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Curran919

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I was doing some research and came across an example of an inverted probe in some old B&K documentation. Has anyone used anything like this? I was surprised that the 'mounted resonance' is 10x higher than the equivalent handheld probe.

 
B&K inverted probe.png 

Walt Strong

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"I was surprised that the 'mounted resonance' is 10x higher than the equivalent handheld probe."

Did you do your own response tests? If you are comparing to "typical published" hand-held accelerometer response, then the data is often with a stinger about 2"-5" that reduces useful frequency response.

Walt
OLi

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It looks like the old CSI 310 "broom handle" maybe from Wilcoxon? It was not that impressive and it had a very short stud where you could mess up things further by attaching a stinger or a magnet or for best result leave it as it was. It worked ok and it had a trigger button that I still remember as a good thing.
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Curran919

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Quote:
Originally Posted by Walt Strong
"I was surprised that the 'mounted resonance' is 10x higher than the equivalent handheld probe."

Did you do your own response tests? If you are comparing to "typical published" hand-held accelerometer response, then the data is often with a stinger about 2"-5" that reduces useful frequency response.

Walt


Aye, I meant a handheld probe with a stinger. You reckon a handheld sensor with no stinger can achieve a 15kHz mounted resonant frequency?
Walt Strong

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"You reckon a handheld sensor with no stinger can achieve a 15kHz mounted resonant frequency?"

Yes, if a suitable accelerometer, the accelerometer base distance to mounting surface is minimal, metal-metal contact surface is the same as for a stud-mounted base, and the g-level is less than 1.0. Higher g-levels that exceed the hold-down force would cause the accelerometer to bounce and affect accurate measurement. The real advantages of a stud-mount is that the accelerometer base is directly on the surface, and the stud allows restraint for high g-levels.

Let's put it another way: if surface acceleration level is less than 1-g, then an accelerometer placed on a smooth metal horizontal surface without any restraint, besides gravity, would measure with the same frequency response as compared to mounting with a stud!

Walt

Curran919

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Quote:
Originally Posted by Walt Strong

Let's put it another way: if surface acceleration level is less than 1-g, then an accelerometer placed on a smooth metal horizontal surface without any restraint, besides gravity, would measure with the same frequency response as compared to mounting with a stud!


Aye, this is true... in theory.

But it assumes uniaxial vibration and a perfect flat-flat contact. If you place an accel, its just going to have a three point contact. It is very liable to rock on two of those contacts (especially with transverse vibe). Any amount of downward force (stud) will prevent this rocking. It will also flatten out the highest asperities and be able to achieve many more than 3 points of contact, which stabilizes it further.

Although I suppose these realisms just decreases the max g until it becomes nonlinear. I suppose the stiffness would not be significantly affected as long as the signal is low. Hmm...
OLi

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I once had a stud mounted hi freq. accel obviously having the resonance at 18kHz as that was 2xGearmesh of the gearbox.......
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