It8 Thermocouple Problem

hello to all,i am parag uike from india working for rockwell automations.just facing a one problem of the IT8 thermocouple module.in which exactly what is happening is when i am connecting thermocouple to the channel of mine module it is not showing any temp. eventhough it is giving properly mv output. but after a some hours it starts to give a display properly. but instead of that thermocouple IF i connect the other thermocouple then it works finely. as well as when i connect the mv source to the channel then also it is showing properly.is there any problem of noise or EMI? COULD U PLZ TELL ME WHAT IS HAPPENING SIR . STILL I HAVENOT GOT A SOLUTION.system which i am using is devicenet adapter on rack2, where all thermocouple and rtd IR8 modules are connected. controller is flex logix.PLZ LET ME KNOW. also other problem is that as soon as u connect the thermocoule to channel its responce time is also low. its actual temp. comes after 8 OR 9 seconds. but for displaying it is ok. but our one pid controller is used through that thermocouple. will it malfuction during pid operation?
thanks in advance.
ur support is needed.
the game of plc
parag uike

What are you trying to measure temp on? A little more info on what you are working with will help to answer your questions.

If the thermocouple is sitting on an isolated/non-metalic table, do you still have the same issue? Wondering about a ground issue since it works with a mV source.

George

Here is a link to check out.http://www.omega.com/config/probeconfig.html
There are different typs of thermocouples. For example

1. grounded- these provide fast response times but they are very susceptible to ground loops and electrical noise.

2. ungrounded- slower response time but they have a better resistance to ground and also electrical noise.

3. exposed- are best used for gases or air as liquids tend to contaminate them.

Probably the first thing I would do is look at the application to determine the best type that you need, I would also test the one giving you trouble to see what type it is, because obviously you would not want to use a grounded probe for water if you are in a vessel that is conductive.

If I understand correctly, one of the thermocouples works fine when connected to the input channel and the other thermocouple does not. Based on what you've said, I don't think the problem is with the PLC module.

I don't know what style TC you have but if it is bare or a grounded junction thermocouple, you could be experiencing the effects of a "ground loop". Ground loops occur when there is a difference of potential between process equipment. The current will seek ground and may travel through process sensors like pH electrodes or thermocouples. It doesn't take much to affect an input that's expecting a millivolt signal.

If possible, swap process locations of the thermocouples and see if the problem follows the thermocouple or the process.

You could also have an induced voltage on the thermocouple extension wire. Is the wire lying beside any high voltage / high current conductors? Again, swapping the location of the thermocouples may help you determine where the real problem lies.

sir, its a temp of the furnace. in which 2 thermocouples are installed. one at start and one at the end of furnace.i am facing a problem is that when i replaced 2nd one by first one. its showing properly.and 2nd gets disturbed bcause of the 1st is connected to 1st channel.that is a problem.also abt shielding of the thermocouple showing a properly done.its a sulphur plant. sulphur is burning inside.
thanks for reply.
your ans. will be appreciated.
the game of plc
parag uike