Re: Dual vs lens-focused crystal advantages? For immersion technique the situation is quite different. In most cases the probe is separated from the test object by a certain water path. The length of this water delay line is adjusted to position the zone to be tested near to the focus point of the probe or to the near field length (the natural focus) of an unfocused probe.
With immersion technique we can use a lens or a curved crystal for direct coupling to the water delay line. In contrast to this, for direct contact probes we need a flat coupling surface. This means, the lens construction is more difficult and we always have to use delay line probes to avoid additional ringing between the crystal and the front face of the probe which would impair the near resolution. The length of this solid delay line must be longer than the time of flight equivalent thickness of the test object to avoid multiple echos from the delay line in the observed time gate. On the other side, the diameter of the delay line must be big enough in relation to its length toavoid spurious echos from the side wall of the delay line. This is often difficult to achieve for thick test objects, when small flaws have to be detected.
Therefore, single crystal focused probes are much better suited for immersion technique than for direct contact applications. In addition, when looking for small flaws the immersion technique is better for scanning under constant coupling conditions than the contact technique.
Of course, there are also many applications for dual probes with pseudo immersion technique. That means, there is only a small water gap of some tenth of a millimeter used for coupling. Typical applications for that water gap technique are plate or billet testing in automatic testing machines. For example, for plate testing many (multi element) paint brush probes are used in one line to scan the whole width of the plate. Each probe has one or more rectangular crystals to produce a broad cross section of the sound beam in one direction for covering a wide scan width. In this case, it is not possible to use single crystal focused probes in immersion technique but dual probes coupled by a water gap are the only choice. These dual probes have a good near and far resolution over the whole wall thickness range of the plate for the critical flaws of this application.
: Thanks for the response.....
: How does your response apply, in the case of immersion Ultrasonics? ...where focus, seems to be used instead of dual crystal design, in most applications...why?
: : Klaus Matthies asked me to reply to this question as he feels not familiar with this
: : subject.
: : 1. I do not agree to the statement that lens-focused single crystal probes are more
: : common for applications where near surface resolution is requested. To my
: : experience, the near surface resolution of direct contact dual probes is much better
: : than for direct contact single crystal probes. Even when the single crystal probe is
: : focused there is still a zone of poor resolution after the transmission pulse or after
: : the interface echo if a delay probe is used.
: : 2. Of course, dual probes can be optimized by choosing the proper crystal material.
: : For example, by using piezocomposite transducers the signal amplitude and
: : resolution will be improved significantly. By taking different kinds of piezocomposite
: : material for transmitter and receiver crystal you can match them to the electrical
: : impedance of the transmitter and receiver electronics individually.
: : 3. I see no reasons why not to use dual probes for critical tasks of near surface
: : resolution.
: : Gerhard Splitt
: : : The original Dual crystal contact probes used for near surface resolution seem to have been superceded by lens-focused
: : : single crystal probes.
: : : Are there new crystal materials which would give Dual crystal probes an advantage over lens-focus probes? i.e. optimized
: : : transmitter and receiver crystal material.
: : : Other than assembly cost ofDual probes, are there clear reasons not to use Dual probes instead of lens-focus single crystal
: : : probes?