Low Frequency Ultrasonic Transducers and Their Applications

Transducers and Leads

The transducers consist of piezo-electric elements mounted in stainless steel cases. The elements are very tightly held on to the inside face of the case to provide highly efficient acoustic transmission. The transducer assembly is very robust and can withstand reasonably rough conditions as normally encountered under conditions of industrial usage.

The high voltage excitation of the pulse generator causes the transducer to oscillate mechanically at its own natural frequency; this frequency depends on the size and stiffness of the whole transducer assembly. Different sizes of piezoelectric elements and cases enable different pulse frequencies to be obtained.

Each transducer is fitted with a cable socket to enable it to be detached from the cable and to allow cables of different length to be used.

Short co-axial cables for connecting the transducers to the V-Meter are supplied with the instrument. Longer cables may be used when access to the areas to be tested is restricted. Although long cables reduce the size of both the transmitted pulse and the receiving signal, these are generally both of adequate magnitude for testing even when the cables are up to 25 ft. long. Cable lengths of up to 20 meters(approximately 60 feet) have been used successfully with the optional pre-amp( V-C-4896 ).

Note: When using very long leads, try to prevent the two leads from coming into close contact with each other when the transit time measurement is being taken. It is possible for the receiver lead to pick up unwanted signals from the transmitter lead resulting in an incorrect display of transit time. Such incorrect displays are readily detected by their instability and may be corrected by simply separating the leads.

Types of Transducers

24 kHz Transducer : Used for larger (softball sized) aggregates. Range is up to 16 feet without Pre-Amp.

54 kHz Transducer: Most commonly used transducer. Used for 12mm (½ inch) diameter aggregates. Range is up to 16 feet without Pre-Amp.

150 k Hz Transducer: Used for sand sized aggregate (mortar and ceramic). Range is up to 1 foot without Pre-Amp. A pre-amp is typically used in conjunction with these units.

500 kHz Transducer: Used for very small aggregate (ceramic and glass). Range is up to 1 foot without Pre-Amp. A pre-amp is typically used in conjunction with these units.

ShearWaveTransducer: Used for finding non-homogenous conditions of a surface. Space the transducers 1 foot apart with the lines facing each other. No Pre-Amp required. Use of the shear wave transducers is limited to indirect mode, i.e. each transducers needs to be on the same surface of the material under test. The flat edge of the transducer denotes the orientation of the internal crystal. These edges need to facing each other for proper operation.

Exponential Transducer: Used for testing samples with non-flat surfaces on either side. Range is 1 foot with Pre-Amp. They were developed for either very rough surfaces or to test specimens where a standard size transducer does not fit. A pre-amp is typically used in conjunction with these units.

ExponentialTransducer Transmitter

ExponentialTransducer Reciever

Underwater Transducer: Used for a substrate with softball sized aggregates that is submerged underwater. Range is 16 feet without Pre-Amp, however a pre-amp is built into the recieving unit. Underwater Transmitters and Underwater Receivers are sold seperately.

Based on the above descriptions, the user should be aware that using the wrong transducer for an application can lead to misleading results. If a low frequency transducer (24 kHz for example) is used where the aggregate being tested is of a finer grade, the V-Meter could claim that there are no issues with the substrate when there could be. Similarly, if a high frequency transducer (150 or 500 kHz) is used where the aggregate is too large, the V-Meter may claim that there are issues in the substrate when there might not be.