1756-IT6 inaccuracy

phuz · Feb 25, 2019

Using a 1769-IT6 module with a Type K thermocouple, we're seeing a +7 degree C offset. We used a calibrated fluke tester and simulated both 0C and 100C and both were reading +7 degree C higher. The CJCs are installed properly. Any thoughts?

Bit_Bucket_07 · Feb 25, 2019

phuz said:
Using a 1769-IT6 module with a Type K thermocouple, we're seeing a +7 degree C offset. We used a calibrated fluke tester and simulated both 0C and 100C and both were reading +7 degree C higher. The CJCs are installed properly. Any thoughts?

See page 122 in the linked document: https://literature.rockwellautomation.com/idc/groups/literature/documents/um/1756-um009_-en-p.pdf

Cold Junction Offset Option

The Cold Junction Offset box on the Module Properties Configuration Tab lets you make module-wide adjustments to cold junction compensation values. If you know that your cold junction compensation values are consistently inaccurate by some level, for example, 1.2 °C (2.16 °F), you can type the value into the box to account for this inaccuracy.

According to the document, there is also a newer, more accurate TC Module available. The 1756-IT6I2.

phuz · Feb 25, 2019

1769, not 1756

James Mcquade · Feb 25, 2019

just a guess based on experience.

from the thermocouple to the plc input.
is the wire designed for thermocouples?
is it the right wire for the thermocouple you are using?
are there any splices in the wire?
are there any terminal blocks in the wire? - if yes, are they designed for thermocouples? had some that were incorrectly wired to a standard terminal block, the thermocouple terminal was the next one down.
are all the terminals correctly wired and seated? - I had some that were loose and also not connected correctly.
james

phuz · Feb 25, 2019

Yes, thermocouple wire. I'll have to check on the terminal blocks that are being used, but I believe they are the proper ones.

Sorry forgot to mention we simulated directly the terminal block, not from the field wire.

Lovell · Feb 25, 2019

You should try from an ice bath using the field sensor and all the wiring to check. this will verify all junctions, wire type, sensor accuracy, etc. Did you check all the input settings on the module? Sometimes the CJC's break (I have broken some while installing them)- are you showing a temperature of the module in the properties?

Try disconnecting all other channels for the test, maybe something is shorted or picking up induced current.

phuz · Feb 25, 2019

There aren't many settings/properties on the 1769-IT6. Not like the 1756 versions.
CJCs look good. This is a brand new system.
No other channels are currently used.

There's a technote on the 1769 version saying the IT6 should be at least two modules away from power supply, which it is.

Lovell · Feb 25, 2019

Do you have type K thermocouple wire handy? just cut off a couple inches, strip the ends, twist one end together and wire into one of the channels. Maybe something is off with the calibrator - what model of fluke calibrator is it?

BAJ · Feb 25, 2019

According to the document, there is also a newer, more accurate TC Module available. The 1756-IT6I2.

That's good, because the 1756-IT6I was a piece of ****. We had multiple failures across multiple systems 6-8 years ago, I won't use them anymore. We'd lose half the channels on the module in most cases, $1k+ to repair from AB as I recall. I installed a brand new system with three 1756-IT6Is and replaced four modules (yes, I replaced one twice).

phuz · Feb 25, 2019

Lovell said:
Do you have type K thermocouple wire handy? just cut off a couple inches, strip the ends, twist one end together and wire into one of the channels. Maybe something is off with the calibrator - what model of fluke calibrator is it?

Yeah, we do.
But the thermocouple itself was reading 7 degrees higher, and so was the calibrator. This is a very large company who calibrates their meters on the regular, so I have no doubt it's fine, especially after comparing to the actual thermocouple.

phuz · Feb 26, 2019

After further investigation this morning, the electrical outfit failed to use thermocouple terminal blocks and thermocouple wire to those blocks. When wiring directly to the card or simulating to the card, the reading is dead on. There you have it. Using the wrong components gave a +7 degree C reading inaccuracy. It pays to use the right parts.

James Mcquade · Feb 26, 2019

thanks for the follow up.

james

mylespetro · Feb 26, 2019

phuz said:
After further investigation this morning, the electrical outfit failed to use thermocouple terminal blocks and thermocouple wire to those blocks. When wiring directly to the card or simulating to the card, the reading is dead on. There you have it. Using the wrong components gave a +7 degree C reading inaccuracy. It pays to use the right parts.

I've seen TCs wired in the field with both ends of the wire terminated on the same side of a normal (non-TC) terminal block in a pinch. I know it's not ideal, but it may help while you wait for parts.

1756-IT6 inaccuracy

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