[Brian]

What type of thermocouple do you guys use for your vacuum furnace control thermocouple? We have been considering changing ours. Currently we have an exposed junction type C thermocouple (OEM design). We are thinking about changing it to a R thermocouple in a vac-rated moly sheath. Currently, we have problems with the temperature reading suddenly jumping several dozen degrees for no reason. It doesn't happen very often, and it corrects itself immediately, but it is a nuisance.

I was curious to see what others' experiences were. Is an exposed junction needed for good temp control response?

Brian

[heattreater]

Brian,
I am not familiar with Type C thermocouples, and have never heard of them being used in vacuum heat treating furnaces. I think most of the furnace OEM's supply sheathed Type S thermocouples with ceramic tips on new vacuum furnaces for both the control and overtemp thermocouples. Please note that AMS 2769 (Heat Treatment of Parts in a Vacuum) states in Table 4 that "Bare thermocouples shall be discarded after being exposed to five heat treat cycles or...".

I think the exposed junction can lead to problems (i.e. contamination or vaporization can lead to faulty readings). I have once heard of a heat treater processing brass in a vacuum furnace, some of which vaporized and contaminated his exposed t/c's. The faulty readings of the contaminated t/c lead to overheating and destroying his furnace hot zone.

Are you instruments even capable of receiving a Type C input?

I would contact the furnace OEM and go with their recommendation on t/c type. The type S are expensive, but they last a long time (if not mechanically damaged first).

When you change the t/c type, make sure you change the controller setting as well to match.

heattreater

[Brian]

Type C is a Tungsten/Tungsten-Rhenium thermocouple. Omega has a handy chart showing it along with the more standard types. Our instruments can be set for C (among many other) thermocouples. Type C was what was originally spec'ed for this furnace (I have the prints) back in the early 80s. I wonder if they did that to save money (tungsten is cheaper then platinum). Type C cannot be used in an oxidizing environment, so you can't use it in a sheath, thus the exposed junction.

I was looking at the ceramic sheathed thermocouples the other day in a catalog. They (Pyromation) mentioned on their website that an alumina protection tube should not be used when the temperature ramp rates will exceed 100C per hour. Perhaps that is only the protection tubes and does not apply to a thermocouple sheath. The ramp rate problem is why I was initially leaning toward a moly sheath. If it's usable, I'd rather have the alumina.

We were interested in moving to S (or R) as we have several of those installed in other pieces of equipment and have never had a problem with them, at least that I can recall.

Brian

[josephmarkgreene]

I would venture that the biggest liability is the exposed junction. As heattreater says, the component is wide open to contamination, as well as electromagnetic interference. I'd at least try a sheathed Type C if it is available. Then again, Type R or Type S will last for a long time without the calibration drifting.

The heat fluxes and heating rates in vacuum furnaces are so moderate that the rate of response is plenty quick even with a sheath.

Even for low-cost work thermocouples, exposed junctions are worthless. Sheathed with ungrounded junctions are the best for reliability and low interference. Type K was always the faithful standby, but Type N is supposed to be superior in some ways.