CCST Calibration & Testing of Control Instruments 2 — Questions and Answers
Question 1: What is the key difference between accuracy and precision in instrument measurement?
- Accuracy is how repeatable measurements are; precision is how close to true value
- Accuracy is how close a measurement is to the true value; precision is how repeatable measurements are (Correct answer)
- Accuracy applies to analog instruments; precision applies to digital instruments
- Accuracy and precision are synonymous terms in metrology
Correct 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.
Question 2: What is 'hysteresis' in a control instrument?
- The time delay between a process change and the transmitter output response
- The difference in output readings when approaching the same input value from increasing versus decreasing directions (Correct answer)
- The temperature coefficient affecting sensor accuracy
- The maximum overrange pressure the instrument can withstand
Correct 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.
Question 3: Why must calibration standards used in the field be NIST-traceable?
- To comply with OSHA regulations for field instrument technicians
- To ensure measurements are linked through an unbroken chain of comparisons to national measurement standards, ensuring accuracy validity (Correct answer)
- To qualify the technician for ISA certification renewal
- To meet EPA reporting requirements for environmental measurements
Correct 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.
Question 4: An RTD (Resistance Temperature Detector) operates on what principle?
- The Seebeck effect — two dissimilar metals generate a voltage proportional to temperature
- The predictable change in electrical resistance of a metal with temperature (Correct answer)
- The expansion of a bimetallic strip with temperature change
- The change in capacitance of a ceramic element with temperature
Correct 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.
Question 5: What is 'deadband' in a control instrument or controller?
- The frequency range where the instrument cannot respond to process changes
- A range of input values over which no change in output occurs, used to prevent unnecessary control action (Correct answer)
- The signal loss experienced in long cable runs
- The minimum pressure required to open a control valve
Correct 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.
Question 6: When calibrating a thermocouple transmitter in the field without a reference junction compensator, which method provides the most accurate result?
- Applying a millivolt source calibrated to simulate the thermocouple output at specific temperatures, accounting for CJC (Correct answer)
- Connecting a known resistance in series with the thermocouple circuit
- Replacing the thermocouple with an RTD and recalibrating the transmitter
- Using the transmitter's internal auto-zero function
Correct 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.
Question 7: What does 'calibration tolerance' define in an instrument calibration procedure?
- The maximum ambient temperature range in which calibration can be performed
- The maximum allowable error between the instrument's output and the reference standard at each calibration point (Correct answer)
- The time interval between required calibrations
- The minimum signal level the instrument can detect
Correct 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.
CCST Calibration & Testing of Control Instruments 2
What is the key difference between accuracy and precision in instrument measurement?