System Performance Drift on a Calibration Block After Probe Change
A UT system is set up on a 1018 steel calibration block with 1.5 mm side-drilled holes at 10, 20, 30, and 40 mm depth. The instrument is a digital flaw detector with a 5 MHz, 12.7 mm diameter contact transducer and a 0-degree wedge. Initial calibration shows a 2 dB spread across the four holes. After a probe change to a nominally identical 5 MHz transducer, the 10 mm hole reads 4 dB lower and the 40 mm hole reads 1 dB higher. Gain linearity check shows a 0.5 dB error at 6 dB steps and 1.5 dB error at 18 dB. The technician wants to proceed because the spread is still within the 3 dB tolerance in the written procedure.
1. Is the 3 dB spread tolerance sufficient evidence that the system is fit for purpose? Explain.
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No. A spread tolerance checks consistency across depth but does not check the individual elements that produce the reading.
- The depth-dependent shift (low near, high far) is consistent with a change in near-field or beam profile, not with a flat gain error. That points to a probe characteristic difference, not an instrument fault.
- Gain linearity error of 1.5 dB at 18 dB exceeds what a competent system should show for amplitude-based sizing. If the procedure relies on DAC amplitude, a 1.5 dB nonlinearity at high gain propagates directly into sizing error.
- A 3 dB spread can be met by two compensating errors (probe low near, instrument high far). Passing the spread test does not prove the system is linear or that the probe is equivalent.
Conclusion: the spread test alone is not sufficient. Linearity and probe equivalence must be verified separately.
2. What specific checks would you require before accepting the new probe?
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Require, at minimum:
- Gain linearity across the full working range, not just at two steps. Document the error at each step and compare against the instrument specification.
- Probe index and beam angle verification on the appropriate block, since a nominally identical probe can have a different index point.
- Beam profile or near-field check: the depth-dependent shift suggests a different effective crystal or lens. A side-drilled hole response curve against the original probe quantifies the difference.
- Repeatability: three repeat readings on the same hole to establish the noise floor of the measurement.
If any check fails the instrument or probe specification, the system is not accepted regardless of the spread result.
3. How should the results be documented, and what is the disposition?
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Documentation should record: instrument serial number, probe serial number, block identification, gain settings, each hole response in dB, linearity error at each step, and the acceptance criteria used.
Disposition: the system should be rejected for amplitude-based sizing until the linearity error is corrected or the instrument is replaced. If the work must proceed, the procedure would need a documented correction or a different sizing method (e.g., time-of-flight) that does not depend on the nonlinear gain region. The Level III should not sign off on the basis of the spread tolerance alone.