Why is mounting not critical (i.e. the use of magnets and contact probes) for ultrasound (high-frequency measurement) when it is so important for vibration measurements?

Good question! I'll be interested to hear the responses.

I am not an expert in this area, but we were told in Level 1 training that when using ultrasound for trending purposes, probe placement and orientation are very important. The transducer needs to be mounted the same way every time. With pillow block bearings, for example, a small 'point' can be drilled for placement of a 'stinger' probe. The probe should be pointed toward the center of the shaft. You are just measuring how much noise the thing makes at a certain frequency, to see if it gets louder in the future. Very early bearing faults and lube problems start out emitting in ultrasound frequencies.

Even though recordings and spectrums can be made, ultrasound is not normally used to pinpoint a particular defect, for example, BPIR. Why? Because this type analysis is tied to mutiples of rotating speed. The further you get from 1X running speed, the greater the potential for error, as far as knowing whether energy at a particular frequency is synchronous. Why go to a lot of trouble gathering this data so precisely, when it cannot be trusted for detailed analysis?

Stinger probes? Maybe ultrasound has no trouble traveling through the probe, because of the tiny short waves that make ultrasound.

Gotta go for now, I am also very interested in hearing responses to this great question.

I would think it has to do with the quality of the crystal being used. Way back, 10 or more years ago, I worked with UE on hooking the 2000 (I think it was) up to a DataPac 1500 and collecting data. We were able to do so, but the best you could get looked like 300,000 Fmax with 400 LOR.
At that time, UE explained the crystal used was "dirty" since they didn't need an ultrafine one for what the instrument did. They were also at that time trying to develop the argument that you could use the UE gun to "trim down" the route of equipment to use the vibration on (I was doing the test underground in the mines at the time).
The newer 10000 uses software to view the spectrrum derived from the TWF they capture, but when you look at it, it's still way "dirtier" than any vibes box I've used.

My thinking is if they haven't upgraded the crystal any, you don't need real good mounting as your not looking for any resolution anyway.

Could be wrong.

Dave

My concern/question is how the high-frequency ultrasound signal gets to the instrument (crystal) when you use a magnet or contact probe.

Pretend we have a magical accelerometer with a Fmax of 30 kHz. I don't think anyone is going to tell you that you can see a 30 kHz signal when using a magnet or stinger. Are the crystals in the ultrasound instruments that sensitive?

Ultra sound via which transducer? Sound microphone? What is your area of interest? Leak, change in bearing noise/vibration or? Mounting and distance may be important and critical factors if resonance or magnitude accuracy is of concern. Several factors to consider.

I believe the mounting is one of the most critical steps when using ultrasound to detect problems when using a magnet or stinger setup.
Repeatablity can only come with consistent locations and operational parameters.
The amount the detectable high frequency waves can be varied in dB level is sometimes a massive amount. For me background influences such as other machines in the area are a big challenge to eliminate whilst you collect your readings.

Hooch

quote:

Pretend we have a magical accelerometer with a Fmax of 30 kHz. I don't think anyone is going to tell you that you can see a 30 kHz signal when using a magnet or stinger. Are the crystals in the ultrasound instruments that sensitive?

Steve, an accelerometer is part of a system that is built to measure acceleration.
In ultrasonics, are the crystals sensitive enough to "see" 30 kHz? Apparently so. I can tune my UE 9000 to 100 kHz, and hear the Warble Tone Generator very easily (using the airborne scanning module). In a quick experiment high on a catwalk, everything tested exactly as expected. The higher frequencies are extremely directional, and the Ultraprobe had to be pointed with great precision to hear 100 kHz at any distance.