The diaphragm located closest the carburetor body is the air metering diaphragm. It measures the difference in air pressure taken from two locations within the carburetor. Chambers A and B are on opposite sides of the air metering diaphragm. The velocity of the air flow entering the carburetor is measured by placing one or more venturi directly in the airflow. The venturi creates a low pressure that changes with the velocity of the air. As the air pressure in chamber A is decreased with greater airflow, the diaphragm is pulled toward the carburetor body. Chamber A also contains a spring that opens the fuel metering valve when the air flow is absent. The mass of the air entering the carburetor is measured by placing a number of impact tubes directly in the airflow, generating a pressure that represents the air density. The impact tube pressure is connected to "Chamber B" on the side of the air metering diaphragm farthest from the carburetor body. As the air pressure in chamber B is increased, the diaphragm is moved toward the carburetor body. The difference in pressure between chambers A and B creates what is known as the air metering force'. The second diaphragm is the fuel metering portion of the regulator, and is located farthest from the carburetor body. It measures the difference in fuel pressure taken from two locations within the regulator itself. Chambers C and D are on opposite sides of the fuel metering diaphragm. Chamber C contains "unmetered fuel", that is the fuel as it enters the carburetor. Chamber D contains "metered fuel", that is fuel that has already passed through the jets, but not yet injected into the air stream. The difference in pressure between the two fuel chambers creates the fuel metering force. The air metering force from chambers A and B is opposed by the fuel metering force from chambers C and D. These two forces combine into movement of the servo valve to adjust the fuel flow to the precise amount required for the needs of the engine, and the needs of the pilot. Operation When the engine started, air began flowing through the boost venturi, causing the pressure (referred to as a partial vacuum as it is lower than atmospheric pressure, but not a full vacuum) in the venturi to drop according to Bernoulli's principle. This causes the air pressure in chamber A to