CCST Calibration & Testing of Control Instruments 3

Question 1

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

Accepted Answer

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

Answer

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.

Question 2

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

Accepted 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

Answer

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.

Question 3

What is 'turndown ratio' for a flow meter?

Accepted Answer

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

Answer

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.

Question 4

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

Accepted Answer

100 PSI

Answer

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.

Question 5

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

Accepted Answer

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

Answer

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.

(CCST) Certified Control Systems Technician Practice Test

(CCST) Certified Control Systems Technician Practice Test

CCST Calibration & Testing of Control Instruments 3