Dan Truitt

This is a bizarre one... CompactLogix, 1769-IF8 with 1492-AIFM8-F-5 wiring system. My client has reported several instances where one of his flow rate displays is frozen at its maximum value.  Each time, I have gone to the site, found that the value in the analog input tag is at full scale, and measured more than 20mA at the input - as much as 28mA!  I measure it using a clamp-on meter and I measure it at the input module (1769-IF8) and the external, pre-wired terminal block (1492-AIFM8-F-5).  I cycle power to the PLC.  No change.  I cycle power on the transmitting device.  No change.  I disconnect and reconnect the input (signal wire at the module or the terminal block) and everything goes back to normal.  This has happened on multiple channels on multiple input modules and with two different types of transmitters. I didn't install this system, but it seems to have been done properly and it had been running fine for many years.  These problems just started happening out of the blue. Has anyone out there had any similar experience?  This is a new one for me after more than 25 years in the business.

Thanks to everyone for the additional input. So far, it has happened with three different transmitters.  Two Grundfos units that I believe are powering their own loops and one other simple, generic device that is getting it's power from the control panel.  I didn't see a nameplate on it to get the mfr/part number.  Like many of you, I had reached the conclusion that the Grundfos flow meter controllers were failing, but when a third device that is completely different failed with the exact same symptoms.  I thought that it was time to reach out to some other experts and see what you guys might have seen in the field. It's just so weird that we can get everything back to normal by simply disconnecting and reconnecting the signal wire.  Of course, that's not a practical solution for the long haul.  Especially since it is at an unmanned facility. Next time it happens, I'm going to take a harder look at the grounding and gather additional information like the exact make an model of the flow transmitters. Thanks again to everyone.  I promise to try to pay it forward!

Thanks for the ideas!  I did check the scaling and configuration of the IF8.  It's scaled for percentage and when the problem occurs, the channel goes to 105%+.  But that matches the current going into the module.  So, I'm pretty sure that this is a hardware issue.  This has happened with multiple transmitters (not at the same time) at different locations (literally opposite ends of the building).  So, I don't think EMF from another (new) device is to blame and I'm not aware of anything new.  As soon as we disconnect the wire at any point, the signal drops from 22mA+ to zero and everything goes back to normal, but I will try to get my clamp-on meter onto the wire at the transmitter next time. I don't recall seeing the shield wires at all, but I wasn't looking for them.  I'll check that next time as well. I'm not sure how this would result from the V+ dropping, but I'll think that through and try to find a means to detect if it does drop out momentarily. Everything is working fine today, but we expect that at some point over the next month, it will happen again.  There are about 16 flow meters of two different types and only one will fail.  It could be one of either type.  It could be one from any part of the building.  It could be one attached to one of three different analog input modules.  The only common denominator is the symptom - actual signal current going way above 20mA and freezing there until we physically break the loop. When it happens again, I'll check some of these ideas. Thanks again!