On a machined light-alloy panel the thickness is not an estimate but a design dimension: the tolerance is of the order of microns, and a precision gauge is needed, not a corrosion one.
- ✓ Selectable resolution 0.01 or 0.001 mm
- ✓ High-frequency single element probes with delay line
- ✓ Interface-Echo and Interface-Echo-Echo modes
- ✓ Non-destructive measurement on the finished part, repeatable
- ✓ A-Scan to check visually which echo is being used
- ✓ NIST-traceable certificates for the metrological chain
Lightening an aircraft structure means removing material down to the limit allowed by the calculation. A panel machined from solid, a titanium-alloy bulkhead, a machined rib: in all these cases the remaining wall is a design dimension with a tight tolerance, and checking it after machining is part of quality control, not a maintenance inspection.
This changes the instrument completely with respect to the world of corrosion. High-frequency single element probes are needed, which resolve thin thicknesses better, and a delay line separating the entry echo from the first backwall echo: without that separation, on a thin part the two signals overlap and the measurement is not possible. The Interface-Echo modes were created for exactly this, and corrosion models do not have them.
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Then there is the question of sound velocity. Different aluminium alloys, and titanium all the more so, have different velocities: setting a generic value introduces a systematic error that at micron resolution is not tolerable. The correct velocity is derived on a sample of known thickness of the same material.
Precision and corrosion: two different worlds
| Aspect | Corrosion instrument | Precision instrument |
|---|---|---|
| Probe type | Dual element | Single element, delay line, pencil |
| Typical resolution | 0.01 mm | 0.001 mm |
| Mode | Pulse-Echo, Echo-Echo, ThruPaint | Interface-Echo, Interface-Echo-Echo |
| Typical surface | Irregular, corroded, painted | Machined, clean, regular |
| Question it answers | How much material is left | Whether the dimension is within tolerance |
The precautions that make the difference
At thousandth-of-a-millimetre resolution, details that are negligible elsewhere become the main source of error.
- The sound velocity must be derived on the actual alloy, not taken from a generic table
- The delay line wears: a worn tip shifts the measurement
- The parallelism between the two faces affects the quality of the echo
- The temperature of the part and of the environment affects the time of flight
- The A-Scan makes it possible to check that the gate has locked onto the right echo
The delay line tip is a consumable: it has to be replaced when readings start to become unstable.
Recommended instruments
| Instrument | Why in this application |
|---|---|
| Dakota PMX3-DL | 0.001 mm with colour A-Scan: visual check at every reading |
| Dakota PMX2-DL | Five modes with delay line and pencil probes, A-Scan and B-Scan |
| Dakota PZX1-DL | Thin materials and quality control in production |
| Dakota PCX8-DL | Top of the precision range with extended data logger |
| Single element probes | Delay line and contact, from 2 to 25 MHz |
Frequently asked questions
Because it uses dual element probes, typically resolves to hundredths of a millimetre and has no Interface-Echo modes. On a machined part with a micron tolerance that resolution is not enough, and without a delay line the entry echo and the first backwall echo overlap on thin thicknesses.
Yes. Different aluminium alloys have different velocities, and titanium differs still more: using a generic value introduces a systematic error that at thousandth-of-a-millimetre resolution is not acceptable. The real velocity is derived by measuring a sample of known thickness of the same material.
It depends on the instrument-probe combination and on the material, not on the instrument alone. With high-frequency delay line probes it goes very low, because the delay line separates the entry echo from the first backwall echo. Tell us the material and nominal thickness and we will confirm the combination that can be used.
Back to Aerospace and Defence. For a check on light alloy or titanium, write to us stating the material, the nominal thickness and the tolerance required.
Further information and contacts
Systems division and sensor integration partner : www.roder.it
More information about RODER SRL : about us
Contact for general information : info@roder.it
Contact for information about these products : info@roder.it
The information contained on this web page is provided for information purposes only. Although it has been compiled with the utmost care, it does not constitute a contractual offer or a binding commitment to supply. It may contain transcription, translation or typographical errors. For precise and up-to-date information, please contact our company directly.
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