Calibration & Testing of Control Instruments Flashcards

What is the key difference between accuracy and precision in instrument measurement?

Answer: Accuracy is how close a measurement is to the true value; precision is how repeatable measurements are

Accuracy describes closeness to the true or accepted value, while precision describes the repeatability or reproducibility of measurements regardless of how close they are to the true value.

What is 'hysteresis' in a control instrument?

Answer: The difference in output readings when approaching the same input value from increasing versus decreasing directions

Hysteresis is the phenomenon where an instrument gives different output readings for the same input value depending on whether the input is increasing or decreasing, caused by mechanical friction, backlash, or magnetic effects.

Why must calibration standards used in the field be NIST-traceable?

Answer: To ensure measurements are linked through an unbroken chain of comparisons to national measurement standards, ensuring accuracy validity

NIST traceability provides an unbroken documented chain of calibrations from the working standard back to NIST's primary standards, ensuring the reference standard's accuracy is known and defensible.

An RTD (Resistance Temperature Detector) operates on what principle?

Answer: The predictable change in electrical resistance of a metal with temperature

RTDs exploit the well-characterized positive temperature coefficient of resistance in pure metals like platinum, where resistance increases predictably and repeatably with temperature according to known equations.

What is 'deadband' in a control instrument or controller?

Answer: A range of input values over which no change in output occurs, used to prevent unnecessary control action

Deadband is an intentional or inherent zone around a setpoint or threshold within which no output change occurs, reducing wear on final control elements and preventing control system hunting.

When calibrating a thermocouple transmitter in the field without a reference junction compensator, which method provides the most accurate result?

Answer: Applying a millivolt source calibrated to simulate the thermocouple output at specific temperatures, accounting for CJC

A precision millivolt source calibrated to the thermocouple's mV/temperature curve, combined with accurate measurement and compensation of cold junction temperature, provides an accurate simulation of the thermocouple signal for transmitter calibration.

What does 'calibration tolerance' define in an instrument calibration procedure?

Answer: The maximum allowable error between the instrument's output and the reference standard at each calibration point

Calibration tolerance specifies the acceptable error band — if the instrument's output falls within this tolerance of the reference standard value, the calibration is considered acceptable; if outside, adjustment is required.