thermo readings

[Robert Coyle]

Well this is pretty annoying... Just for laughs I compared a brand new K  thermocouple with the one in my kiln. The one in the kiln still has lots of metal fused at the junction so i figured that they would be pretty much the same.

SURPRISE! one read 34.6 mv and the other 36.4mv. So I thought i would try a third as a tie breaker... 33.9 mv.  The third one was just a wire probe rather than the beefy kiln probes. but it was a K. type
we're talking about a 300 deg F+ spread at around 1600F ( who knows now what it really is)

I measured in the same kiln with the same meter with all well into the kiln and at equilibrium.

The moral of the story...  never trust the absolute accuracy of your controller. don't ever fire without witness cones.

[Robert Coyle]

Yes Marina, I was shocked to find the spread of readings I got from one K thermocouple to the next. At this point, I pay little attention to the temperature reading and only go for consistency  once I have developed a good set of  ramp/hold parameters. The readings on the same thermocouple seem to hold pretty well as it ages. I have even shortened old burnt thermocoulpes and welded the ends back together with a welding torch. They worked just fine for the second go around. 

[Marina Reijsmeijer (Kleierij)]

I recently tested some 5 different k-type thermocouples at 100C (boiling water) and found about 5C difference. I also tested a bisque fire with 2 thermocouples in the same hole and found 30C difference (cone 05 1/2, 1020C 1860F). I was disappointed, but reading your observations I should be content :). We are building a raspberry pi controller and using a temperature compensation chip. The chip implements a linear conversion from mV to T, but at higher temperatures the conversion is no longer linear, so we needed to recalculate.   My conversion table states 

34.6 mV = 830C (1526F) and 36.4 = 880C (1616F)which is a spread of 50C (90F) not 150F, but still not good enough.

I have been using 3 different type-k thermocouples with the old home built controller ( in 10 years) and found the firings consistent, I always use a cone 8 in the disabled kiln sitter so I can know when the kiln reaches cone 8 and it was pretty much the same each firing.

[Robert Coyle]

Yes Jeff, I do the same thing. I move the TC from one kiln to the other. My  home brew controller is also portable and I move it along with the same TC. That way I have the maximum chance of getting the close to the same ramp on my large kiln as on my small test kiln. 

[George Lewter]

My L&L measures and controls 3 sections. Already in for $3 G, $540 extra for 3 platinums seemed -- extravagant. I am a one man, part-timer. Hope I outlive my Type K's and get to complain about them.

[Jeff Poulter]

I will never go back to a "K".  I have one outlet for both kilns to share in my shed so I can only fire one kiln at a time, but when they are both at rest, when I unplug one & plug in the other, they always read read identical temperatures.  I guess I could do a bisque in each one and then check the controller to see what the top temp was on each firing doing the exact program, although they are different controllers, so they probably wouldn't handle it identically.  Both "S"s have always been spot on when I have thrown cones in. Just my $.02!!!!   jhp

[Norm Stuart]

Just from memory I think the price for thermocouple and cable insulated with foil was $180 or so.

We bought it from Cress about 3 years ago.

Switching the jumper on the Bartlett controller and changing the hidden menu option from K to S took less than a minute.

It was something like 1/2" longer so I used a short piece of cordierite kiln furniture post as a spacer between the outer kiln wall and the thermocouple base - the two screws were long enough to span the extra space so I there must have been something very much like that cordierite piece missing.

Type-S thermocouple

the Kiln furniture I used as a spacer - cost about 58 cents

foil shielded cable for Type-S due to lower voltage produced

As installed

L&L Kilns sells a different model Type-S Kit for $225 which includes a metal spacer which backs-out the thermocouple about 1/2" which ours didn't include. - http://hotkilns.com/type-s

[Robert Coyle]

Good to know Norm...  I really just go for consistency from run to run. I use the same ramp on the same kiln with the same TC that gives the results I want.   When in doubt, I always believe the witness cones.

What's the best price you ave managed for an S?

[Norm Stuart]

I'm liking our Type-S platinum thermocouple more with each passing year.

Every once in a while I'll throw in a cone which always comes out dead-on.