Both controls slide echoes across the A-scan, but only one changes the reading. Watch the animation, then try it yourself in the sandbox below.
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Try it — Epoch sandbox
Gate A is parked on the reflector and reports its depth. Use ZERO OFFSET, DELAY and RANGE to finish each task. Velocity is fixed at 5920 m/s (steel).
Zero offset (probe delay) 0.00 µs
Delay (display delay) 0.00 mm
Range 60.00 mm
Hint
Key points
Epoch: ZERO OFFSET · µs
Probe delay moves zero
- Removes the time sound spends in the wear plate, delay line and couplant, so zero is the test surface.
- Changes every reading by the same amount.
- Always used. Set it in every calibration (CAL ZERO on the thin step, CAL VEL on the thick step).
- Recheck when you change the probe, cable, delay line or couplant, and as the wear face wears.
Epoch: DELAY · mm
Display delay moves the window
- Slides the screen's left edge deeper into the part, like panning a camera.
- Readings do not change. Only what you can see changes.
- Used when needed: zooming into a depth band on thick parts, pushing the water path off-screen in immersion, spreading out the region near the back wall.
- Set it back to 0 when you're done, or near-surface flaws on the next job will be off-screen.
Immersion
Both controls can handle the water path
- At steel velocity, water looks about 4× longer than it is: 50 mm of water shows up as about 200 mm.
- DELAY pushes the water path off-screen, but readings are still measured from the probe and include the water.
- ZERO OFFSET can absorb the water time, like a very long delay line. Depths then read from the front surface. Calibrating on a reference block in the tank at the same water path does this for you.
- It only holds while the water path is fixed: 1 mm more water shifts every reading about 4 mm in steel. Re-standardize, or use an interface gate if your instrument has one.
- Setup check: water path ≥ ¼ × steel thickness, so the 2nd interface echo lands beyond the back wall.