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mhowasso

MicroLogix 1400 with 1762-IT4

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Hi All, I am using a MicroLogix 1400 with a 1762-IT4 Thermocouple module with an industrial type K temp probe. My issue is if ANY portion of the probe itself touches the metal of the furnace the output fluctuates +/- 5 degrees. IF it does not touch any metal portion of the furnace (ie. laid on brick, wood, etc.) it is steady as a rock. We have isolated grounds, added grounds, taken grounds off, done the whole gounding thing, and nothing we do makes it any better or worse. Has anyone had an issue like this before..?? This happens on all four channels of the module and we have tried a new module and we get the same results. Any advice or ideas would be greatly appreciated. Thanks

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Does your thermocouple have a grounded junction? If so get one that is ungrounded. See link for more info on that. http://www.omega.com...rmocouples.html Edited by Mickey
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I am not familiar with your equipment, but it sounds like you are making the DC power source unhappy. Is it possible to reverse polarity of the sensor?

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The MicroLogix thermocouple input module is nonisolated, so if you use a grounded thermocouple or accidentally ground the junction of your thermocouple, you're pretty much guaranteed to have a noisy signal. Thermocouple voltages are very small; a 5 degree C variation is only 0.0002 volts. If you can't get a probe with a ceramic or other insulated jacket, I think your best option is to add a thermocouple transducer and take in the data over an ordinary 4-20 mA loop.
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Here are pictures of the probe. I dont see where I would gound it.

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By grounded thermocouple, Mickey and Ken are talking about the thermocouple tip itself. You want one that is ungrounded, and it is not possible to tell which type you have from a picture. It is likely that you can swap out just the probe in your housing to one that is ungrounded to solve the problem. Follow Mickey's link to the Omega site which gives a good explanation of the different types (bottom of the page has a nice illustration), and they are a great source for replacement parts. Edited by OkiePC

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Thanks guys... This makes sense. But I cant tell from looking at the probe tip either I guess. Im using a Watlow probe so I got a call in to them to find out. I did not know there was grounded and ungrounded probes. Thanks for the info. Mark

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I was told that this is an UNgrounded probe.

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Check it with a meter.

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I just did and they are deffinently NOT grounded. In fact I checked all eight and none of them are grounded. WATLOW PN: ARJEF0F180UK000 OMEGA PN: NB3-CAIN-14U-18 Any other ideas or things to try...??? Thanks, Mark Edited by mhowasso

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Ken, Is it possible to get the same accuracy using the 4-20ma converter..??? Any device mfg suggestions..?? Thanks, Mark

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If not ground issues, then perhaps induced noise on the T/C wire? Can you ensure that it is physically isolated from all power (especially VFD) cabling? The 4-20mA signal would be much more immune to noise and the resolution would depend on the transmitter, scaling and analog input card being used.

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This is a pretty simple application. Only about six feet of teflon coated type K wire from the probe going straight to my PLC. And there are no VFD's within 200 feet. Maybe the answer is to just make some sort of sleeve to keep the probe from touching. Thanks

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It appears from your picture the thermowell is screwed into a placstic coupling .Have you tried a metal couple?

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Yes, it is a PVC coupling. This seemed to help the situation. It seems the more I can isolate the probe from the metal of the furnace the better it works. I think if I can get a ceramic insert to slip the probe down thru this might just resolve the issue. But just so I am clear, from the posts above, I DO NOT want to try a Grounded probe..is this correct..??? Thanks, Mark

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I dont think you can reverse a thermocouple wire without adverse effects. And there is no DC supply involved with this. Thanks

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Ken, Paul, Micky... I noticed in the 1762-IT4 installation manual, it says to use twisted/shielded thermocouple wire. Have you had to use this before..?? Im just using normal TC wire. Just curious. Edited by mhowasso

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This usually helps reduce noise issues. If you have exhausted all efforts to reduce the noise you can always apply a filter, see text below Filtering.txt Edited by Mickey
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Not clear what you mean here..??

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Which part of my post is unclear?

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The filtering text. Do I use a CPT instruction to do this..?? F8:10 + F8:11 = F8:12 F8:10 + F8:11 (F8:12 - F8:10) Im not sure what you want me to do here.. Edited by mhowasso

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You can use a CPT instruction or math instructions, your choice. That's not the formula I posted, see below Filtered Value= Filtered Value+(Constant*(New Value-Filtered Value)) New Value = Unfiltered Value Constant= 0.00-1.00 The smaller the number the more damping. This will dampen or filter the noisy reading. Create a trend with the unfiltered value and the filtered value to compare the difference as you adjust the constant to get what you want. The 1762-IT4 module has different filter settings you might try and see if that helps first. Edited by Mickey

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I tried both the 50hz and 60hz filter setting on the module. I did not see any difference in my readings with either one.

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Why not try all the settings not just 50 and 60.

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Mickey, I tried all the values (10Hz, 50Hz, 60Hz...500Hz, 1Khz) and I cant see any difference in my count fluctuations. If it would not make you too mad, could you please show the filter formula in an actual CPT instruction. This is not soaking in with me for some reason. Thanks Edited by mhowasso

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