05:23 Aug-29-2005 Ed Ginzel R & D, - Materials Research Institute, Canada, Joined Nov 1998 1208
Re: Near Field for TR Probes Near field is a result of interference effects from the pulsed tranducer surface. Therefore the near field would apply to the transmitter. In the receive mode a piezo element is passive and sound pressure must impinge on it to generate a voltage indicated as the "received signal" so the interference effects associated with the near field are not applicable.
Assuming a single element for each of the transmit and receive sides of the probe, the near zone calculation should apply using the standard calculations for the transmitter element only.
----------- Start Original Message ----------- : How to calculate Near Field lenght for TR Probes? ------------ End Original Message ------------
Ed is right and I want to add some thing more,
TR probes has focal spot and and it is adjustable with roof angle of transmitter and receiver crystal by manufacturer design to get maximum response of reflector at desire focal point.focal spot is also affecte by near field of each crystal and as you know there is maximum response at the end of nearfield of single crystal so focal spot should be set at end of near field of transmitter crystal by manufacturers.you can measures focal spot for TR probes and near field for single probes by making DAC curve with special reference block.
Although a TR probe does not exhibit a near field, it may have a dead zone. A small reflector such as a corrosion pit that is too close to the test surface will not necessarily produce a reflection that is well detected by the receiving element. Some TR probes are stated by the manufacturer to have an optimum range, with the minimum and maximum useful limits being somewhat beyond these. If the object is to measure very thin sections a single-element probe with a delay line may be better. But with this type of probe there is less sensitivity to corrosion pits because the beam is spreading beyond the near field, where with a TR robe the beam has effectively a focused zone.
09:44 Dec-29-2014 John Norman Consultant, owner of business NTS Ultrasonics Pty Ltd, Australia, Joined Oct 2012 108
Re: Near Field for TR ProbesIn Reply to Biju Varghese at 06:54 Aug-19-2005 (Opening).
Mark is correct in that a good way to work it out is to generate a DAC curve for the probe. My concern with calculation based on the transmitter element is that TR probe elements usually have a complex shape, often a "half moon" or something like that. Nearfield calculations using simple text book formula are a bit difficult to apply, even for TR probes with rectangular elements.