Dead Zone 

DZ – Dead Zone is the name given to the period immediately after the transmit spike has been applied to an ultrasonic crystal. The crystal oscillates for a number of cycles and these oscillations prevent the detection of received echoes during this period. The period referred to as the DZ can be described in units of time or, more commonly in contact testing, by how far the DZ extends below the surface of the component in mm. In single-crystal contact testing, the transducer ringing, and hence the DZ, is observed at the start of the A-scan display. BS EN 12668-2:2010 gives a definition of the DZ of a probe and 
BS EN ISO 16811:2014 refers to the effects of the DZ when setting a distance amplitude curve (DAC).

Twin-crystal probes, with separate transmit and receive crystals, are used to overcome the problem of the DZ and to allow inspection for defects close to the surface of the component.  
In the time-of-flight diffraction (TOFD) technique, a backwall DZ is encountered, where the relatively large echo from the backwall can obscure smaller defect signals. BS EN ISO 16828:2014 gives a definition of this TOFD backwall DZ.

For a diagrammatic representation of the DZ see:

What the hec?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: