On a very thin material the echo returns before the crystal has finished transmitting. The delay line buys time, and makes visible a signal that would otherwise remain hidden.
- ✓ Separation between transmit pulse and return echo
- ✓ Measurement of thicknesses that could not be read without a delay
- ✓ Protection of the crystal from direct contact with the part
- ✓ Dedicated Interface-Echo modes on precision instruments
When a crystal transmits a pulse it keeps vibrating for a short interval, during which it cannot distinguish an incoming signal. On substantial thicknesses this is not a problem, because the echo returns when the crystal has already settled. On a very thin material, however, the echo arrives while transmission is still in progress and remains indistinguishable.
The delay line places between crystal and part an element of known material, which the pulse must cross on the way out and on the way back. The time spent in that stretch shifts the echo forward: when it reaches the crystal, transmission ended some time ago and the signal is clean and measurable.
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The delay introduced must however be excluded from the measurement, because it is not part of the piece. Precision instruments therefore have dedicated modes, referred to as Interface-Echo, which recognise the interface between delay line and material and calculate the thickness from there.
When the delay line is needed
| Situation | Reason |
|---|---|
| Very thin materials | The echo would return during transmission |
| High-resolution measurements | The separated signal is cleaner and better defined |
| Hot surfaces | The delay keeps the crystal away from the part |
| Abrasive surfaces | The replaceable element protects the transducer |
| Thin plastic materials | Combination of low thickness and attenuation |
An accessory that wears out
The delay line is the part of the probe that touches the part, so it is also the one that wears. On abrasive surfaces it wears down and must be replaced: it is a spare part, not a permanent component, and it pays to keep some in stock as one does with cables.
Replacement requires the probe zero to be redone, because the delay introduced changes with the element fitted. On models with automatic probe zero the operation takes place without a manual procedure when the transducer is changed.
Instruments and probes
| Element | Use |
|---|---|
| Dakota PMX2-DL | Interface-Echo and Interface-Echo-Echo modes |
| Dakota PZX1-DL | Precision measurement on thin materials |
| Dakota PCX8-DL | Top-of-the-range precision gauge with colour A-Scan |
| Precision transducers | High-frequency delay line and contact probes |
Frequently asked questions
Because the echo would return while the crystal is still vibrating from transmission, remaining indistinguishable. The delay line shifts the echo forward, making it readable.
No. Precision instruments have Interface-Echo modes that recognise the interface between delay line and material and exclude the stretch travelled in the delay.
Yes, it is the part that touches the piece and wears, especially on abrasive surfaces. After replacement the probe zero must be redone.
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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
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