How does a differential pressure transmitter infer flow when used with a primary element such as an orifice plate?

How does a differential pressure transmitter infer flow when used with a primary element such as an orifice plate?

• A. Measures flow directly with a turbine sensor.
• B. Uses differential pressure to measure temperature.
• C. Does not require any calibration.
• D. It measures DP across the orifice; using fluid properties and DP, computes flow rate via the flow equation; requires periodic calibration.

Answer: (D)

The main idea is that a differential pressure transmitter doesn’t measure flow directly. It senses the pressure drop created as fluid flows through a primary element like an orifice plate, and that pressure drop is then translated into a flow rate using the fluid’s properties and the element’s geometry. When fluid passes through an orifice, its velocity increases and static pressure drops, producing a differential pressure (ΔP) that is related to how much fluid is moving. The transmitter provides this ΔP signal, and a flow calculation uses the appropriate flow equation with known fluid density (and, for gases, expansion corrections) and the orifice’s characteristics (area, beta ratio, discharge coefficient, installation factors). A common form ties the flow rate to the square root of ΔP: Q is proportional to sqrt(ΔP/ρ), with constants that account for the orifice geometry and fluid behavior. Because those factors change with temperature, pressure, viscosity, and installation conditions, a flow computer or calculation step applies the equation to compute flow from the measured ΔP and must be calibrated periodically to maintain accuracy. So, the differential pressure across the orifice is measured, and, using fluid properties and the flow equation, the system computes the flow rate, with periodic calibration ensuring the derived flow stays accurate over time.

The main idea is that a differential pressure transmitter doesn’t measure flow directly. It senses the pressure drop created as fluid flows through a primary element like an orifice plate, and that pressure drop is then translated into a flow rate using the fluid’s properties and the element’s geometry.

When fluid passes through an orifice, its velocity increases and static pressure drops, producing a differential pressure (ΔP) that is related to how much fluid is moving. The transmitter provides this ΔP signal, and a flow calculation uses the appropriate flow equation with known fluid density (and, for gases, expansion corrections) and the orifice’s characteristics (area, beta ratio, discharge coefficient, installation factors). A common form ties the flow rate to the square root of ΔP: Q is proportional to sqrt(ΔP/ρ), with constants that account for the orifice geometry and fluid behavior. Because those factors change with temperature, pressure, viscosity, and installation conditions, a flow computer or calculation step applies the equation to compute flow from the measured ΔP and must be calibrated periodically to maintain accuracy.

So, the differential pressure across the orifice is measured, and, using fluid properties and the flow equation, the system computes the flow rate, with periodic calibration ensuring the derived flow stays accurate over time.