As the differential range is approached, what causes the diaphragm to move and the differential metering valve to open?

The diaphragm's movement and the opening of the differential metering valve are caused by the pressure differential between reference pressure and atmospheric pressure.

In a system where the differential metering valve operates, there is typically a controlled pressure reference established within the system. As the conditions change—particularly if the atmospheric pressure decreases while reference pressure remains relatively stable—the pressure differential increases. This increase in the pressure differential results in a force that acts on the diaphragm. When this force exceeds a certain threshold, the diaphragm moves, thereby opening the differential metering valve.

Opening this valve allows for the regulation of flow in response to the changes in pressure, ensuring that the system can adjust to maintain equilibrium or desired performance levels under varying conditions. This mechanism is critical for systems that require precise pressure management and control, which is often the case in aviation environments.

In contrast, increased atmospheric pressure, decreased cabin temperature, or increased airflow from the supercharger do not directly relate to the mechanism of diaphragm movement or the opening of the metering valve in the context of pressure differential management. Each of these factors would affect system performance differently but does not establish the fundamental principle of how the diaphragm and metering valve interact through pressure differentials.