Calibration & Testing of Control Instruments Flashcards

What is 'instrument drift' and why is it significant in calibration programs?

Answer: The gradual change in an instrument's output over time without any change in the actual process input, indicating calibration interval adequacy

Drift is the slow, unidirectional change in instrument output over time due to component aging, mechanical wear, or environmental effects, and its magnitude between calibrations determines whether the calibration interval is appropriate.

When performing a 'wet leg' calibration on a differential pressure level transmitter, what is the purpose of the wet leg?

Answer: To maintain a constant static head on the high-pressure side of the transmitter, providing a stable reference for measurement in closed or pressurized vessels

A wet leg fills the high-pressure impulse line with a reference liquid (often water or process fluid) to create a constant, known static head on one side of the DP transmitter, enabling accurate level measurement in closed or pressurized tanks.

What is 'turndown ratio' for a flow meter?

Answer: The ratio of the maximum measurable flow to the minimum measurable flow within the instrument's accuracy specification

Turndown ratio (also called rangeability) expresses how wide a flow range the meter can measure accurately — a 10:1 turndown means the meter can accurately measure from 10% to 100% of its maximum rated flow.

In a 4-20mA signal, what process value does a signal of 12mA represent when the transmitter range is 0-200 PSI?

Answer: 100 PSI

12mA is exactly the midpoint of the 4-20mA range (16mA span ÷ 2 = 8mA above zero = 12mA), representing 50% of the 0-200 PSI span, which equals 100 PSI.

What is 'repeatability' in instrument testing and how does it differ from reproducibility?

Answer: Repeatability is the closeness of agreement between successive measurements under the same conditions by the same operator; reproducibility involves different operators or conditions

Repeatability measures how consistent an instrument is when the same operator makes multiple measurements under identical conditions, while reproducibility extends this to different operators, labs, or measurement conditions.

According to ISA-5.1 and calibration best practices, what information must be recorded in a calibration record?

Answer: As-found readings, as-left readings, reference standard used, date, technician ID, calibration tolerance, and environmental conditions

A complete calibration record must capture as-found and as-left data, the traceable reference standard used, tolerance criteria, environmental conditions, technician identification, and date to satisfy quality and regulatory requirements.

What is the correct procedure when the 'as-found' calibration error of a transmitter exceeds the calibration tolerance?

Answer: Document the as-found error, investigate the cause, adjust the transmitter to within tolerance, record as-left data, and assess whether out-of-tolerance readings affected process control or safety decisions

An out-of-tolerance as-found condition requires full documentation, root cause investigation, corrective adjustment, and a retroactive impact assessment to determine if incorrect measurements affected product quality, safety, or regulatory compliance.