S-type thermocouple

[Dick White]

When installing a new K-type thermocouple (or buying a new kiln with a K), there is some calibration of the controller required to adjust the offset so that a witness cone bends at the stated temperature per both the Orton chart and the display on the controller. I am considering a switch to S for greater accuracy and am wondering if these typically are accurate out of the box, or will I need to run several firings to tweak the controller offset until everything matches?

Thanks

[Bill Kielb]

1 hour ago, Dick White said:

When installing a new K-type thermocouple (or buying a new kiln with a K), there is some calibration of the controller required to adjust the offset so that a witness cone bends at the stated temperature per both the Orton chart and the display on the controller. I am considering a switch to S for greater accuracy and am wondering if these typically are accurate out of the box, or will I need to run several firings to tweak the controller offset until everything matches?

Thanks

K type really ought to be reasonably accurate for the use. I am not sure that dialing in a temperature offset is as much an indication of the accuracy of the tcouple or just a way to compensate for control programming. Most controls were built around PID logic, (proportional, integral and derivative terms) which if you ever tuned one can be simple or make you pull most of your hair out.

I would be surprised if thermocouples differed by more than a few degrees actually, but I suppose it’s possible. They do make higher grade thermocouples and high end modules to read them that are certified in accuracy. With respect to the built in interface to read the thermocouple, many kilns have three or more channels, none certified. Again though not usually an issue for kilns. S are more accurate but not sure it solves the other possibilities for difference.

Interesting though, I don’t think I have a single answer. Once dialed in with a new thermocouple, I would think it would remain reasonably dialed in when replacing with a new thermocouple.

[Dick White]

Thanks @Bill KielbMy thoughts about the S-type TC derive from the conventional wisdom that K-types have uncertain (but probably not huge) inaccuracy above 2000F, and will drift over time as they wear. I don't know if K-types are slightly variable at manufacture or if a new TC were properly dialed in for cone 6 with the controller, one could put a different new one in every day and each one would be spot on every time? Certainly, it would seem that replacing an old one where the controller has been recalibrated from time to time to compensate for age-drift would require the offset to be tweaked back. But, conventional wisdom purports that the S-type suffers neither inaccuracy at mid- and high-fire temps nor drift. Just sudden failure at EOL. As I am upgrading my kiln and would like to get back into some crystalline glazes that require a fair degree of temperature accuracy, I'm trying to decide if the S- route is worthwhile.

 

[Mark C.]

S are more accurate as noted above 2000 and have much longer life in a protection tube. My guess is the drift is also less. I do know my platinum wire reduction/temp probe is rock solid on temps aboove 2000.

My firend who did cystaline  at cone 10 has  in an electric masde for it had 3-  S type TCs  in a three zone setup. Thats the cats meow if you ask me. Of course they cost more on the upfront end and seem to always be like mini thermos so easy to break.

[Bill Kielb]

8 hours ago, Dick White said:

I'm trying to decide if the S- route is worthwhile.

My speculation goes to I am never sure if constant dialing in is for thermocouple wear or element wear. Offsets are a means to an end but I think they compensate for other properties of the kiln and controller, not necessarily a drifting thermocouple. Most kilns in my experience rarely make their final rate so the controller (within limits) needs to terminate at the appropriate time to account for the reduced rate. As far as sensing accuracy protection tubes will reduce the speed at which temperature is sensed but for kiln firing speeds, usually not an issue.
Total speculation, but as the elements wear and  TC offsets are changed to compensate my sense has always been to leave the offset even with the change of a new thermocouple until elements have been changed at which time everything had to go back to the original setup offsets.

S type are very cool but I get the feeling that the same compensating offsets will be made for the elements as they wear. If that is the case, then is it a tcouple accuracy thing or element wear compensation. I think the latter, but only way to have a better idea is try it, it would be interesting.

There was an interesting thread here with guys building their own control to which they ended up having two or three controls all reading different temperatures throughout and even at the top end of the firing which I believe actually stabilized a bit between the three at the higher temp. . To which the surprise was, for non certified stuff, we really don’t measure things with as much accuracy as we like to think. I blame the digital era.

Final story, sending students to do their first lab duct traverse is always fun as every piece of cool equipment they own  and their best technique probably gets them within 10% of actual. Then for fun, have them all swap digital voltmeters.......... and if you have 20 students you probably get 10-15 significantly different voltmeter readings (1-10%) Lots of long (digital) faces on that day of lab!

[neilestrick]

Type K are plenty accurate for most pottery work. Type S are slightly more accurate, but like Bill said, any gains in accuracy are likely going to be negated by inaccuracies within the controller itself. IMO the reason to use Type S is for longevity and durability, like when firing to cone 10 regularly or doing long, high temp holds like with crystalline glazes.

[CactusPots]

I always describe cones as measuring temperature and time the way a speedometer measures speed and distance.  If there was a meter that measured heat work that way, that might be useful,  but otherwise the thermocouple probe itself is only giving half the information.  I suppose with a programmable controller on a newer electric kiln, you guess the controller is doing this for you if you use their cone fire program.

My question would be if anyone has actually verified the programmed climb rate and  temperature against cone packs.  How close is it to expectations?   Dead on at cone 6?   I have zero experience with electric glaze fire, so this is interesting.  In my gas kiln, I watch the cones above stated 2000 and 10 is usually down around 2150 or so.

As usual, if I need correcting in stated facts, please bring it.

[Mark C.]

At cone 10 standard thermocouples can vary my cone 11  at half way bent glaze today was 2340 . Next time it could read 2290. They also age quickly and readings vary. I have been firing every week lately and a trend occurs at times and I only trust cones never the thermocouple reading

[Bill Kielb]

1 hour ago, Mark C. said:

I have been firing every week lately and a trend occurs at times and I only trust cones never the thermocouple reading

It should most closely coincide with your speed in the last 200 degrees. Slower longer = lower (in the last 200) faster = higher (in the last 200) pretty much as the cone chart says. All that said nothing  is better at showing a fluxed reaction as something made of glaze with a flux ratio of 0.3:0.7 , ahh yes a cone.