Thermocouple Scaling/Calibrating

[Wildesy]

Hi all,

Probably a bit out of my depth here as I'm not an instro but just wondering if someone could provide some clarity on calibrating/scaling the thermocouples a customer has on some equipment they've purchased overseas. It's a heating oven that typically operates in a 150-300°C range, it has have 4 Type-K thermocouples (-270 to 1370°C) wired back to a 1769-IT6 thermocouple module.

Currently, each input is just scaled in their absolute raw values, -32,767…32,767 scales to -270 to 1370. Putting a Fluke dry-well calibrator on each probe at 0°C and 300°C ends up in a pretty significant amount of error and difference in each probe (assume this is because probe readings aren't linear, although the IT6 thermocouple card might compensate for this and have formula-based scaling in it?).
It looks like a previous tech tried to correct errors in readings by making slight adjustments to the whole span, e.g. he changed the RawMin to -32500 to try and offset an inaccuracy they were getting when testing at 250°C, but I can't see how this was ever going to work properly.

Ultimately, I've got a couple of queries;

1) Should we just be scaling it within our expected range? This seems to make more sense to me considering we can't test the readings at the real absolute values of -270 and 1370. Should I just put the probe in the dry-well and find the reading at say, 0°C, set that point as the RawMin then check the probe at 300°C and set that analog reading as the RawMax? This then gives us a basis to work with and an easy point with which to make minor adjustments if scaling is out?

2) How do you calibrate the full span of a thermocouple without equipment that can actually test the full range? If you have a freezer at -40°C, is it actually necessary to ensure you can test it down at that range? Our dry-well can only cover from -15 to 350 to interested to see how it's done. Does it rely on multiple testing points in the range you can test and then constructing a coefficient to extrapolate it out?

3) Should I just be looking at installing probes/thermocouples that are more closely specced to the actual operating range of the oven?


Appreciate any insight people can provide. Just note, I acknowledge if we really want to complete the calibration properly I probably need to engage a specialist. Just for the basic setup and testing though to get it moving in the right direction I'm hoping somebody here can help out.

Thanks

 

[parky]

A couple of things, are the cold junction compensation thermistors connected or is there a faulty one (I believe there are two.
Also, the lower temperature seems wrong I think it should be -230 Deg. C for a K type.
Perhaps look at the manual

https://literature.rockwellautomation.com/idc/groups/literature/documents/um/1769-um004_-en-p.pdf

 

[Rson]

The very first thing to check is that when they wired it to the panel they used Thermocouple wire. If they landed on a terminal block, it must be a thermocouple terminal block and wire back to the PLC.

I spent a day going through a similar situation only to find out the electrician used regular AWG wire at the thermocouple.

Additionally, there are two CJC cables that need to be installed. I've seen people remove them in the past.

 

[danw]

Caveat - I've never used A-B components, but I've done a lot of thermocouple work.

First question relates to the perceived problem: what is does "pretty significant amount of error and difference in each probe" mean? Please explain with numbers. Does it mean that with the elements in a dry block calibrator stabilized at 250°C that the Module reads 249.9°C or 278.7°C? Or are you reading mV with DVM and from a probe in dry block? Or something else? What is the test setup and what are the numerical results?

Given the propensity to miscalculate engineering units of degrees from raw counts, why are you using raw counts when engineering units are available?

A skim review of the IT-6 user manual shows that it can have an auto-calibration enabled that performs on a periodic basis. Is auto calibration turned on?

Why do you think the thermocouples are Type K and not some other Type? Color codes vary by country.

T/C Factoids:

1. Thermocouples are non-linear and become more non-linear (drift) with exposure to high temperature. Any reasonably competitive commercial thermocouple module 'compensates' for the base non-linearity, typically with a polynomial calculation. I assume A-B has a competitive T/C module design, and their user manual reflects that.

2. Cold Junction (CJ) compensation is mandatory. Lack of cold junction is evident as a lower than expected reading (by the difference between the terminal block temp in Deg C and 0 Deg C), because the cold junction temperature measurement of the terminal block temperature has not been added to the thermocouple gradient temperature to produce a reported temperature value.

It is not clear from a skim read of the manual whether 'raw counts' includes CJ or not. It could be the A/D output of the gradient mV only because that's what the words "raw count" mean. But A-B might put their own spin on "raw count". When values get converted to Engineering units, like Deg C, it is generally assumed that the CJ is taken into account, although local CJ can frequently be disabled for various reasons or dysfunctional (removed/disconnected).

3. Thermocouple measurements REQUIRE the use of the same type extension wire as the Type thermocouple. The use of copper wire or the wrong extension/lead wire will produce both a fixed error and errors that change with ambient temperature.

4. MgO sheathed thermocouple assemblies with some extension/lead wire are not very expensive. If there is any question about the health of the sensors, replace them with new assemblies, Pay the $2 premium and get the limit-of-error option (halves the inherent error)


Experience
In my experience, the results from efforts to improve 'accuracy', with the exception of an offset value added across the range, produce worse results than the original values. How hard is it to just read eng units without any 'adjustments'?

Expectation
Calibrated accuracy for a Type K is ±1.5 Deg C. How much worse than that are the readings at 25°C, 150°C and 300°C, that cover the range of operation?

 

[lfe]

Note that a thermocouple does not generate a signal equivalent to the absolute temperature in the hot junction, what it measures is the difference between the hot junction and the cold junction.

