Auditor states my Oven Thermocouples must be Calibrated

T

timmlaww3256

#1
Hello all,
We, are an AS9100 registered Organization.
I have been issued an NCR due to customer auditing. Although I understand much of what has been said here, I'm sorry to say I remain very confused, and need some additional guidance on the particulars...

The exact wording on the NCR is as follows:
"Supplier's thermocouples located on drying ovens do not have identifications or traceability to a lot or roll nor the type of wire that is being used during the processing of parts."

First, Is there a recognized standard that speaks directly and specifically to thermocouple calibration? I ask because I need to know what my third-party cal house's Certifications need to say (reference), and what requirements I need to reference in my POs to them.

Second, My oven controllers are calibrated annually, should I put the thermocouples on the same cycle? FYI-My ovens top out at 500F, but are never set higher than 270F.

I really just want to get out from under this NCR without having to spend too much time becoming an "expert" as we outsource all our calibration needs.

Any help would be greatly appreciated.

Thank you,
timmlaww3256
 
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bobdoering

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#2
Re: Do Thermocouples need to be Calibrated?

"Supplier's thermocouples located on drying ovens do not have identifications or traceability to a lot or roll nor the type of wire that is being used during the processing of parts."
Each time you make up a thermocouple, you should verify it is reading correctly prior to use. You may have a faulty weld, etc. Over time it may also degrade, depending on the atmosphere and temperature of the oven.


First, Is there a recognized standard that speaks directly and specifically to thermocouple calibration?
Actually, yes...ASTM E2846-14 Standard Guide for Thermocouple Verification

Second, My oven controllers are calibrated annually, should I put the thermocouples on the same cycle? FYI-My ovens top out at 500F, but are never set higher than 270F.
You can start there, but realize the controller does not see the rough service that the thermocouple does!
 

BradM

Staff member
Admin
#3
Re: Do Thermocouples need to be Calibrated?

Hello all,
We, are an AS9100 registered Organization.
I have been issued an NCR due to customer auditing. Although I understand much of what has been said here, I'm sorry to say I remain very confused, and need some additional guidance on the particulars...

The exact wording on the NCR is as follows:
"Supplier's thermocouples located on drying ovens do not have identifications or traceability to a lot or roll nor the type of wire that is being used during the processing of parts."

First, Is there a recognized standard that speaks directly and specifically to thermocouple calibration? I ask because I need to know what my third-party cal house's Certifications need to say (reference), and what requirements I need to reference in my POs to them.

Second, My oven controllers are calibrated annually, should I put the thermocouples on the same cycle? FYI-My ovens top out at 500F, but are never set higher than 270F.

I really just want to get out from under this NCR without having to spend too much time becoming an "expert" as we outsource all our calibration needs.

Any help would be greatly appreciated.

Thank you,
timmlaww3256

Hello Tim!

Question.... are these thermocouples expendable (from a thermocouple roll) or are they stainless steel (permanent type) thermocouples?

If they are a roll, yes, you need to have the roll calibrated, and then depending on time and temperature of use, you have so many uses. While it's an Aerospace specification, AMS 2750 might be a good reference for this.

If your thermocouples are permanent, you will need to establish a calibration cycle for them. I would keep it simple. Depending on its cost effectiveness and how much you wish to take on, buy a calibration furnace, and calibrate the thermocouples and your controller/high limit/ recorders as one loop. :)
 

Jerry Eldred

Forum Moderator
Super Moderator
#4
Re: Do Thermocouples need to be Calibrated?

As I didn't have time to fully read the whole thread, I'll just say that thermocouples installed in an oven need to be verified (some will say that you can't "calibrate" a thermocouple, as you can't adjust it). The AMS and ASTM specs are some of the better/tighter in calibrating ovens and their associated thermocouples.

I have seen many temp control thermocouples in many ovens over the years drift considerably. We have a set of high temp Type K's that we use for some high temp ovens. I just had to throw one away because at somewhere around 1000 F, it had drifted down to about a -37 Degree F error at 1000 F. In baking or curing ovens, thermocouples will commonly degrade, and certainly should be checked.

The worst "method" for "calibrating" an oven (not acceptable) is just simulating into the controller. This verifies that the controller is working (but controllers rarely drift very much).

