2-2 Product Overview 5700512-C
Advanced Energy
®
When gas flows through the MFC it is split into two streams. A larger stream is routed
through the bypass assembly, and a smaller stream that is proportional to the larger
stream is routed through the flow sensor. The MFC is designed so that the ratio
between these two streams remains constant over the entire measurable flow range of
the instrument.
The flow sensor measures the flow of the smaller stream which represents the total gas
flow through the MFC. The flow sensor consists of a small stainless steel capillary
tube with two thermal resistors wound around the outside. Current passes through the
thermal resistors that function as heaters to warm the sensor tube. The thermal
resistors form part of a bridge circuit that senses the resistors’ resistance balance.
When gas is flowing, heat is transferred from the upstream to the downstream thermal
resistor. The degree of imbalance is proportional to the rate of the gas flow.
The control valve adjusts gas flow through the MFC based on the flow sensor reading
and the set point to ensure a stable gas flow from the MFC.
For more detailed information regarding the components in the MFC, see “MFC
Components” on page 2-2.
MFC COMPONENTS
The main components of the MFC are the flow sensor, bypass, electronics (bridge,
amplifier, and comparator circuits), and valve assemblies. Figure 2-2 illustrates these
major components.
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