9.3 Pressure Difference Flow Sensors

This flow sensing principle relies on the measurement of the differential pressure p in a flowing fluid. Pressure sensors can be used to measure flow by sampling the pressure drop along a flow channel with known fluidic resistance, R _f , and calculating the flow Q from the fluidic equivalent to Ohm's law: Q = ? p/ R _f. It is comparable to measuring the current ( Q) in an electric circuit by sensing the voltage drop ( ? p) over a fixed resistance ( R _f).

The sensor presented by Cho et al. [81] uses a silicon-glass structure with capacitive read-out [Figure 9.22(a)]. Fluid enters the chip through the inlet at pressure p ₁, flows through a channel and leaves the sensor with pressure p ₂. If the flow channel is small enough to create a resistance to the flow, a pressure drop ? p appears across the channel. The pressure above the membrane and the pressure at the inlet are kept equal. The pressure difference is measured by a capacitive pressure sensor, which is switched at 100 kHz.

Figure 9.22: (a, b) Schematic drawings of pressure difference flow sensors (a) ( After [81].) (b) ( After [82, 83].) The silicon membranes are 25 ?m thick, 1.5 mm long, and 1.5 mm wide. The flow restriction channel is between 200 and 570 ?m wide, 2.9 mm long, and...