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failing transducer output

failing transducer output

failing transducer output

(OP)
Im having an odd problem with an ultrasound driver and transducer combination.  The driver produces a an waveform output from 500hz to 5MHz (tested with 10ohm resistive load) which we are trying to use to drive transducers ranging 500khz too 3MHz (500KHz, 1MHz, 1.35Mhz, 2MHz, and 3MHz) at the resonant frequency.  We are using a driving voltage from 5 to 35Volts on each. Each transducer has a capacitance of around 5nf and each is the same size except for thickness which of course, varies with frequency/wavelength, the slower frequencies needing to be thicker.

In a test  water bath, the 1.35Mhz, 2MHz, and 3MHz transducers work great, producing a very powerful output which creates a force of around 1megapascal or larger as measured on a hydrophone. The output power can also be verified outside of the water bath as these transducers can levitate water.  The output pressure wave of these transducers stays constant over time (ie 20min).
The unexpected problem is that for the lower frequency transducers cannot sustain their high energy output.  The 1MHz produces a large amount of pressure but then this energy fades after about 5 seconds (at 30V). If I let the setup rest for about a minute and turn on the power again, it runs again for 5 seconds. If I run it at a lower voltage, the transducer will run at the higher power for a longer amount of time but eventually will fade out.  With the 500KHz, the effect is even more pronounced, and it can be run less than half the time the 1 MHz can run.   During the entire time that the transducer is failing, the voltage intput it stays constant as seen on an oscilloscope.

Im guessing that either the circuit is heating up or otherwise the transducers.   I would have expected more heat loss at the circuit during high frequency operation though due to frequency response losses at the higher bandwidth.  The lower frequency transducers, which are thicker, don't feel hot so Im confused about what is going on.  Im guessing they are not meant to be driven in a continuous fashion?  Any ideas?
 

RE: failing transducer output

I am certainly no expert in this and it has been 40 years since I experienced this when we were using water to simulate liquid sodium.  The signal produced tiny bubbles in the water that built up in a short time.  That deadened signal transmission.  Wait a little while for the bubbles to go away and you could repeat it.  And no,you won't see any bubbles.

RE: failing transducer output

(OP)
that pretty interesting.  Ill have to look into this next week. I have seen bubbles on the lower frequency transducers but just assumed they form on the higher frequency ones too and would get blown away with the large force they produce which shoots water though the tank..  Im still not sure it explains why the higher frequency transdeucers work and the lower dont, unless cavitation was somehow dependent on frequency but i dont think it is.

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