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

Random electronic noise in the measurement signal

Answer

The physical movement of an instrument mounting due to vibration

Answer

The change in instrument reading caused by ambient temperature fluctuation

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

To provide cooling water for the transmitter in high-temperature applications

Answer

To flush contaminants from the process impulse lines

Answer

To provide a liquid seal preventing process gas from entering the transmitter

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

The angle at which the flow meter must be oriented relative to gravity

Answer

The reduction in flow achieved by partially closing a downstream valve

Answer

The time required to recalibrate after a process fluid change

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

75 PSI

Answer

120 PSI

Answer

60 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 is accuracy under varying conditions; reproducibility is accuracy under identical conditions

Answer

Repeatability applies to analog instruments; reproducibility applies to digital instruments

Answer

Repeatability and reproducibility are identical terms used interchangeably in ISA standards

Question 6

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

Accepted Answer

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

Answer

Only the final as-left readings if the instrument passed calibration

Answer

Only the calibration date and technician signature

Answer

The process operating conditions at the time of calibration only

Question 7

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

Accepted 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

Answer

Immediately replace the transmitter as it has failed

Answer

Simply adjust the transmitter and record only the as-left data

Answer

Notify management and leave the instrument in service until the next scheduled calibration