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Engineering Toolbox/Control Valve Deadband / Reversal Check

Instrumentation & Calibration

Control Valve Deadband / Reversal Check

Screen the amount of command-signal reversal required before valve travel first responds in the new direction.

Reversal test

Reverse direction and creep the command until movement is first observed.

mA
mA
% signal span

Do not use a generic acceptance limit. Use the applicable OEM, site, loop-performance or maintenance criterion.

Practical field test

Measure the reversal before movement.

Stabilize the valve, reverse the command direction, and change the input slowly enough to identify the point at which stem/shaft travel actually begins in the new direction.

Calculation basis

Deadband (% signal span) = |Signal at first movement − Signal at reversal| / nominal signal span × 100

For 4–20 mA the nominal span is 16 mA; for 3–15 psi it is 12 psi. This is a field screening calculation. Positioner resolution, friction, backlash, packing load, actuator behavior and test rate can influence the observed result. It does not replace a diagnostic valve signature.

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4–20 mA Signal Scaling Convert loop current to percentage of span and engineering units, or calculate the expected mA from a process value. 3–15 psi Pneumatic Signal Convert a standard pneumatic control signal between psi and percentage output without hiding underrange or overrange conditions. Five-Point Calibration Check Evaluate 0, 25, 50, 75 and 100% points against a defined tolerance in percentage of calibrated span. Calibration Error Calculate signed error in engineering units and percentage of span, with a clear pass/fail result against tolerance.