What is the role of the primary element in a differential pressure flow measurement using a DP transmitter?

• A. The primary element supplies power to the transmitter.
• B. The primary element stores calibration data.
• C. The primary element displays output on a local indicator.
• D. The primary element, such as an orifice or venturi, creates a pressure drop that the DP transmitter measures to infer flow using a calibration curve.

Answer: (D)

In differential pressure flow measurement, the primary element’s job is to create a controlled pressure drop in the flowing fluid. This element—the constriction such as an orifice plate, venturi, or flow nozzle—causes the velocity to rise as fluid passes through it, which results in a higher pressure drop between upstream and downstream sides. The DP transmitter then senses this differential pressure and, using the known relationship (the calibration curve) for that element and the fluid properties, converts the measured ΔP into a flow rate. So, the primary element establishes the link between how much fluid is flowing and the pressure difference that the transmitter reads, enabling accurate flow indication.