Phantom Thermocouple Spikes

I am experiencing an increasing temperature on one, or two, or even 3 of my load thermocouples in a high heat vacuum furnace. Normally, this is desired behavior except it is happening when the furnace switches to its quench (plunge cool) segment.

In the trend you can see the temperature reading rapidly increase for 16 to 18s before it peaks and begins decreasing as expected. The increase seems to be initially triggered by the backfilling of Argon, which begins at the red cursor. This is triggered by the opening of a pneumatic solenoid valve. Five seconds after the valve opens, a cooling blower spools up which is shown as the cooling blower amps in lime green. To note: the argon is room temperature up to the inlet.

To summarize the problem, a 2050F furnace under is backfilled with room temperature gas and the Load TC temperature increases, but the control thermocouples decrease as expected.

This particular order/part/recipe has some sort of spike ~85% of the time, while the others probably less than 5%.

The first thought was noise of some sort so we tried a few different things:
1) Grounded the TC to the load by soldering a short pigtail to the hot junction that made contact with the load.
2) Used an AB TC signal conditioner
3) 100mcF capacitor between TC(+) and TC(-) and jumped TC(-) to GND at the PLC.
All three were done alongside the load TCs and nothing showed positive results.

Having thought about it more, I have a hard time believing it is noise causing the issue because the temperature reading never has an exaggerated decrease and the duration of the increase is too long. This issue only occurs with this order, independent of furnace.

With that said, I do not have any other good explanations. Any other ideas would be much appreciated.

Other information:
PLC - HC900 via AI card
Type K TC

Trend Legend:
Green, Blue, Brown - Load TCs
Silver, Gold - Control TCs
Lime Green - Cooling Blow Amps

1st 2 questions...
Is the Control TC enclosed in a sheath and the Load TCs open ended (ceramic fiber or insulators on bare wire)?

I assume this is a vacuum furnace?

This is indeed a vacuum furnace. Control TCs are sheathed and Load TCs are bare wire fed through ceramic beads.

Probably your Control Tc is working as expected is because it is shielded from the environment inside the chamber.

Do you always locate your load TCs in same location on every load?
I am wondering about the location of the green pen in the furnace compared to the brown and blue...
The gas could be pushing or conducting hotter air to these locations during initial backfill.
Does the back fill gas come in through the cooling nozzles or directly into the chamber from outside the hot zone?

What is the condition of the plug in TC strip (mounted between the hot zone and chamber cold wall) for the Load TCs? If it is in bad condition it may be injecting this problem due to "plating" of the strip by metals that are "outgassed or vaporized" during the process and then collect on the load Tc terminal strip..
Remember when you go into Gas/Fan cool (or plunge cool as you term it), you are now moving hot gas from the hot zone out into the area between the hot zone and cold wall. I have seen this in the order of 800F and higher.


Next we will look at the material used in this process cycle compared to other process cycles....


I have done a lot of HC900 jobs on vacuum furnaces (with Type S Control and Type K or N load) without seeing these issues...so I do not expect it to be an AI card issue (Honeywell TC cards are one of the best).
That does not mean I have not seen this type of reaction., but it has been on fiber covered wire and not ceramic beads. It is usually visible on initial pump down. The fiber is moist from the humidity of the day and when the furnace pumps down it cause the fiber to chill due to the vacuum pulling the moisture out of the wire, so the load tcs go downscale during pump down.

is the signalcard isolated and are the channels isolated from each other.

Good question, yes the TCs are always in the same location. Blue - Front, Brown - Middle, Green - back, running diagonally. This orientation stays the same. The green(back) TC is usually the most troublesome, but sometimes varies. Attached is an example of the front and middle TC spiking, while back TC performs as expected.

I have not personally examined the TC port/strip, but I know the plant monitors these closely as part of their PM regimen.

Whats most interesting is that the higher and lower temperature cycles do not see this almost at all, and are all ran in this furnace as well.

@shooter - The AI card is isolated and I believe channels are isolated by pairs.

Are your load TCs in the part or on the part?
Have you tried using a different plugs on the strip? like 6,7,and 8 instead of 1, 2, 3?

The load TC's are in the part itself.

When I ran my first test I used plugs 4, 5, and 6 for my hopeful solutions. We ran it like this for a couple days but 4, 5, and 6 behaved no differently than the others.

Maybe your purge gas isn't as pure as you believe, and may be introducing some oxygen to the process.

Is the part material different on this process cycle than the other process cycles that work?

I would think if that were the case that this issue would have presented itself running a cycle at 2200F or on one of the other furnaces but it doesn't seem like that is the case. I will have to check on that.

I'm not sure about the part material. I will have to do some digging and get back on Monday.

You forgot to answer this question...
Does the back fill gas come in through the cooling nozzles (directly into the hot zone) or directly into the chamber, not directly into the hot zone?

Sorry, I'm waiting on clarification on that. I'm thinking it goes to the outer chamber first but I'm only 60% confident.

Ipsen, GM Enterprises, ABAR, VFS or who made the furnace?