A single crystal probe transmits and receives ultrasound with one crystal: the crystal transmits the pulse and vibrates when the pulse returns from a backwall echo or a flaw.
However, when a single crystal probe is used, a signal appears on the screen at the beginning of the time base. It is caused by vibrations immediately adjacent to the crystal and is called by several names: initial pulse, transmission signal, crystal strike or main bang.
The signal on the screen caused by the initial pulse may mask signals from flaws close to the top of the parent material. For this reason the area on the CRT (cathode ray tube) screen in which flaws may be masked is called the DEAD ZONE.
A twin or double crystal probe is designed to minimise the problem of dead zone. A twin crystal probe has two crystals mounted on perspex shoes angled inwards slightly to focus at a set distance in the test material. If the crystals are not angled, the pulse would be reflected straight back into the transmitting crystal.
The perspex shoes hold the crystals away from the test surface, so that the initial pulse does not appear on the CRT screen. The dead zone is greatly reduced to the region adjoining the test surface, where the transmission and reception beams do not overlap.
There are other advantages:
- The double crystal probe can be focused from 3 - 25mm
- It can measure thin plate
- It can detect near-surface flaws
- It has good near-surface resolution
But a double-crystal probe has disadvantages:
- Good contact is difficult with curved surfaces
- It is difficult to size small defects accurately as the width of a double-crystal probe is usually greater than that of a single-crystal probe
- The amplitude of a signal decreases the further a reflector is situated from the focal distance - a response curve can be made out.
Therefore single and twin crystal probes are complementary.
Please contact us for more information.