Thermocouple Fluctuating readback

Hey all new to the forum so bare with me. I am having some issues with some thermocouple readings on some elements which are meant to be running at 245 degrees celcius. The issue is that every now and then i am getting a reading flick up of either 20 or so degrees above or below the actual temperature. (only for a second or so)). The Thermocouples are connected to a Allen Bradley 1794-IT8 Flex I/O which is communicating to a PLC5/80. I have replaceed the flex I/O and checked the cold junction compensator connections and all other connections, still no good. Any Help on possible causes for this would be greatly appreciated. Probably something basic but my knowledge is only basic.

thanks

Welcome to the forum..Jayden....."Bare with me.."..i am sure you meant "bear"..anyway.

Looks like interference to me..

do you have any Variable speed drives output cables or heavy current carrying coductors near your thermocouple wire..a good 300 mm distance is recommended between them...also ground the screen of the thermocouple cable at the panel end.
If it is the variable speed drive which is causing it then you will notice a substantial rise in interference during start/stop of the driven motor.

http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1794-in021_-en-p.pdf

FLEX I/O Thermocouple Input Analog Module and RTD Module
[font=KOJEO H+ Univers,Univers]Cat. No. 1794-IT8 and 1794-IR8[/font]
[font=KOJEO H+ Univers,Univers]
[/font]pg 2 lower left warning:



If you are using grounded thermocouples the shield connection might be different than for ungrounded thermocouples:



The output map indicates a filter setting, but the note indicates a Fixed Digital Filter (FDF) for thermocouples. This deserves some investigation because filtering affects noise.



variable scan rate might mean a variable filter rate


The spec on page 5 says to use shielded thermocouple wire:



Dan

Note that the default condition of the filter is disabled ! !

The table from the 1794-IT8 User's Manual

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1794-um007_-en-p.pdf

shows the filter frequencies when enabled.



Dan

I've seen a simular situation if the unused T/C inputs (analog)are not referenced to the ground/reference terminal...

I agree about unused thermocouple inputs. It is best to short them all out + to -.

I have seen the problem described where the thermocouples were also wired in parallel to a recorder. Some devices including recorders do horrible things to the signal in the name of detecting bad thermocouples. Sometimes if this is the problem you can disable such facilities or else disconnect from the troublesome device.

Hi Jayden


If possibble try a new thermocouple. I have had a similar problem before and replacing sorted the problem out.

I also live in Camperdown, only in South Africa

Brian

John Gaunt said:

I have seen the problem described where the thermocouples were also wired in parallel to a recorder. Some devices including recorders do horrible things to the signal in the name of detecting bad thermocouples. Sometimes if this is the problem you can disable such facilities or else disconnect from the troublesome device.

Click to expand...

If you trend the T/C signal normally you would see a pattern when a second device tests the T/C. As long as you don't have a motor on a timed cycle causing the interference. Is the T/C junction grounded or ungrounded? Is it opposite of what it is suppose to be?

John Soltesz said:

Is the T/C junction grounded or ungrounded? Is it opposite of what it is suppose to be?

Click to expand...

Hello John, when talking about grounded T/C, do you mean that measuring point (hot end) is short circuited to the metal body of T/C?
I've noticed that there are two types of T/C, one type hase short circuited measuring top to the metal body, while other has measuring top (point) isolated.

In some cases where there was short term noise we put a capacitor (220 uF) across the T/C leads. This reduces noise, but note it will also reduce response time.

Yes, grounded thermocouple elements are electrically connected (shorted) to the sheath.

Ungrounded elements are isolated from the sheath.



http://books.google.com/books?id=6pSXSvGwOqAC&pg=PA682&lpg=PA682&dq=construction+of+grounded+thermocouple&source=web&ots=DgYeYi-Xg2&sig=F5XtQiljrqP0zpXWTNzuoFhTm8o#PPA682,M1

Dan

mikas_m said:

Hello John, when talking about grounded T/C, do you mean that measuring point (hot end) is short circuited to the metal body of T/C?
I've noticed that there are two types of T/C, one type hase short circuited measuring top to the metal body, while other has measuring top (point) isolated.

Click to expand...

Yes, the hot junction. Some sheathed T/C's are gounded, that can give faster response. All that we ever used are ungrounded. If the T/C is failing and starts to short to the sheath and ground you can get some odd signals.
Is this, the noise spikes, something new to the process? Can you monitor when the elements are powered and compare that to the T/C spikes? Are the elements controlled by the PLC? Can you use the trending in RSLogix and monitor those signals, the T/C and command reference for the elements. Element amps would be better.

Jayden,

If these are new thermocouples, then be sure to address the issues that have been brought up, interference, noise, filtering, shielding, and so on.

If it is some thermocouples that have been operating okay in the past, and suddenly have developed these spikes, then look for what has changed. Often, as Briancr implied, it means that the thermocouple is failing, either shorting to ground or the junction is opening up, or a lead is broken but still touching, but sometimes moves and opens the circuit for a split second. T/C failure happens often, as you are dealing with a device that operates in hot conditions. I have had such spikes and erronous readings on a T/C, and go and check it (after it has cooled) and find it good, but the next time it is up to operating termperatue, the same problems appear. High temperatures can make a BAD T/C read intermittently and incorrectly, even though it reads okay at room termperature.

A sudden change in the T/C readings could also mean that some new equipment has been added, or old equipment has been moved or re-wired, so that some new noise or spikes are now being induced on the T/C leads.

Flex I/O TC Input readings.

From my recent experience......

Is there also a possibility that "open thermocouple detection" is causing an intermittent reading to be incorrect?

Also if a ground is detected (such as an intermittent short in an Analog Input) in the Flex I/O Drop, all grounded TC inputs will have "improper" readings, while the Ungrounded TC inouts will read normal.

....webranger

If the flex I/O is powered from another 24v psu make sure the 0v is grounded, had this once it even foxed AB.