CCST Calibration & Testing of Control Instruments 1 — Questions and Answers
Question 1: What is the standard live zero signal range for a 4-20mA transmitter used in industrial process control?
- 0-10 mA
- 4-20 mA (Correct answer)
- 0-20 mA
- 1-5 VDC
Correct answer: 4-20 mA
The 4-20mA range is the industry-standard signal range, where 4mA represents 0% of span and 20mA represents 100% of span, with the live zero at 4mA allowing detection of broken wire conditions.
Question 2: What does 'zero suppression' mean when calibrating a differential pressure transmitter for level measurement?
- Removing the zero adjustment screw
- Setting the zero point below the actual process zero to account for a static head offset (Correct answer)
- Disabling the zero trim function in the transmitter
- Calibrating the transmitter output to read zero at atmospheric pressure
Correct answer: Setting the zero point below the actual process zero to account for a static head offset
Zero suppression shifts the transmitter's zero point downward to compensate for a positive static head caused by wet legs or elevated mounting, so the transmitter reads correctly across the true measurement span.
Question 3: During a 5-point calibration of a pressure transmitter, at what percentages of span should calibration points typically be checked?
- 0%, 20%, 40%, 60%, 100%
- 0%, 25%, 50%, 75%, 100% (Correct answer)
- 10%, 30%, 50%, 70%, 90%
- 5%, 25%, 50%, 75%, 95%
Correct answer: 0%, 25%, 50%, 75%, 100%
A standard 5-point calibration uses 0%, 25%, 50%, 75%, and 100% of span to adequately characterize linearity and identify any non-linear errors across the full range.
Question 4: What is 'as-found' data in the context of instrument calibration records?
- The calibration data recorded after adjustments are made
- The calibration data recorded before any adjustments are made during a calibration event (Correct answer)
- The manufacturer's factory calibration certificate data
- The data recorded during the initial instrument installation
Correct answer: The calibration data recorded before any adjustments are made during a calibration event
As-found data captures the instrument's actual performance at the time of calibration before any corrections or adjustments, providing a record of instrument drift and compliance history.
Question 5: A loop calibrator is used primarily to:
- Measure insulation resistance of signal cables
- Source and measure 4-20mA signals to test and calibrate transmitters and I/P converters (Correct answer)
- Generate 120VAC power for loop-powered devices
- Measure the resistance of RTD sensors in the field
Correct answer: Source and measure 4-20mA signals to test and calibrate transmitters and I/P converters
A loop calibrator can source a precise 4-20mA signal to simulate a transmitter output and also measure the milliamp signal produced by an actual transmitter, making it the primary tool for analog loop calibration.
Question 6: When performing span adjustment on a transmitter, what are you changing?
- The minimum output signal at 0% of process input
- The relationship between the input signal range and the output signal range (gain) (Correct answer)
- The location of the transmitter's mounting bracket
- The process variable engineering unit displayed on the HMI
Correct answer: The relationship between the input signal range and the output signal range (gain)
Span adjustment sets the gain or slope of the transmitter's input-to-output transfer function, defining how much output change occurs per unit of input change across the full measurement range.
Question 7: What is the primary purpose of performing a 'leak test' before calibrating a pressure or differential pressure transmitter?
- To verify the transmitter's output signal is stable
- To ensure there are no leaks in the calibration tubing or manifold that would cause incorrect applied pressure readings (Correct answer)
- To check that the transmitter housing seal meets IP65 requirements
- To verify the process isolation valves are fully open
Correct answer: To ensure there are no leaks in the calibration tubing or manifold that would cause incorrect applied pressure readings
A leak test confirms that the calibration test equipment connections and tubing are leak-free so that applied pressure is accurately delivered to the transmitter's sensing element without loss.
CCST Calibration & Testing of Control Instruments 1
What is the standard live zero signal range for a 4-20mA transmitter used in industrial process control?