How does a DP Transmitter measure flow?

• A. Measures the velocity of the flow in the pipe.
• B. Measures the pressure difference between high and low pressure connections.
• C. Measures the density of the fluid.
• D. Uses thermal expansion to infer flow.

Answer: (B)

A differential pressure (DP) transmitter measures flow by detecting the pressure change across a flow-restricting element such as an orifice plate, venturi, or flow nozzle. When fluid passes through the restriction, the upstream pressure is higher and the downstream pressure is lower. The transmitter sits across these two taps and outputs a signal proportional to the pressure difference, ΔP. That pressure drop is related to how much fluid is moving, through the geometry of the restriction and the fluid’s properties. With the known discharge coefficient and density, you can convert ΔP into a flow rate. So, the key idea is that flow is inferred from the measured pressure difference, not from direct velocity measurement, density measurement, or thermal expansion-related methods.