Ultrasonic Immersion Bath Sensors

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Ultrasonic immersion bath sensors

Schematic configuration

Immersion bath sensors are similar in design to vertical or standard sensors, although they are slightly longer than sensors for direct coupling. To ensure watertightness and optimum impedance matching to water, they are usually equipped with special sealing or matching layers, which also serve to protect the sensor against electrical short circuits. Immersion technique sensors can be used for vertical and oblique (angle) sound transmission and can be partially or completely immersed in water. Technical variants of water or immersion coupling include flowing water coupling, immersion bath coupling, the so-called puddle technique, and guided and free water jet coupling, also known as squirter technique (Fig. 1) [1].

Fig. 1:

Variants of the immersion bath technique arrangement with (a) flowing water coupling, (b) immersion technique arrangement, (c) puddle technique, (d) guided and (e) free squirter technique according to [1, 3]

Examples of implementation

For flat structures, not only a single sensor is often used, but rather a sensor field, whereby this arrangement is used in particular for passive scanning, i.e. for a moving test object. Especially for metal or plastic pipes, rotating immersion sensor can also be used, whereby these are fixed at a defined distance [2].

Lead metaniobate transducers (PbNb2O6) have the lowest acoustic impedance of all piezo ceramics and, like polyvinylidene fluoride (abbreviation: PVDF), are particularly suitable for constructing high-resolution immersion technique transducers with extremely short pulses. In PVDF sensors, the damping element consists solely of highly absorbent plastics. At identical frequencies and with the same transducer diameter, highly damped immersion sensors made of PVDF and lead metaniobate, for example, exhibit approximately comparable sensitivity at very short pulses [1].

Piezoelectric plastic films made of PVDF ensure highly effective sound radiation, especially in water and plastics, which is why PVDF is suitable for high-frequency delay line or immersion sensors (f up to 150 MHz) [2, 4].

Due to their acoustic impedance, piezo composites (composite sensors) are also very well suited for sound radiation in liquids and plastics, whereby the plastic filler of the damper should only be moderately filled (impedance). This also allows the production of very broadband immersion technique sensors whose sensitivity is higher than that of piezoelectric ceramic transducers. Examples of immersion technique sensors are shown in Fig. 2.

Fig. 2:

(a) 2 MHz composite immersion bath sensor and (b) 5 MHz piezoceramic immersion bath technique sensor from Fa. SONOTEC Ultraschallsensorik Halle GmbH, Halle (Saale)

References

[1]	Deutsch, V., Platte, M., Vogt, M.: Ultraschallprüfung – Grundlagen und industrielle Anwendungen. Springer Verlag, Berlin (1997), (ISBN 3-540-62072-9; siehe AMK-Library under M 45)
[2]	Matthies, K. u. a.: Dickenmessung mit Ultraschall. DVS-Verlag GmbH, Berlin, 2. Auflage, (1998), (ISBN 3-87155-940-7; siehe AMK-Library under M 44)
[3]	Deutsch, W. A., Deutsch, K., Schuster, V.: Automatisierte Ultraschallprüfanlagen – Überlegungen zu Durchsatz, Überdeckung und Sensorik. DGZfP-Berichtsband 87, FA-Seminar des FA Ultraschall 2003 Download as pdf
[4]	Schuster, V., Lach, M., Platte, M.: Die Qual der Wahl: Welcher Prüfkopf für welchen Einsatz. DGZfP-Jahrestagung „Zerstörungsfreie Werkstoffprüfung“ 2004, Salzburg, Österreich, Sonderdruck Karl Deutsch, SD 1/51 Download as pdf

Schematic configuration

Examples of implementation

See also

References