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".
(Before you ask, this is NOT an ISO, QS, AS or any other type of 9001 organization!) (Except for their electronic calibration lab! )
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
Let the learning begin! (Thanks in advance.)
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