The second slightly better method is a comparison with a thermocouple (or RTD for temps and accuracies where applicable) placed next to the temp controller thermocouple in the oven. This at least verifies that the controller is measuring the control spot in the oven correctly.

The next better method is checking at least in the middle of the chamber where product will be placed (a little better, but still not the best). This verifies center of chamber. And it should be done at a range of temperatures.

The next better (pretty good, but not perfect) method is mapping or profiling the chamber. A thermocouple of known accuracy in all eight corners, and in the center of the chamber. This verifies pretty well the chamber uniformity, and particularly in larger chambers, is a necessity (the larger the chamber, the more critical this is - as they tend to be more non-uniform).

The perhaps best is the AMS procedure which is all of the above mapping, plus there are some soak time requirements, etc. And some requirements even for the calibrations done on the thermocouples used to do the mapping.

My two cents.
 

Big Jim

Super Moderator
#5
Re: Do Thermocouples need to be Calibrated?

Hello all,
We, are an AS9001 registered Organization.
I have been issued an NCR due to customer auditing. Although I understand much of what has been said here, I'm sorry to say I remain very confused, and need some additional guidance on the particulars...

The exact wording on the NCR is as follows:
"Supplier's thermocouples located on drying ovens do not have identifications or traceability to a lot or roll nor the type of wire that is being used during the processing of parts."

First, Is there a recognized standard that speaks directly and specifically to thermocouple calibration? I ask because I need to know what my third-party cal house's Certifications need to say (reference), and what requirements I need to reference in my POs to them.

Second, My oven controllers are calibrated annually, should I put the thermocouples on the same cycle? FYI-My ovens top out at 500F, but are never set higher than 270F.

I really just want to get out from under this NCR without having to spend too much time becoming an "expert" as we outsource all our calibration needs.

Any help would be greatly appreciated.

Thank you,
timmlaww3256
I assume you mean AS9100, not AS9001.

What the auditor is asking for reaches beyond what AS9100C requires. Unless there is a customer requirement to do more you are not required to do what he is after.

That said, that may not be the best way to handle a customer's audit although a discussion of what the standard actually requires is never a bad idea.

I call on some heat treat shops. Some calibrate the wires alone, others calibrate the controllers and wires together. Either way meets the requirements of AS9100C. If you calibrate the wires by themselves you will still need to calibrate the controllers.

To ensure that the ovens / furnaces are working properly it is common to perform an oven survey. That means taking readings at several positions in the oven so you can see if there are any hot or cold spots. Sometimes they can be made more uniform from repairing or improving the insulation. Sometimes the oven / furnace needs a design change to re-position the heat source.
 

BradM

Staff member
Admin
#6
Re: Do Thermocouples need to be Calibrated?

I view a uniformity/ mapping study on an oven to be less about calibration/accuracy; and more about oven capability. The uniformity study is critical to knowing how uniform the chamber operates, any extremes in the chamber, and how well the chamber operates with respect to the temperature controller.

Barring industry requirements...

I would calibrate the temperature; high limit; monitoring/ recording probes first, as a loop. I would not recommend using expendable thermocouples for these.

Once this is done, make thermocouples from a calibrated roll and place into the oven according to a known volume/ size requirement/ location grid. Connect the thermocoules to a calibrated logger. Start from cold, and set the controller on the first operating temperature. Once the temperature has stabilized, review for uniformity and control parameters.
  • Did the temperature overshoot? (Could be a tuning problem or control probe location problem). (depending on governing specifications) a bias is allowed to be added to the controller.
  • Is the temperature inside oscillating? (most likely a tuning issue).
  • Does the permanent monitoring/recording probes represent the hot/ cold spot?
  • If the mapped temperature is way off from the controller (and the controller is calibrated), the control thermocouple may be located in the wrong spot. Typical installation has the tip of the control thermocouple extending just a bit past the discharge ducts in the oven.
  • If the uniformity is off, stop the mapping study. Check the tips of the mapping thermocouples and make sure they aren't touching metal, fallen off in a corner or something; and aren't damaged. Also, verify there are no bare spots on the wires. If the two bare wires of the thermocouples are touching metal, that becomes a measuring junction.
  • If the oven uniformity continues to be off, there may be more involved issues with the oven such as recirculating fan issues, sufficient static pressure, ducts/louvers that need adjusting, etc. Also, the oven may not have been designed to achieve the desired uniformity. The tighter the spec., the more air needs to be moved, which means more $$$$.
When the uniformity is sufficient, take the readings and go to the next test point.

