Scientific Oven Calibration

[mmkraftwerk]

Good morning friends,

My department has recently started a lab for flammability testing. Among other issues getting it started, I'm conflicted on what to do with the calibration/certification of our oven for heat treatment for OEM specific (FMVSS 302 Compliant) testing that require a specified amount of time in a scientific oven, before commencing the flammability testing. My boss is of the opinion that instead of certifying the thermocouple in the oven, that we should just purchase a scientific thermometer/thermocouple to verify it against, my worry is that this isn't going to hold up in an IATF audit. Her concern is more related to cost save rather than what is best practice. I think a record of verification of the thermocouple with a secondary NIST traceable unit would be good, but I don't think it would replace an annual calibration/certification of the oven thermocouple itself.
Can anyone here provide some guidance on what a good practice would be?

 

[Miner]

I have experience with two types of ovens (hydrogen furnace and solder reflow ovens). In both cases, we calibrated all the temperature controls, but we also would periodically check the oven profiles using a data logger.

 

[ChrisM]

This could be a can of worms.... how large is the oven and what temperature variations could exist within it? If I understand correctly, the "thermocouple in the oven" is the oven's "built-in" temperature indicator.. where is that taking measurements from, top or middle or bottom (width and height-wise)? For the tests, what temperature variation is acceptable (e.g. test pieces must be placed in an oven for 1 hour at 200 degrees F - tolerance on the time, could be 1 hour minimum, and the temperature, 200 deg plus or minus what?)

 

[John Predmore]

The probe(s) which feeds temperature to the oven control circuit may be distant from the point where the heat treated parts sit in the oven. Using a set of thermocouples, we were surprised by the variation between the center of the oven and the sides and the corners. This activity is called a Heat Map. Over time, electric heating elements may get encrusted with crud, natural gas orifices can become partially occluded, which increases internal variability.

 

[Bev D]

Agree with John - you need to calibrate and verify your thermocouples AND you need more than one - you really should create a heat map that validates that your oven is heating properly throughout the part locations. I’ve surprised many an engineer with these

 

[John Predmore]

check the oven profiles using a data logger

To echo what @Miner said, a data logger shows dynamics. Depending on criticality, you might care about how long a preheated oven takes to get back up to temperature after the door is opened, and the difference between the thermal mass of a single part versus a full load of room-temperature parts. We found the temperature cycle inside our ovens was affected by the exhaust fan speed (which the operator had to remember to adjust) and whether any other ovens on a common exhaust vent were running or not. A full study would fall in the realm of equipment validation.

 

[AllTheThings]

Most test oven validation requires mapping with a minimum of 9 thermocouples...At least once, but preferably on some interval. I'm sure there are use cases where this isn't required, but it is fairly industry standard. This is a good reference, and notes some IEC 60068 standards that provide the methods used by a lot of labs, chiefly IEC 60068-3-5.

https://www.euramet.org/Media/docs/Publications/calguides/EURAMET_cg-20__v_3.0_Calibration_of_Climatic_Chambers.pdf