One who is continually learning is now on another quest for the knowledge of the assembled gurus of The Cove. This young fool (well, middle-aged at least) rashly overturned a long-dormant can of snakes, and I am now seeking a policy useful to return them to a state of control. Then I can retreat to the safety of the electronic calibration lab and hope all of the production managers forget who I am ...

The problem is temperature measurements in a number of repair, rebuilding or low-volume remanufacturing processes. For example, one process is something to do with curing adhesives in composite material lay-ups. Temperature is an important parameter, often a critical one. (I don't actually know much about this process or any of the other problem processes.) The operators monitor temperatures with an array of thermocouples placed on the work and connected to digital thermometers. Some thermocouples are purchased, but most are manufactured in the shop as needed, from bulk thermocouple wire. They are replaced when they "wear out" or if the readings "look funny".

The digital thermometers are calibrated. The thermocouples are not calibrated.

I did a back-of-the-envelope MSA and discovered that, using the standard conformance specification for the thermocouple type, their measurement system uncertainty is way more than what is specified in the process instructions. But, "that's the way we've always done it". :mad:

(Before you ask, this is NOT an ISO, QS, AS or any other type of 9001 organization!) (Except for their electronic calibration lab! :bigwave: )

I would like to discover some policy examples y'all may be able to share covering (a) local manufacture of thermocouples by the shop that uses them; and (b) calibration of thermocouples and other temperature sensors. Here are my initial thoughts for your gracious nit-picking.

( **** Definition: Ice Point = the freezing point of pure water at "normal" atmospheric pressure, defined as 0.0 degrees Celsius.)

A: Local fabrication of thermocouples is acceptable provided the performance is correctly verified before use. This means the thermocouple output should be measured at the ice point and at the expected working temperature of the process. The thermocouple should be tagged with the date is was made, the lot number of the wire spool it was made from, and the measurements recorded at the verification temperatures. If needed, the tag should have room to tally the number of times the thermocouple has been used, and the maximum temperature it has been exposed to.

B: Other types of thermocouples and temperature sensors should be regularly calibrated. Typical instruments include but are not limited to Thermocouples, either open-junction or sheathed probes Thermistor or PRT probes Liquid-in-glass thermometers Bimetal thermometers
If thermocouples are mounted in process equipment they must be calibrated in place. Sensors that are not permanently mounted should be calibrated in the calibration lab on a regular basis. They should be calibrated at the ice point, the fixed point closest to the lower of the highest process temperature or the maximum usable temperature, and at other fixed points sufficient to verify the linearity.


Let the learning begin! (Thanks in advance.)

It's been years since I worked in a lab environment but as I remember we never calibrated thermocouples - we calibrated the sensor/indicator at specific voltage inputs. Each type of thermocouple wire had a different voltage range so that had to be accounted for.

Jerry may be able to shed some light on this.

Marc,

I have seen both calibrated and uncalibrated thermocouples used, but most organinzations I have been with or visited require thermocouples to be calibrated if they are used in a process that determines product quality. But then, most of those organizations have been military, or defense contractors.

Here is an example of why it may be a problem if an uncalibrated thermocouple is used.
The type conformance specification for a standard Type-J thermocouple (iron vs copper-nickel) is an accuracy of +/- 2.2 deg.C or 0.75% of the output, whichever is larger.
The performance specification for a typical digital thermometer in wide use (Fluke 51 or 52, for example) is +/-(0.1% of reading + 0.8 deg.C). It has a best resolution of 0.1 degree.
Since these values are obtained independently and are essentially random (any thermocouple can be connected to any meter) the values can be combined by the RSS method.
Axiom: The total uncertainty can never be smaller than the largest component.
Even though the digital thermometer can display to 1/10 degree, the system accuracy can never be less than +/- 2.3 deg.C when the numbers above are combined. (If you do a worst-case combination, it is +/- 3.0 degrees!)
Even if the digital thermometer is the fabled "perfect" one with zero uncertainty, the system uncertainty will still be +/- 2.2 deg.C unless the thermocouple is calibrated.
An inadeqautely trained operator will usually accept the thermometer display as "truth" and use that to control or monitor the process. This can result in an in-tolerance measurement display but an actual process temperature that is out of tolerance by as much as 2.2 degrees.


