CCT Calibration Techniques & Procedures 5 β Questions and Answers
Question 1: A pressure transmitter calibrated at sea level is relocated to an installation at 5,000 feet elevation. For a gauge pressure transmitter, this change will:
- Increase the output by an amount proportional to the absolute pressure change
- Shift the zero reading because gauge pressure references local atmospheric pressure (Correct answer)
- Have no effect because the transmitter measures differential pressure internally
- Decrease the span due to reduced air density
Correct answer: Shift the zero reading because gauge pressure references local atmospheric pressure
Gauge pressure is measured relative to local atmospheric pressure; at higher elevation, lower atmospheric pressure causes the zero point to shift, requiring re-zeroing at the new location.
Question 2: During calibration, a technician applies 50% of input range and observes a reading 2% of full scale higher than expected consistently across three trials. This error is best classified as:
- Random error due to noise
- Systematic error (bias) at mid-scale (Correct answer)
- Hysteresis between up-scale and down-scale passes
- Resolution error of the display
Correct answer: Systematic error (bias) at mid-scale
A consistent, repeatable offset at the same input point across multiple trials indicates a systematic bias rather than random scatter or hysteresis.
Question 3: The primary reason calibration procedures specify a stabilization period after changing the input to the unit under test is to:
- Allow the technician to record ambient temperature
- Allow transient responses to decay so the instrument reaches steady-state before reading (Correct answer)
- Give the calibration standard time to auto-range
- Purge any air bubbles from hydraulic test lines only
Correct answer: Allow transient responses to decay so the instrument reaches steady-state before reading
Instruments have response time constants; reading before steady-state is reached introduces a dynamic error that is not representative of the instrument's static accuracy.
Question 4: When calibrating a torque wrench, the applied reference torque is usually generated by hanging calibrated weights at a known distance from the drive axis. The formula used is:
- T = F / d
- T = F Γ d (where F = weight force, d = moment arm length) (Correct answer)
- T = F Γ dΒ² / 2
- T = F / (d Γ Ο)
Correct answer: T = F Γ d (where F = weight force, d = moment arm length)
Torque equals force multiplied by the perpendicular distance (moment arm) from the axis of rotation; T = F Γ d is the fundamental torque equation.
Question 5: A calibration technician must calibrate a volumetric flask. The gravimetric method determines volume by:
- Measuring the flask dimensions with calipers and calculating volume geometrically
- Weighing a known mass of water delivered by the flask and dividing by water density at that temperature (Correct answer)
- Comparing the flask to a reference flask of identical nominal volume
- Timing the flow rate of water through the flask opening
Correct answer: Weighing a known mass of water delivered by the flask and dividing by water density at that temperature
The gravimetric method uses the relationship V = m / Ο, where mass is measured precisely and water density at the measured temperature is obtained from tables to calculate true volume.
Question 6: Which of the following actions would invalidate an instrument's calibration status and require recalibration before further use?
- Storing the instrument in its original carrying case
- Dropping the instrument or subjecting it to mechanical shock (Correct answer)
- Connecting the instrument to a load within its rated specifications
- Replacing the battery with the same battery type
Correct answer: Dropping the instrument or subjecting it to mechanical shock
Mechanical shock can alter the physical characteristics of sensors, springs, or electronic components, introducing unknown errors that cannot be corrected without recalibration.
Question 7: In a calibration procedure that requires 'overlapping range points,' the purpose is to:
- Check that the instrument reads correctly beyond its rated range
- Verify consistency at points that fall within the uncertainty intervals of adjacent calibration points (Correct answer)
- Reduce the total number of calibration points needed
- Test the instrument's response to out-of-range inputs
Correct answer: Verify consistency at points that fall within the uncertainty intervals of adjacent calibration points
Overlapping points detect non-linearities or inconsistencies between segments of the calibration curve that would be missed if each point were measured only once in a strict sequence.
CCT Calibration Techniques & Procedures 5
A pressure transmitter calibrated at sea level is relocated to an installation at 5,000 feet elevation.
For a gauge pressure transmitter, this change will: