DP (Square-Root) Flow Transmitter 4-20 mA Scaling

ScopeThe flow a differential-pressure (orifice/venturi/nozzle/averaging-pitot) transmitter's 4-20 mA signal represents - a square-root, not linear, relationship. The transmitter outputs DP linearly, but flow ~ sqrt(DP), so flow% = sqrt((mA - 4)/16): 12 mA (50% signal) is 70.7% of flow, 8 mA (25% signal) is 50% flow -- the classic DP-loop error. value = flow_low + flow% x (flow_high - flow_low). A low-flow cutoff (5-10%) zeroes the noisy square root near 4 mA. Loop linearization only (the element's Cd is in the transmitter calibration); a smart transmitter doing internal extraction is linear instead. NAMUR NE43 fault band. The transmitter range and calibration govern.

Formula or tablefraction = (signal_ma - 4)/16; flow_percent = sqrt(fraction) x 100 (0 if fraction<=0 or below the low-flow cutoff); flow_value = flow_low + (flow_percent/100)(flow_high - flow_low). NAMUR NE43 fault band as in the linear tile.

Source and editionDifferential-pressure flow transmitter square-root extraction on a 4-20 mA loop, by name (ISA / instrumentation practice; NAMUR NE43 fault-signal levels); the transmitter's range and calibration govern.

Free accessThe square-root relation between flow and differential pressure and the 4-20 mA live-zero scaling are public instrumentation practice; the transmitter's engineering-unit range comes from its configuration.

What governsEstimate. AHJ and licensed professional govern.

API field namessignal_ma, flow_high, flow_percent, flow_value, linear_percent

• Square-root extraction flow% = sqrt((mA - 4)/16); flow ~ sqrt(differential pressure)instrumentation practice
• Loop linearization only the flow element's Cd and beta are already in the transmitter calibrationscope of this tile
• Low-flow cutoff reading forced to zero below the cutoff percent to reject near-zero square-root noiseinstrumentation practice