Type J Thermocouple readings are jumping.

[Andy Patel]

Hay everyone,

I am using a Micrologix 1400 with 1762SC-IF8 module to read temperature from different parts of a plastic extrusion line. I am using Type J thermocouples for this. The control system was already there and was using individual temperature controllers to control the temperature. My task was to replace it with a PLC and HMI, as I am doing it, my PVs from all the thermocouples are jumping like crazy. up to 20 degrees F plus and minus. which is giving me some hard time to control it.
Can anyone help?

Thank you.

 

[Holmux]

Hi Andy

There are a few things, actually if you search the forum, you will find lots of treads discussing this subject

Here is a quick fix, until you have time to go deeper:

My guess is that you large jumps comes from you module reading nothing from your probe, if your input range is from 6195 to 31104, try limiting your temp update, so your input needs to be above 6500.

If you add an update timer as well (2 sec) you will see a more smooth temp reading.

/Holmux

 

[Firejo]

The first thing you want to do is go to Spectrum Controls website, the product page for the 1762sc-IF8U and download the manual (if you don’t have one handy already). Section 2.6 talks about field wiring and has some things that need to be done to ensure an accurate and stable reading. Part of that section also talks bout noise prevention and the same thing holds true there. Noise is probably what you are seeing and is one of the biggest challenges when dealing with very low signal levels. Also try playing with the input filter setting. Sometimes you can find a “sweet spot” where the filter eliminates the “bounce” effect. Having said that, making sure the installation is good and follows the guidelines is very important regardless of what you can get by using a filter.

 

[Firejo]

I forgot to mention, Spectrum Controls I/O products are covered by Rockwell's Tech Connect however you don't need a contract with Rockwell to get the service (they use Spectrum's account). You have to make sure you call the right number thou, the general number doesn't get you to the right support group. You can all Spectrum's main number and ask for support on a 1762sc-IF8U and they will connect you or you can get the correct number to call on Spectrum's website (under "support")

 

[Old No. 7]

There are filter settings in the Spectrum card you can try adjusting, but the issue is almost certainly noise. How many t/c's do you have connected? If more than one, try disconnecting all but one and see if it goes away. Are you using grounded or ungrounded t/c's? Are you using shielded t/c wire? Do you have any "noisy" devices on/near the equipment - VFD's, SCR's, etc.?

 

[Andy Patel]

Thank you everyone for the instant replies, it looks like that we have a long extension cable from a panel near extruder to the main control panel. And that is not shielded, and that’s causing the problem. Right now management has decided to run the line without correcting the issue because 10-20 degrees F do not matter for a plastic extrusion line. To try all the tips you guys gave I need to wait until we stop the production on that line.