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Danny Harvey

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Reply with quote  #1 
Just found out why we had such a hard time balancing a big fan. 

Two of the channels on the 4 channel mux were crossed at the factory!

GRRRR!
RustyCas

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Reply with quote  #2 
Believe nothing you hear, half that you see, but all that you do.
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RustyCas

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Reply with quote  #3 
Actually, Danny, after thinking about this, I don't see why it would matter if 2 channels were crossed, unless you were getting a 'zero' input and that should be immediately obvious.  What many people don't realize is that with Fastbal, none of the information you enter is used in the calculations except the amplitudes, phase readings, and trial weight amount.  All of the other setup info (tach location, rotation direction, sensor orientation & location) is just window dressing.  Software just looks at the change in amplitude and phase and the trial weight amount (not even the location is really necessary).

Were you using all 4 channels (4 sensors)?

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OLi

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Reply with quote  #4 
If right and left in minimum 2-plane  was swapped it could make a mess out of it? Olov
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RustyCas

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Reply with quote  #5 
OLi, yes that I think would be a problem. I so seldom do 2-plane balancing that I didn think of that. It's not something we should have to worry about. I guess we have to assume there might be problems and thoroughly test every piece of equipment we buy, but we shouldn't need to.
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Walt Strong

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Reply with quote  #6 
Rusty,

"I guess we have to assume there might be problems and thoroughly test every piece of equipment we buy, but we shouldn't need to."
I do, since it is the best way for me to learn how to use an instrument new/used/rental, find out limitations, and verify that it functions for the needed job. Skipping this task can prove to be embarrassing or worse!

Walt
Danny Harvey

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Reply with quote  #7 
I'm not really sure of the origin of this mux. It may have been in what I bought directly from CSI or it may have been part of the equipment I got when I bought out another provider in 2011. Now that I think about it, it probably was since I never had that problem until a job around then.  Maybe it had been repaired incorrectly.  

One of my guys just found it and fixed it during a routine check of a bunch of adapters and it provided an explanation for the unresolved job from 2012 and another from earlier this year so there is some degree of uncertainty. The one from this year was one of those where everything we did resulted in what was the opposite effect. Don't recall any details other than extreme heat and frustration about the other.
Danny Harvey

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Reply with quote  #8 
Rusty,

Yes it was 2-plane and 4 channel.

I'm not sure about which channels were crossed.
Danny Harvey

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Reply with quote  #9 
Thinking back about when I bought that other business I recall getting a balance job and going out with the other guy to assess his skills. He told me sheepishly that he "hadn't had much luck balancing" as we prepped. Maybe the backwards mux was why but maybe not.

I watched him do the set-up and it was pretty obvious that he wasn't properly trained so I took over. I never balanced the fan because when we started it up I found that it was running backwards and had been for at least 17 years. He had "balanced" it several times during that time frame.  After a 6 hour wait reversing rotation, balance was not required so I never even got a reference run to verify that the signal was running speed and not 2/3 x from turbulence due to backwards flow.

It was perceived as magic and I a Wizard because I had fixed the shaky fan with only the wave of a wand though and that's certainly good for business!
RustyCas

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Reply with quote  #10 
I saw a reference to 2/3X somewhere else. Why does a fan produce 2/3X when running backwards? I've never heard that before, but have a number of documented cases of fans running backwards. So now I gotta dig up that data to see if there's a 2/3X. Is it dominant, or just noticeable?
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Danny Harvey

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Reply with quote  #11 
Rusty,

I can't say that running a fan backwards will cause 2/3 because I immediately had them shut this one down but it seems like it would almost certainly cause flow turbulence which I believe can show up at 2/3.
Mike C.

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Reply with quote  #12 
There is a reference in Art Crawfords book related to rotating stall in fans.  If I remember right the indicating frequency is 0.6X ts. With a fan running backwards it may produce the same flow characteristics as running a fan way back on its curve.
Danny Harvey

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Reply with quote  #13 
Mike,

I remember reading that but obviously not that well.  Thanks.
fredyarevalo

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Reply with quote  #14 
I believe the same occurred  to me this morning. I was trying to take cross phase reading to confirm misalignment and notice zero on Chanel B. when i switch input 2 for input 3 it worked and i finally had readings from both sensor. I m wondering if it is the same that happen to you!

i am using a 4 channel mux csi i beilive is a 648
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