Reader Feedback: Measuring Junction Construction in AMS2750 Rev. G

Readers are checking out recent AMS2750 Rev. G changes and want some more information from Heat Treat Today about a specific clarification. Read the correspondence about the implications of AMS2750 Rev. G paragraph 3.1.1.5 about how to measure junction construction.

Douglas Shuler, lead auditor at Pyro Consulting, has written numerous articles with Heat Treat Today about AMS2750 standards. Check them out by searching "Doug Shuler" at www.heattreattoday.com.

Submit your question, comments, thoughts, or queries here or email Bethany Leone at editor@heattreattoday.com.

READER QUESTION: After combing the new AMS2750 Rev. G, I found that paragraph 3.1.1.5 no longer allows thermocouples to be tack welded directly to parts, OR to representative dummy parts. This has been standard practice for decades. So I dug into it further with the folks from PRI and it turns out to be true. They’re now expecting load thermocouples to be either placed inside of a part (ends twisted and inserted), or inside the hole of a dummy block.

I’d done some searching online and there isn’t a single source talking about this major change. This could lead to a lot of failed upcoming Nadcap audits.

Douglas (Doug) Shuler (Pyro Consulting) for Heat Treat Today: Historically (i.e. prior to Rev. F), AMS2750 was silent on measuring junction construction. In Rev. F, the construction of the measuring junction was as follows:

Measuring junctions shall be made by any combination of twisting and/or welding the thermal elements provided there is no addition of filler metal.

This raised concerns about both the use of quick tips and spot welding to make the measuring junction. The AMEC AMS2750 revision team engaged with Cleveland Electric Laboratories to perform testing on these measuring junctions as compared to the twisting/welding combinations.

The conclusion of the tests were that both quick tips and spot welding to a part/heat sink became unstable at temperatures above 2000°F. The quick tip crimping point and the spot welds showed rapid oxidation and increased errors in a short period of time. Based on the results of these tests the AMEC AMS2750 revision team put forth the following update in Rev. G:

Measuring junctions shall be made by either of the following methods:

• • • • Any combination of twisting and/or welding the thermoelements provided there is no addition of filler metal (including ungrounded and grounded MIMS).
      • Spot welding the thermoelements directly to a part, simulated part, or heat sink is permitted for temperatures ≤2000°F or 1100°C.

This allows spot welding measuring junctions for process temperatures at or lower than 2000°F. The team and AMEC members decided that quick tips were to unstable to permit their use going forward.

READER FOLLOW-UP: Our current method that we’re using is to twist the thermocouple using a set of Twister Pliers, then tack-weld that twist onto the part (first photo below). We’ve been doing this for parts up through brazing temperatures (~2150°F) without issue. We recently ran a furnace run around 2100°F with parts tack-welded as I’ve described AND had T/Cs that were just twisted with no tack weld. We noticed there was no significant difference in the temperature the TCs were reading. This was also our technique that we used at my previous company.

The way I read AMS2750 Rev. G was: You may twist and weld thermocouples to a part, but only for temperatures less than or equal to 2000°F.

To comply with Rev. G, we have gone ahead and made heatsink blocks to make sure we’re in compliance. Our new method of temperature measurement is twisting the wires and sticking the twisted end down inside of a block of solid metal (like the one I’ve shown below).