The meters are calibrated as you describe -- we do a lot of them. My concern is that the production shops are taking a calibrated high-accuracy meter, connecting it to an uncalibrated, relatively coarse sensor, and believing they are making quality measurements. In fact, in an effort to "improve the quality" some of them recently got a large batch of new meters with accuracy specifications of +/-(0.05% of reading + 0.3 deg.C)! I have plotted all of this several ways in Excel, and with an uncalibrated thermocouple this new meter makes virtually Zero difference. (The difference it does make is in the calibration lab! It takes 4 to 6 hours to calibrate the new ones manually, using our high-end DC standard (Datron 4808). The less accurate ones can be done in 30 minutes on our automated system using a Wavetek 9100.)

A thermocouple can easily be calibrated to a k=3 accuracy of +/-0.5 deg.C at the ice point and the process temperature. Combining that with the Fluke 51, the system uncertainty is now +/-0.9 deg.C -- a major improvement, and within the process requirements.

I need to beat them with the math (assuming my reasoning is correct) but I also want to present a viable solution that will make everyone happy. (Or at least equally miserable?) It's much better to tell the bosses "you have a problem and here's how to fix it" than to just slap them with the problem & leave! Thus the quest for knowledge.

Graeme

You're way ahead of me! I'll have to sit back and see if there are any other experts lurking about.

Our calibration policy is only to calibrate measuring equipment that determines the conformity of the product.

We have thermocouples in our injection moulding machines, but these are not calibrated because we determine the conformity of the product at subsequent in-process inspection stages.

If we relied on temperature as an important process parameter that was critical to product quality, and we could not subsequently verify the product (i.e. a special process) then we would need calibrated thermocouples.

You might compare this to the lead screws and dials and/or NC code on a lathe - should these be calibrated ? No, because we verify the product that comes off the lathe using other calibrated equipment.

We do have calibrated thermocouples in our test lab, which is ISO17025 approved, as clearly temperature is an important parameter in environmental testing.

So you need to look at the application of the thermocouple.

Hope this helps.

Not too long ago we had an adhesive curing operation that was mildly temperature sensitive. Full cure could be achieved, for example, with 1 hour at 100 C, or 40 minutes at 120 C, or 20 minutes at 140 C, with a max. allowed temp of 150 C. precise temp measurement was not needed, but it had to be "in the ballpark". We decided to calibrate the thermocouple and instrument for this particular application together by using boiling DI water and looking for a readout of 100 +/- 2 degrees C. It was a fast, easy way to do an in-house cal that was suitable for this application. Once this was done, we cured some parts at 140 C for 20, 30, and 40 minutes and did tensile testing of the bond. Strength was the same on all 3 sets so we verified that the total process was working. To allow some room for Murphy, we set the process at 140 C for 30 minutes and every 3 months we re-calibrated the unit/thermocouple and watched the process output for anything strance and had no problems. Just remember, the clock starts once the bond line reaches temp, not when the oven reaches that temp.

Mike S.

I just found out my wifef's car died, and so I am headed out. Not to mention there are a lot of replies already.

This happens to be one of my strong areas. I'll give some preliminaries for the moment... more later.

We calibrate all thermocouples. If you're using them in a non-quantitative diagnostic mode, you can probably get by without cal. But we use them in numerous applications from 50 deg C to about 1200 deg C. And they do have errors, and at higher temperatures they do degrade.

The typical low temp problems are the impurities in connector materials (which cause an induced secondary junction of unknown error). This needs to be compensated for with a good zero reference. If you use detachable thermocouples in handheld meters (such as Fluke 51/52), a good zero in crushed ice is important. Gain and linearity checks are important as well. We've had numerous applications with type T and K in small oven chambers with as much as 1 to 2 degrees error on a new TC at temps around 150 to 250 C. Type K has a nasty habit of changing significantly at certain temps, and has to be rejuvenated through an anneal process.

High temps with type R and S (I won't even talk about B - they need to be removed from existence) cause outgassing of the Rhodium and subsequent emf degradation, and also causes precipitation of oxides. This causes the linearity and gain to drift.

