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John from PA

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Posts: 963
Reply with quote  #16 
Originally Posted by Rang
We are using Accelerometers on this site.

I'm well aware of that.  But OLi made the statement "My velocity probes go down to at least 1Hz but there rarely are any energy down there IMHO. Noise level from a copper coil suspended in air is seriously lower than a amplified and integrated pico coloumb signal also IMHO" and I believe he is referring to a moving coil velocity transducer.  Such a device eliminates the need to integrate as you are doing to get velocity from an accelerometer. 

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Posts: 1,917
Reply with quote  #17 
I was answering this at the same time
Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.

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Posts: 599
Reply with quote  #18 
I've got some questions to the setup here, but I feel their are some clues that I am not catching on to that answer most of these questions.

Is the assumption that these are loop powered self-integrating sensors that give an RMS/overall proportional 4-20mA signal?

Indeed very strange signals. It doesn't make sense with any sort of reasonable scaling to get values like that. Furthermore, with a normal ground loop, you expect to have a rather constant offset, but there is nothing constant or even periodic about this, assuming that they aren't messing with the system during those measurements.

Posts: 57
Reply with quote  #19 
Sorry, not loop powered. These are for portable collection only. I have found that the shield should be connected to the negative wire, will try this when next we have access.

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Posts: 1,810
Reply with quote  #20 
I have a couple of CTC 4-channel boxes that worked fine for a while, but then I started getting "Sensor not connected" errors or erratic data which looks like connection errors. I could press sideways on my cable connector, and usually get it to read. I tried a different cable because sometimes the ground tabs on the BNC connector break off or get weak and the ground connection is erratic. But a brand new cable did the same thing.

Looking closely at the jack (in the box ha) I could see that the socket (female) was too big. Called CTC then sent the boards (with the jacks) back. They "tested" them and said they were OK. I said no, send some new ones, and they did.,same problem soon after. I don't have a set of "pin gauges" and they're expensive, so I can't prove the sockets are the problem, but they are. I just this week replaced the jacks with Kings brand (far superior to anything I've seen). I suggested to CTC that they use Kings brand, but they don't. (And the BNC connectors on my cables still fail as well)

As for the "high reading", I just today replaced an IMI right-angle sensor that was consistently reading 0.26 ips when it should have read 0.05 which the one on the opposite side of the bearing did. I replaced it but saved it, so I'll test it tomorrow.

"The trend is your friend"

Sr. Member
Posts: 382
Reply with quote  #21 
I have have had the same issue with CTC 12 position boxes. I would get the sensor not connected error, but the cable and accelerometer would test fine. Sometimes when I reconnected them they would work again, sometimes not.

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Posts: 69
Reply with quote  #22 
I have been told that my site has had signal problems in the past (before me time) when collecting data from a switch box. Since we stopped connecting the shield (screen) to anything in the box we have had no problems. We get all of our boxes, wires and accelerometers from CTC.

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Posts: 88
Reply with quote  #23 
We are currently having a similar problem, readings say the sensor is not connected.  We turn off the fan and connect the sensor to a shaker and everything is fine.  We actually monitored it as the fan was coming on and it was fine as it came up to speed and then at some point it goes wonky on us.
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