black dust from thermocouple

[Seth A]

In my last firing I noticed some black dust on my kiln shelves that I suspect are coming from the exposed end of my thermocouple based on their location.  The kiln was purchased new, and has only been fired 5 times.  I'm wondering if the thermocouple is a reasonable suspect, and if so is this normal or am I doing something wrong.

thanks

[GEP]

Totally normal! It’s called “spalling.”

[neilestrick]

Yep, it's normal. That's one reason why some kiln manufacturers put a protection tube over the thermocouple. You can always add one if it's a problem. You just have to put an offset into the controller to compensate for the insulation of the tube.

[Bill Kielb]

Yep, from first firing to last they will corrode. One side generally corrodes faster than the other (The slightly magnetic side (Chromel) I believe) and often is a good indication that you are nearing replacement time. The corrosion will grow so large it will begin cracking the ceramic insulator and  often observing this small crack is a decent indicator that replacement time is near.

 

[Seth A]

Awesome! Just the answer I was hoping for.  Thanks for the info!

[Babs]

Just a question.

I have only ever used the platinum???? Thermocouples, obviously different and they last for absolute years unless I manage to break them usually.

How long do these above ones tend to last?

Going to C 6 nowadays, ahould I be using the above ones?

Why the difference in thermoc.?

An obvious $ difference?

[Bill Kielb]

Cheap, type K thermocouple. Good for maybe 100 firings. Off gasses kill them the most so can be variable on lifespan. Two wires welded together. Simple one chromel, one alumal. 

[neilestrick]

@Babs Type S platinum are $270. Type K are $18.50. Type K in a protection tube can last 150 firings of cone 6 and bisque, but even at 100 firings, you'd need to get over 1,400 firings of the type S for them to pay off. For most people, type K are accurate enough for cone 6 firings, and type S are only necessary if you're regularly firing to cone 10.

[Babs]

Right here in Aus. I haven't come across  the cheaper ones I think the thermocouple I have in my new electric is type R . but I've stopped reading that part of the program. Which is more expensive again.  I DO know that the porcelain sheath on my old therm. on my gas kiln was $80 Aus to replace. You guys get stuff so so  cheaply.

Hey I could flog this therm r and buy a life times supply of type K but you just put up with the spalling? What radius is the fallout? If contained in sheath does the spalling affect the life of it,

[neilestrick]

1 hour ago, Babs said:

Right here in Aus. I haven't come across  the cheaper ones I think the thermocouple I have in my new electric is type R . but I've stopped reading that part of the program. Which is more expensive again.  I DO know that the porcelain sheath on my old therm. on my gas kiln was $80 Aus to replace. You guys get stuff so so  cheaply.

Hey I could flog this therm r and buy a life times supply of type K but you just put up with the spalling? What radius is the fallout? If contained in sheath does the spalling affect the life of it,

Protection tubes here are about $35. The contain the spalling, and keep the thermocouples from corroding as quickly since they're protected from the fumes in the kiln. I haven't noticed any lifespan difference between dumping out the spall periodically or just leaving it in the tube. Both ways they last longer than without the protection tube. The tube is big enough that there's room for the spall to collect in there. The radius from the spall can be quite large, like 6 inches out or more. Take a look at L&L's Thermcouple Test. They tested a whole bunch of thermocouples to see which was the best setup for the price. You can see the spalling all over the bottom of the kiln. 

[Bill Kielb]

Ya gotta love all the wiring to that thing it looks like a NASA project.