Say Graeme,

You are correct, the navy uses the standard thermocouple tolerances and adds them to the tolerance of the meters in all their procedures where a thermocouple is HOT checked with a meter. You can find the wire uncertainty in the Omega Temperature Catalog. 2.2 deg C for Type J sounds right. :biglaugh:

We use thermolcouples in our lab for monitoring our ovens and water baths and freezers. They are attached to a yearly calibrated Omega monogram. Any suggestions as to validity of these wires? They were purchased certified from Omega.
Also we use a Data Paq to measure our cure and bake ovens for paint. If anyone knows of an umcomplicated way of verifying them please advise.:ca:

I could use some help on calibration of a temperature controller. I've always had an established Calibration dept to handle thermocouple and controller calibrations but now I"m faced with preparing a procedure for an new company.

I know that I can input a source to controller as for the monitoring thermocouple to check that portion, but to check the output to the controlling thermocouple; whats the best approach? Does anyone have an old procedure that they have used that could be shared? :confused:

As to thermocouples: in the past we would purchase a "calibrated" thermocouple and maintain it. We would used this to make a "Ref" calibration thermocouple that was checked against the purchased unit and use the Rerference unit for calibrating operating thermocouples. This prevented the degradation of the purchased calibrated unit.

hello GW! Welcome to the Cove!

I'll start off with some questions.

1. What is your accuracy requirements for the equipment?
2. Is this something this company has been doing in the past, or is it new?
3. Are the thermocouples as such they can be removed, while still being attached to the controller/indicator?
4. What kind of calibration standards do you currently have?

You can simulate a standard signal into the controller. Then, purchase/calibrate thermocouples separately. You can calibrate them as a loop in a dry block setup. Or, you can perform a direct comparison with a standard probe.

There are a number of different scenarios that can play out, depending on your requirement, your application, and your capability.

There are a number of methods for thermocouple calibrations. Timne doesn't permit me to go into too much detail. But here are a few brief notes:

CALIBRATING A CONTROLLER
METHOD A. Using a thermocouple calibrator which simulates the millivolts and with the correct corresponding cold junction compensation (for the particular thermocouple type; such as J, K, T, E, etc..), generate millivolts into the controller. This verifies the accuracy of the controller but not the control thermocouple attached to it.

METHOD B. Using a certified thermocouple with certified meter, place the certified thermocouple next to the thermocouple under test in the oven/chamber (or what ever is being controlled). Set the chamber to at least three temperatures, and allow adequate time for it to stabilize. Verify the controller/thermocouple under test measured chamber temperature matches the certified meter/thermocouple to within acceptable tolerances (not as accurate a method).

METHOD C. Calibrate the controller as in Method A above, then remove the thermocouple under test, and verify it's accuracy separately using a certified temp meter and a drywell calibrator.

These are just three simplified methods. There are numerous other methods. I have been calibrating thermocouples and temperature controllers for many years, and there are methods which may be considered based on the various specifics of your requirements and manufacturer/models of temp controller/thermocouples.

I am accustomed to only having the measuring instruments calibrated. However, strict guidelines from your accreditation agency or National Certification Body (NCB) can dictate the size & type of thermocouple wire to use, its preparation, as well as how it is to be attached to the test sample.

The NCB's I have dealt with over the years have accepted information provided by the IECEE / CTL (Committee of Testing Laboratories of the IECEE). http://www.iecee.org/ctl/Default.htm

There are some Operational Procedures (OP) that can be referenced in your own lab's procedures as to be followed.

For example: CTL-OP 108 and CTL-OP 109 (attached) are excellent procedures to be adopted when working with thermocouples, and your accreditation agency should have no problem accepting these methods.

This may be a little off the subject matter, but maybe it can be helpful to someone.

-D_Wood-

P.S. I came across another document more in line with your question.
Attached is document EAL-G31, Calibration of Thermocouples.

-D_Wood-

I have used the same method as Jerry stated. Our Omega Thermomters are calibrated, then we verifiythe thermocouples using a Hart Dry Block.

Can a thermocouple be calibrated (adjusted), or can it only be verified (meets the specification)?

Phil

Brad,

1) Accuracy: +/-2C is sufficient
2) This is new for this company.
3) Yes, we could remove the thermocouples and validate separately
we have purchased a calibration unit but not a calibrated thermocouple. The thermocouples are new (<1yr) but the controllers are older models so my immediate interest are these. We will eventually purchase a calibrated TC and use as I outlined above.