The cold junction in your case is the terminals of the PLC module. As the module is not really at 0ºC there is a thermistor that measures the temperature at the terminals and compensates for it.

To have continuity of the thermocouple wires, it is essential to use a specific compensation cable for the type of thermocouple from the probe to the module. If connectors are to be used on the cable, they must also be of the appropriate type. Using normal copper wires creates an error.

Using the Fluke calibration thing, you must also mount the thermocouple and wiring as described.

 

[forqnc]

I may be stating the obvious, but when I use the 1762-IT4 modules I set them up for "Engineering Units" not Raw/Proportional. This gives a direct temp reading. You don't have to mess about with scaling.
Only errors I get doing this, as stated above, is when the installers cut corners and use regular wire and not thermocouple wire.

 

[mylespetro]

Also want to raise the flag about the CJC jumpers on the card, if they're not installed or not installed correctly it will not function correctly with thermocouples. A quick Google has shown me that these cards come with the CJC jumpers pre-wired, but something to keep in mind. This bit me on a ControlLogix RTD/thermocouple card once and now I'll never forget it.

 

[lfe]

I apologize for bumping this thread up, but I came across it in a google search and it's relative to the information I'm looking for and having a hard time finding elsewhere. Any advice or help is greatly appreciated.

This is about thermocouples; not PLC's. I am developing a loudspeaker and am trying to integrate basic type k thermocouples into my voice coils to directly measure coil temperature, and I am having trouble finding resources about connections for it. I am looking for an alternative way to connect the thermocouple as I think using a thermocouple wire will fatigue after some time with all of the speaker's excursion. So I have been looking at using a spider with integrated tinsel leads to bridge the connection from the thermocouple at the voice coil to the connector on the basket since tinsel leads are flexible. These tinsel leads are made from copper, but I am reading that copper is not suitable as an extension wire for thermocouples.

However, does anybody know if there is a way to calibrate a thermocouple using these tinsel wires as extentions from 0-250c? I want to say that I've heard about 3d printers having a way to calibrate the thermocouples in the hot end as they use awg copper wire to connect it to the control box. The accuracy in temp readings for this speaker project is not that critical, +/- 5 degrees is more than plenty. I will be using a max6675 amplifier with an arduino nano to log and display the temps from the thermocouple.

Any advice or help would be much appreciated!

The extension wires must go continuously from the thermocouple to the instrument and, in addition, the instrument must have ambient temperature compensation. If this is not fulfilled, the measurement will not be correct, especially at low temperatures.

The K thermocouple is suitable for temperatures up to 1200ºC but it is not suitable for low temperatures, that is less than 400ºC.

I recommend that you use a PT100 thermistor, it is more precise at low temperatures, it measures the absolute temperature and you can use normal copper wires; but choose a sensor with 3 or 4 wires. the extra wires are used to nullify the error due to the resistance of the wires.

 

[5618]

Two dissimilar metals generate a tiny voltage at a junction. A type K thermocouple is a junction of two alloys: alumel and chromel. These have a predictable and repeatable voltage that varies by temperature. The correct thermocouple cable is these two metals so that the only dissimilar junction is at the point of measurement. Terminals in the path should be made for the type of thermocouple used and made of the same alloys.

Panel meters used for thermocouples might not have terminals of the matching alloys, but compensation is built in to the meter. If you were to use copper in the path, sometimes it might not be too bad. The alumel - copper junction will create a voltage, but that would be offset by the opposite voltage generated at the reverse copper - alumel junction. Same with the chromel side. However, it seems like the coil end will be warmer than the frame end. I have no idea how much the offset voltages would vary with each end of the copper at different temperatures. Theoretically you could add another sensor near the frame junction and do some compensation, but that's getting somewhat crazy.

Another problem is how will you connect the copper to the thermocouple wires? If you solder them, now you're introducing two dissimilar junctions at each end from copper to solder and solder to chromel. (Unless the copper and chromel are solidly touching inside the solder) I don't know how to best bond that to withstand the vibration of a speaker. Maybe capacitor discharge or tig welding?

A thermistor or RTD is a solution to eliminate the dissimilar metals problem, but then you have some mass and volume of a sensor to deal with. A stationary infrared temperature sensor pointing at the coil would probably be my choice for this.

 

[JTCat]

Different metal junctions change the Seebeck coefficient / voltage measurement.
Calibration is simple, and is usually done in two or three points, one for low, medium ,and high end. Calibration of the entire span is not necessary. You are only concerned about the expected operating range of the device and should calibrate for that range.
You can use extension wire as long as the junctions are of the same metal and alloy composition, and they should be welded together, not twisted, not soldered, not wire nutted or taped. They must be welded in order to maintain the correct Seebeck voltage, which is what anything that takes a reading from a thermocouple looks for. Different metal junctions produce this voltage, which is why thermocouples have two different metals welded together at the probe end. If you need some more direct advice, feel free to send me a PM. I spent a lot of years in metrology for pharmaceutical companies.

 

[Steve Bailey]

@hurrication, if your goal is to simply prevent the voice coil from overheating, a thermistor might be easier to implement than a thermogouple.

 

[parky]

Hm.. A thermocouple giving mv on a coil where a ac voltage (switching DC i.e. Audio) and a magnet that could reach 100v pp what about the induced magnetic currents flowing round that, never tried it but there must be quite a bit of noise generated, are you sure you will get accurate readings.