compensation, another diaphragm with a spring and a chamber have been added. See Fig 1 .
The chamber maintains a constant low altitude (below 4000 ft.) pressure while pressure outside it varies according to altitude. The pressure difference causes diaphragm to exert stronger or weaker pressure on inner spring.
At increasing altitudes, decreasing pressure outside diaphragm chamber causes lower diaphragm to rise. The resulting increase of pressure on upper diaphragm increases control pressure.