Brad,

1) Accuracy: +/-2C is sufficient
2) This is new for this company.
3) Yes, we could remove the thermocouples and validate separately
we have purchased a calibration unit but not a calibrated thermocouple. The thermocouples are new (<1yr) but the controllers are older models so my immediate interest are these. We will eventually purchase a calibrated TC and use as I outlined above.

Jerry,

Thanks... this will be a great help to get started... let me absorb everything and I may get back to you

Thanks D.... I appreciate the help... I will read through these Documents.

[COLOR=indigo]
One who is continually learning is now on another quest for the knowledge of the assembled gurus of The Cove. This young fool (well, middle-aged at least) rashly overturned a long-dormant can of snakes, and I am now seeking a policy useful to return them to a state of control. (Thanks in advance.)

My dear friend.....once they are out, they don't go back in the same size can!!!!!

Seriously, we had a similar issue at my previous position with respect to glue.....our solution was fairly simple, as we had an - essentially - controlled environment.....we had a chart that outlined the time to cure (X-axis) to Temperature (Y-axis) which was sufficient for our use.....that being aluminum tables mounted on shock absorbers.....

Hopefully, this gives you at least a starting point........

Hershal

Marc,

I have seen both calibrated and uncalibrated thermocouples used, but most organinzations I have been with or visited require thermocouples to be calibrated if they are used in a process that determines product quality. But then, most of those organizations have been military, or defense contractors.

Here is an example of why it may be a problem if an uncalibrated thermocouple is used.
The type conformance specification for a standard Type-J thermocouple (iron vs copper-nickel) is an accuracy of +/- 2.2 deg.C or 0.75% of the output, whichever is larger.
The performance specification for a typical digital thermometer in wide use (Fluke 51 or 52, for example) is +/-(0.1% of reading + 0.8 deg.C). It has a best resolution of 0.1 degree.
Since these values are obtained independently and are essentially random (any thermocouple can be connected to any meter) the values can be combined by the RSS method.
Axiom: The total uncertainty can never be smaller than the largest component.
Even though the digital thermometer can display to 1/10 degree, the system accuracy can never be less than +/- 2.3 deg.C when the numbers above are combined. (If you do a worst-case combination, it is +/- 3.0 degrees!)
Even if the digital thermometer is the fabled "perfect" one with zero uncertainty, the system uncertainty will still be +/- 2.2 deg.C unless the thermocouple is calibrated.
An inadeqautely trained operator will usually accept the thermometer display as "truth" and use that to control or monitor the process. This can result in an in-tolerance measurement display but an actual process temperature that is out of tolerance by as much as 2.2 degrees.


The meters are calibrated as you describe -- we do a lot of them. My concern is that the production shops are taking a calibrated high-accuracy meter, connecting it to an uncalibrated, relatively coarse sensor, and believing they are making quality measurements. In fact, in an effort to "improve the quality" some of them recently got a large batch of new meters with accuracy specifications of +/-(0.05% of reading + 0.3 deg.C)! I have plotted all of this several ways in Excel, and with an uncalibrated thermocouple this new meter makes virtually Zero difference. (The difference it does make is in the calibration lab! It takes 4 to 6 hours to calibrate the new ones manually, using our high-end DC standard (Datron 4808). The less accurate ones can be done in 30 minutes on our automated system using a Wavetek 9100.)

A thermocouple can easily be calibrated to a k=3 accuracy of +/-0.5 deg.C at the ice point and the process temperature. Combining that with the Fluke 51, the system uncertainty is now +/-0.9 deg.C -- a major improvement, and within the process requirements.

I need to beat them with the math (assuming my reasoning is correct) but I also want to present a viable solution that will make everyone happy. (Or at least equally miserable?) It's much better to tell the bosses "you have a problem and here's how to fix it" than to just slap them with the problem & leave! Thus the quest for knowledge.

Graeme

The other thought here is the calibration of the thermocouples.....in adherence to ANSI/NCSL Z540-1-1994 Paragraph 10.2.b, the 4:1 rule applies and so a hand-held Fluke thermocouple meter is an adequate instrument.....I suspect this may also be supported by ANSI/NCSL Z540.3-2006 though I will have to find the applicable clause.....

In reality, the best concept is to characterize thermocouples, not calibrate them, and then track their drift which can - depending on a number of variables - be substanstial.....

And hence, the uncertainties can also be substantial......

