The bottom line
Most “the gauge is broken” calls are misjudgements. Following the order coupling to surface to settings to part to gauge locates the problem within minutes.
Check order and judgement criteria
Breaking a “suspected fault” into items you can rule out one by one avoids mistaking an operating issue for hardware damage, and avoids letting a real gauge problem slip past the calibration cycle.
- Coupling and contact — re-apply couplant, is the probe face seated steadily. This solves most “no reading” cases and takes the least time, so it goes first.
- Surface state — scale, oil, uncured coating and rough faces all cause abnormal readings; magnetic/eddy models are especially sensitive to surface state.
- Settings — sound velocity, unit (μm / mil) and measurement mode match the current part? A wrong mode setting often looks much like hardware fault.
- Part and gauge — only when the first three are normal consider substrate difference or gauge state. Then verify with a foil; if out of tolerance, send for calibration or repair — do not open it yourself.
Paired with an instrument log (serial, calibration date, certificate number, user), this order also feeds back criteria: if the same gauge keeps failing at step four, it is time to send it for calibration early rather than keep adjusting on site.