When you're done with the mapping study, verify the high limit controller actually turns the heat off when the set temperature is exceeded.

Store the thermocouples in a known/ safe place (where they aren't ran over by forklifts or used for bailing wire. :) ). You should have a log of some kind recording # of uses and at what temperatures, and when you cut them off the roll.
 
T

timmlaww3256

#7
Re: Do Thermocouples need to be Calibrated?

I assume you mean AS9100, not AS9001.

What the auditor is asking for reaches beyond what AS9100C requires. Unless there is a customer requirement to do more you are not required to do what he is after.

That said, that may not be the best way to handle a customer's audit although a discussion of what the standard actually requires is never a bad idea.

I call on some heat treat shops. Some calibrate the wires alone, others calibrate the controllers and wires together. Either way meets the requirements of AS9100C. If you calibrate the wires by themselves you will still need to calibrate the controllers.

To ensure that the ovens / furnaces are working properly it is common to perform an oven survey. That means taking readings at several positions in the oven so you can see if there are any hot or cold spots. Sometimes they can be made more uniform from repairing or improving the insulation. Sometimes the oven / furnace needs a design change to re-position the heat source.
Big Jim,
When you say "What the auditor is asking for reaches beyond what AS9100C requires. Unless there is a customer requirement to do more you are not required to do what he is after."

How can that statement be justified? The standard doesn't speak to specific types equipment/devices/components. The main problem I'm having, is that she says I do have to have these calibrated, and we didn't know that. So then she says "You should've". Then I say "How?", and she has no answer other than "It's required."

SAYS WHO? GIMME SOMETHING IN WRITING THAT SAYS THAT!
If I use a calculator to trig out the beginning of a radius, or add up tolerance stacking, must I have that calculator calibrated?
I know those aren't great examples, it's just that I'm a "Show me Chapter and Verse" kind of guy.

Are thin film RTDs treated the same way?
 

BradM

Staff member
Admin
#8
Re: Do Thermocouples need to be Calibrated?

Big Jim,
When you say "What the auditor is asking for reaches beyond what AS9100C requires. Unless there is a customer requirement to do more you are not required to do what he is after."

How can that statement be justified? The standard doesn't speak to specific types equipment/devices/components. The main problem I'm having, is that she says I do have to have these calibrated, and we didn't know that. So then she says "You should've". Then I say "How?", and she has no answer other than "It's required."

SAYS WHO? GIMME SOMETHING IN WRITING THAT SAYS THAT!
If I use a calculator to trig out the beginning of a radius, or add up tolerance stacking, must I have that calculator calibrated?
I know those aren't great examples, it's just that I'm a "Show me Chapter and Verse" kind of guy.

Are thin film RTDs (resistance temperature detector) treated the same way?
I think the point here is that you shouldn't have to be coming here to get the answers. The auditor should be able to clearly present the requirements that the thermocouple needs to be calibrated.

Is the auditor citing a requirement?
Is it an observation/suggestion?

To your other question, RTD's are temperature sensors just like thermocouples(tc). TC's use dissimilar metals of known properties; RTD's uses resistors. If your standard/requirement states that temperature sensors on a given process be calibrated, then whether it was TC or RTD, it would need to be calibrated.

Are your thermocouples expendable wire (from a roll) or permanent probes?
 

BradM

Staff member
Admin
#9
Re: Auditor states my thermocouples must be calibrated

:modcop:
Tim, I think your topic is very interesting, and slightly off the subject of the original thread.

So I created your own thread here. :agree1:
 

Marc

Hunkered Down for the Duration
Staff member
Admin
#10
Re: Auditor states my thermocouples must be calibrated

Back in the MIL-STD-883 days, a 9-point profile had to be done with the oven fully loaded because profiling an empty oven does not replicate conditions with product inside. If I remember correctly (I did this stuff in the 1980's in DoD work - So my memory *might* be a bit rusty) ASTM E-145 defined a 9 point "zone" check to verify that the temperature is consistent throughout the oven as well as the air flow.

I used to have some old graphics - If I can find them I'll post them. At the time I made them for ESS (environmental stress screening) of electronic equipment.
 
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