Hershal

GW:

Sorry, I guess I am more questions than answers. :)

It sounds like you have a fundamental knowledge of thermocouples and their operation.

When you say "old", are you talking older digital, or the ooold analog controllers (lots of Honeywell ones still out there)? If they are non-digital, I would get them out of there. The new digital ones are so accurate, and also have PID algorithms for assistance of tuning.

I guess I am still wondering about your application. If it is a harsh or high temperature application, I might stick with an approach of purchasing calibrated thermocouples and replacing them. Still, I would be interested in obtaining some level of confidence with the old thermocouples you pull out.

Or you can get a dry block furnace and verify them as a loop. Keep some new replacement thermcouples on the shelf should you find one acting strangely.

What is your maximum temperature? If it is not too high, you could purchase an RTD temperature standard fairly reasonably. If you could swing it, get two so you can offset them.

For the temperature controllers, you could purchase an Altek simulator for $600 or so to simulate to the controllers.

Just one method:
1. Calibrate the controller first.
2. Calibrate the thermocouple.
3. Run a verification of the loop.

+/2C is fairly tight with a thermocouple loop. It will require Special Limits of Error wire, minimally.

:2cents:

Hi everyone,:thanks:
Thank you very much for sharing a policy on the calibration of Thermocouples, because this could be one of our weakness in calibration of equipment.
Thanks again.
Best regards,
raffy :cool:

Brad,

I probably have enough understanding to get me in trouble; :)

The application is the sintering of ceramics. These are fired in an air atmoshpere at temperatures between 1250-1375C.

The controllers are digital but of the 80's vintage (remember Microstar's?). Granted the new vintages controllers offer quite a bit of updates and refinement of controls but these work well and I have plenty of backup and source of repair (and I'm lazy and don't want to retrain or confuse operators).

The Tech purchased a Fluke 714 as the calibration unit. It looks adequate.

High temps with type R and S (I won't even talk about B - they need to be removed from existence) cause outgassing of the Rhodium and subsequent emf degradation, and also causes precipitation of oxides. This causes the linearity and gain to drift.

can you talk about type B for me?

I'd like to get a better understanding about what you mean exactly and the implications of its usage in the glass melting (combustion) markets

thanks,

gd

Types B, R and S thermocouples are considered the "Noble Metal" thermocouples - they are made from different combinations of Platinum and Rhodium. Following are the metal compositions of all three types:

Type B = Platinum with 30% Rhodium vs. Platinum with 6% Rhodium
Type R = Platinum with 13% Rhodium vs. Platinum
Type S = Platinum with 10% Rhodium vs. Platinum

So..... let me get myself into some trouble here. In numerous papers I've read over the years, along with some experimentation work I have personally done, it has become apparent (in my opinion) that in high temperature usage of noble metal thermocouples, Rhodium vaporizes over time. Therefore, the mV output versus temperature changes over time (I have data that I am not able to share on this).

In both types R and S, this gradually lowers the measured temperature (compared with actual temperature).

In type B, in addition, the linearity also unpredictably changes with prolonged exposure to high temperatures. Therefore, type B has problems that are worse than in type R and S.

On top of that, Rhodium is much more expensive than Platinum (todays ASK price for Rhodium is $9610/oz, and Platinum is $1907/oz.).

I understand that the stated operating range of R and S is 1450 Deg C, whereas type B is 1700 Deg C. I would not want to recommend use of R or S at between 1450 and 1700 Deg C, as I can not guarantee what the actual melting point would be.

I was just doing some searching and found that the liquidus temperature of pure Platinum is 1770 Deg C. However, adding 5% Rhodium raised it to 1820 Deg C. And when Rhodium is increased to 13%, liquidus point raises to between 1840 and 1865 Deg C.

It has been a few years since I've delved into this topic. So forgive me if I ramble a bit.

My bottom line point is that because increased Rhodium increases melt point; and increased Rhodium increases long term instability, there is an apparent conflict. However, as the rated operating temperatures are all substantially less than the actual melting point, unless the application is at the edge of operating range, I believe Type B is an excessive cost that lowers long term accuracy in the application.

IMPORTANT: I'll repeat once more that if the use is near the top end of the specified temperature range, there is a risk. As I don't know the typical operating temperatures in the glass industry, I'll not suggest use of R and S over B. If, however, the application temperature is less than 1450 C, it may be worth considering.

