Transducer testing

[Porthandbuoy]

I have a number of echosounder transducers in the shed. Is there any way of establishing their operating frequency? (I have a very good multimeter and an old spinning LED display if that helps)
With the boat afloat I could hook each one up in turn and dangle it over the side, great if it works, but if it didn’t work it could be for a different system.
If I knew what their operating frequency was I’d stick them on the For Sale forum.

 

[AntarcticPilot]

There's no way a multimeter will help, I'm afraid. I think you'd need an oscilloscope to determine the frequency of operation, but you'd have to figure out a way to couple the output of the transducer to the oscilloscope. But there are TWO frequencies to consider - the frequency of the sound, and the pulse repetition frequency (PRF). The PRF is relatively slow; you may just be able to hear a ticking when the transducer is operating; each tick is a pulse being sent from the transducer. It's the PRF that allows the spinning LED display to spin slowly enough to be workable; one rev of the display takes the same time as the interval between pulses. But the point is that BOTH the sound frequency AND the PRF are needed. The PRF may not be determined by the transducer; it may be determined by the instrument head.

 

[jbweston]

Is it possible to work from the other direction - any identifying model number or failing that identifying features that will let you look up the transducer specs on the internet? Or find other people who have them in service and can tell you what they have them connected to?

 

[William_H]

The transducer will be made to resonate at the frequency the sounder produces. The way to determine this would be with a variable frequency oscilator and some way of detecting the output so determine resonance. However probably not practical for you. The PRF does not affect the transducer suitability.
You could try the transducers one at a time connected to the old rotating display sounder. They will probably work ok at shallow depth. You may need to go to deeper water to find proper performance. It may well be that if they are old they are all suited to the old sounder. You could then market them as such. ol'will

 

[AntarcticPilot]

The transducer will be made to resonate at the frequency the sounder produces. The way to determine this would be with a variable frequency oscilator and some way of detecting the output so determine resonance. However probably not practical for you. The PRF does not affect the transducer suitability.
You could try the transducers one at a time connected to the old rotating display sounder. They will probably work ok at shallow depth. You may need to go to deeper water to find proper performance. It may well be that if they are old they are all suited to the old sounder. You could then market them as such. ol'will

Is that correct? Surely transducers will resonate at the design frequency, even if the driving waveform is provided by the instrument? Both moving coil and piezoelectric transducers will have resonant frequencies, and it would be inefficient for them to have a broad frequency response.

 

[Porthandbuoy]

Interesting bit about the PRF. I hadn't considered the display triggering pulses at the transducer resonating frequency.
Would I be correct in thinking the display head will also be tuned so that it matches the transducer? i.e. it would fail to make an incompatible transducer resonate at all?

 

[greeny]

Post pictures of them all. Then let members identify them to match their systems. It my get a few of them identified for you.

 

[andsarkit]

If they are old they are probably 150kHz or 200kHz. 150kHz usually match the old rotating sounders and have a 30mm diameter head with a long stem like the ones sold by NASA. 200 kHz Airmar ones used by many manufacturers are about 55mm diameter and have a thin flange. The Airmar website has much information and diagrams of the various transducers.
If you put them on your rotating echo sounder and point them at right angles to a window you will get a reflection with a reading about 5 times too large. Increase the distance to give and idea of how well they work. If you can get 10m you will be doing well.

 

[William_H]

Is that correct? Surely transducers will resonate at the design frequency, even if the driving waveform is provided by the instrument? Both moving coil and piezoelectric transducers will have resonant frequencies, and it would be inefficient for them to have a broad frequency response.

Yes quite so. Perhaps a little misunderstanding of my post. The transducer will have a resonance ie the frequency at which it vibrates best. The sounder oscilator will be set to match the resonance of the transducer. Yes inefficient to have a transducer which does not resonate at the same frequency as that supplied by the sounder head (oscilator) but still may work to a degree. ol'will