I am no stranger to vibration but was studying over this information for my upcoming CAT II test and ran across this problem that I cannot understand how they got the answer... it seems so simple but I have yet to find out how to work it out correctly. 2016-07-27 11.32.58 Can anyone help me to see how they arrived at this answer?

Attachments

Photos (1)
Original Post
Walt Strong posted:

First of all, the only close answer is 11.59 Hz. My quick estimate was 5 cycles in about 0.5 seconds or 10 Hz. I would not be concerned about 1/100 of a Hz or even 1/10 Hz when trying to visually convert a waveform to frequency.

Walt

Walt, stick to the game of horseshoes where being close counts!

See below for my take on the question

Attachments

Photos (1)

John,

I did not prepare the test and apparently neither did you; although your attachment differs from OP! No test instructions were provided by OP. The question appears to be multiple choice with no 5th choice for "none of the above". You can argue that the question is poorly worded or that you are more correct than Answer-B. You also assumed that a peak occurred at Time = 0.0, but that cannot be proven without seeing the what occurred before that. I choose Horseshoes on this one!

Walt

Walt Strong posted:

John,

I did not prepare the test and apparently neither did you; although your attachment differs from OP! No test instructions were provided by OP. The question appears to be multiple choice with no 5th choice for "none of the above". You can argue that the question is poorly worded or that you are more correct than Answer-B. You also assumed that a peak occurred at Time = 0.0, but that cannot be proven without seeing the what occurred before that. I choose Horseshoes on this one!

Walt

What I sourced and the OP sourced can be found in the Vibration Institute sample exam for Cat II.  I added the red vertical line which indicates time at 0.5++ and didn't mention that it was "my edit".  But remove that line and you have exactly what the OP posted.

As far as making the assumption that a peak occurred at t = 0, many instruments use the 1st negative going portion of the signal as t = 0.  But you make a good valid point so lets start at the zerro-crossing at the right axis as t = 0.  Count back 5 cycles (at the zero crossing) and you will see that the time is still numerically 0.52 (more or less) or 9.6 Hz.

I see approx. 5.75 cycles in 0.6 seconds. A reasonable answer would be 9.6 Hz. Is this question still a practice question for Category II certification? From the V.I.? If so, I hate to beat a dead horse, but it is no wonder that so many vendors opted to no longer pursue a V.I. certification from 1994 onward for a number of years. One way to fix this error, would be to add another answer "e.  None of the above".

Lou Pag posted:

I see approx. 5.75 cycles in 0.6 seconds. A reasonable answer would be 9.6 Hz. Is this question still a practice question for Category II certification? From the V.I.? If so, I hate to beat a dead horse, but it is no wonder that so many vendors opted to no longer pursue a V.I. certification from 1994 onward for a number of years. One way to fix this error, would be to add another answer "e.  None of the above".

I agree that the correct answer is not one of the choices given. I was unaware of people not pursuing a V.I. certification.

I need to get a CAT II Certification that is ISO certified. Who would you suggest?

Huddle_House posted:

Is Technical Associates of Charlotte ISO certified? any thoughts on them?

Check http://www.technicalassociates.net/certification.html.   I do find interesting that when you look at the schedule, items are referred to as Analysis I, Analysis II, Analysis III, etc.  I wonder why they aren't using the common terminology Level I, Level II, etc.?

Be advised as well that this whole process can be confusing.  See the thread at https://www.maintenance.org/top...5#399590942963090365 for a discussion that should be of interest.  

 

I believe that at present there is only 3 organizations that are accredited in the world to ISO. The V.I. , Bindt, and Möbius Institute. Technical Associates, Full Spectrum Disgnostics, Update international and any others, and CMVA in Canada, are not accredited as far as I know. They align or conform their certification schemes to ISO documents but no one seems to care one way or the other that they are not accredited. Candidates from Non- North American countries put more importance on accreditation than anyone else. Certificates of certification keep getting handed out and customers keep coming back and everyone is making money at it and the vast majority of customers seem to be very happy each way. 

I don't always provide comment to these non-technical remarks.  But why not; it is the political season here.

One does need an accrediting body in their country or get accreditation elsewhere.  This can limit some places.  However, I see that the Vibration Institute is giving accredited training around the world.  Mobius does the same.  

I do not oppose the marketplace that provides these services.  When the customer obtains or believes he receives value and the vendor helps by participating in the market, needs are met.

 

 

An Opinion Walt,

As a very basic engineer, Mobius was more accessible to myself when I was learning vibration analysis, and basing on what I saw written about VI it comes across as more suited to or having favouritism towards more decorated engineers, gets hung up on convoluted high theory, (the question asked at the beginning of this entire thread as good an example as any). But like I said its an opinion. I have not sat the VI exams Just what I've seen written.

Tweed's terribly impractical when your working onsite and itches like hell.

 

From the last comment it sounds like DBTCMP is familiar with tweed.  From the earlier comment and lack of direct knowledge of the VI tests, he must be familiar with smoking something as well. 

        I have not sat the VI exams Just what I've seen written.

What is it that you have seen written that troubles you about the VI exams?

I am sorry your tweed itches.

 

3530 cpm is approximately 17 msec per cycle so A & B can be ruled out.  C & D both show a cycle as 0.017 sec and 17 msec respectively so they are possibilities; they have just expressed time in slightly different units.  C comes close to 2.59 0-pk if the arrow head of the vertical scaling is 5.2 units and the horizontal axis represents "0" as shown.

I like your explanation, John. The question calls for a set of skills to correctly answer the question:

1. Estimating period form the waveform and recognizing the relationship between second and second.

2. Using the formula of frequency and paying attention to the units.

3. Reading the amplitude from the waveform and being able to recognize 0-p and p-p.

4. Selecting the best answer by accepting minor approximations.

A practitioner would "visually" answer this question without going through "calculations". Some of these questions are answered based on the best available choice, which opens doors for discussions/disagreements.

Regards- Ali M. Al-Shurafa

Add Reply

Post

HACKATHON in association with IMC-2019

×
×
×
×