The above is my opinion and based on some experimental work.

Jerry, that was an incredibly authoritative post on thermocouples. Thank you very much for spending the time lending us your knowledge. I learn so much from you.


CALIBRATING A CONTROLLER
METHOD A. Using a thermocouple calibrator which simulates the millivolts and with the correct corresponding cold junction compensation (for the particular thermocouple type; such as J, K, T, E, etc..), generate millivolts into the controller. This verifies the accuracy of the controller but not the control thermocouple attached to it.

METHOD B. Using a certified thermocouple with certified meter, place the certified thermocouple next to the thermocouple under test in the oven/chamber (or what ever is being controlled). Set the chamber to at least three temperatures, and allow adequate time for it to stabilize. Verify the controller/thermocouple under test measured chamber temperature matches the certified meter/thermocouple to within acceptable tolerances (not as accurate a method).

METHOD C. Calibrate the controller as in Method A above, then remove the thermocouple under test, and verify it's accuracy separately using a certified temp meter and a drywell calibrator.

These are just three simplified methods. There are numerous other methods. I have been calibrating thermocouples and temperature controllers for many years, and there are methods which may be considered based on the various specifics of your requirements and manufacturer/models of temp controller/thermocouples.

I am not sure, but it is possible the poster was wondering about your Method B you have listed here; not sure though. In my experience, Method A is the most common, Method B is the best method (assuming you have adequate standards); and C is second to B (with A being the least preferred).

What are your thoughts/opinions?

My bottom line point is that because increased Rhodium increases melt point; and increased Rhodium increases long term instability, there is an apparent conflict. However, as the rated operating temperatures are all substantially less than the actual melting point, unless the application is at the edge of operating range, I believe Type B is an excessive cost that lowers long term accuracy in the application.
.

that post was incredible, thanks

is there an alternative to type B t/c at higher temps?

Jerry, that was an incredibly authoritative post on thermocouples. Thank you very much for spending the time lending us your knowledge. I learn so much from you.



I am not sure, but it is possible the poster was wondering about your Method B you have listed here; not sure though. In my experience, Method A is the most common, Method B is the best method (assuming you have adequate standards); and C is second to B (with A being the least preferred).

What are your thoughts/opinions?

i think that method B sometimes would not be possible based on physical limitations or time limitations

method C should surely be better than A, which i think is vastly more common

This getting old nonsense is awful; ;I can't even remember my own posts (why I posted my own quote below).

Let me try to answer the discussion about Method A, B, and C. I will post separately a little later regarding the other question about alternatives to type B at higher temps.

Method C is definitely the best method. There is the assumption by many that the thermocouple does not change, and so does not need to be calibrated. This is sort of sometimes true (definitely not always).

Matter of fact, to truly verify the accuracy of a temp controller, the thermocouple is half of the equation. I could argue that thermocouple accuracy drift is often more significant than the electrical drift of the controller.

I would say Method C (cal controller and thermocouple separately) should be done when ever possible. Method B should be used (as allowed by site quality requirements) only when it is not practicable to remove the thermocouple or controller, and not practicable to disconnect the controller for electrical simulation.


------------------------------------------
Originally Posted by Jerry Eldred

CALIBRATING A CONTROLLER
METHOD A. Using a thermocouple calibrator which simulates the millivolts and with the correct corresponding cold junction compensation (for the particular thermocouple type; such as J, K, T, E, etc..), generate millivolts into the controller. This verifies the accuracy of the controller but not the control thermocouple attached to it.

METHOD B. Using a certified thermocouple with certified meter, place the certified thermocouple next to the thermocouple under test in the oven/chamber (or what ever is being controlled). Set the chamber to at least three temperatures, and allow adequate time for it to stabilize. Verify the controller/thermocouple under test measured chamber temperature matches the certified meter/thermocouple to within acceptable tolerances (not as accurate a method).

METHOD C. Calibrate the controller as in Method A above, then remove the thermocouple under test, and verify it's accuracy separately using a certified temp meter and a drywell calibrator.

These are just three simplified methods. There are numerous other methods. I have been calibrating thermocouples and temperature controllers for many years, and there are methods which may be considered based on the various specifics of your requirements and manufacturer/models of temp controller/thermocouples.