View Full Version : thermocouple

Chris Lowry
12-30-2011, 02:02 PM
So why do my Thermocouple connections keep corroding? I get this rust, battery connection type crap on the wires and then they fall apart. What can I do to keep this from happening?

Plus anyone have a good resource for new Thermocouples?

Rollin Karg
12-30-2011, 03:23 PM
Talk to Richard Roberts he'll have a price on a new one and also give you some advice about the corrosion.


Dave Bross
12-31-2011, 08:23 AM
I've got a spare I'll sell. Same price as mentioned below, $500. More info here:


Rick Sherbert
12-31-2011, 08:35 AM
What type is it and where is it? Before I knew better I used a type K in the furnace. Got the same results you are describing. First thing you can do is put it in a sheath if it's not already and if it's the furnace, pop for the type R. Had the same one running for many years now. Just got one from Precision Engineering in Richmond Va for $400 ish for the new furnace.


Patrick Casanova
12-31-2011, 09:03 AM
This might help with general issues on Thermocouples


John Riepma
12-31-2011, 09:03 AM
For what it's worth to anyone who is currently in the process of changing out a thermocouple: we had some problems with the TC in our moly furnace over the last few years cracking the alumina or mullite outer sheath. I suspect that this was happening when we shut down and expansion/contraction of the furnace walls was occurring. The outer sheath was welded to the inner wall of the furnace by melted batch dust and the broken tip had to be cracked off cold to be removed. Then the next one did the same thing. I think that this let hot gasses travel along the inside of the sheath and reach the connections in the weatherhead fitting when we started up again. This last time when we replaced the crown I got a 1-1/4" OD alumina tube from Omega and drilled the furnace wall to allow this to slip through. That tube is simply supported in the wall and the TC is slid inside it so that it can't be welded to the inner furnace wall any longer. The readings appear to be the same as when it was just in the integrated sheath that it comes with.

Pete VanderLaan
12-31-2011, 10:59 AM
I never let my thermocouples actually extend into the furnace at all. The tip is recessed in the wall about 1/2 inch. I have some that are 25 years old at this point.

Also, mullite tubes do conduct electricity, alumina ones don't.

Steven O'Day
12-31-2011, 03:09 PM
mullite tubes do conduct electricity, alumina ones don't
Would this be a problem with mullite tubes as element hangers?

Pete VanderLaan
12-31-2011, 03:33 PM
I would view it as a potential problem. Mullite is 76 percent Al203 and 26 percent silica. It's the silica that allows the conducting to take place. In mOly furnaces, the voltage is alwys stepped down. Most of the ones I build now are wired in series and the elements draw between 12 and 20 volts. The human body does not conduct under 50 volts which is why you can pick up a car battery by the posts and not get nailed.

SiC systems on the other hand are line voltage if I understand them correctly. That being said, as you move away from the serious heat, the conductivity of the silica drops away.

I would surmise that Chris is having problems because of the salt air. I would put some noalox on the connections.
Thermocouple sheaths should be replaced once a year.

Steven O'Day
12-31-2011, 07:20 PM
I was thinking in a kiln roof with wire elements. Any chance of shorting out the elements?

Pete VanderLaan
12-31-2011, 07:58 PM
I have never shorted out elements held with Kanthal or in tubes going as high as 1600F but then again, I break new things almost every day. I would think that the conductivity would crap out as it fell below 1300F.

Chris Lowry
01-06-2012, 11:34 AM
Thanks for the suggestions, we are having problems on the backside where the wires connect. It will get so bad the wire will break. I'll try the noalox...

John Riepma
01-06-2012, 12:31 PM
Chris, do you know for sure if your TC protection tube is intact? The only time I've seen corrosion inside our Weatherhead fitting is when the outer tube was cracked which made a passageway for the hot gasses straight out through there. You may benefit from leaving the Weatherhead cover cracked open if it's out of the way of harm.

Alexander Adams
01-15-2012, 08:56 PM
If the tube is solid, is there any chance of Galvanic Corrosion? Is the lead wire from the housing to the controller/temperature sensor the correct type?

Sandy Dukeshire
01-16-2012, 08:03 AM
I never let my thermocouples actually extend into the furnace at all. The tip is recessed in the wall about 1/2 inch. I have some that are 25 years old at this point.

this statement caught my eye because i rolled my annealer away from the wall to work on saturday and in that process the tc probe pulled out unnoticed by me. it was recessed in the kiln wall so the temp went up higher than what was showing on the readout and of course i had some slumping action going on..not true for a furnace?

Pete VanderLaan
01-16-2012, 09:30 AM
Quite true for a furnace. I simply don't see a need to expose the whole end of the unit to high heat. i can extrapolate what my furnace is doing based on looking at the glass proofs. If it's reading off 25 degrees, it doesn't matter. What matters is that it is melting well.

With slumping, you're talking a lot lower temps, around 1500 for the most part. That takes a chrome Alumel thermocouple and it will last quite a long time stuck in the heat if it goes no higher than that. The thing that wears out in a platinum unit is the sheath, which when you try prying it's melted lifeless body off of the inner wire, breaks the wire way too frequently. The wire itself rarely fails. It's whacking it that does it in.