Need Thermocouple help

[fitzo]

Hi, Folks,

I have an old (1986) Paragon KM14 HT furnace, which has operated well for years since I bought it new. I had a thermocouple short out, finally. Paragon no longer uses the same thermocouple, and also no longer has the specs on my furnace.

I installed the thermocouples they suggested/sold as replacements. (the folks I spoke with were unaware of ever having a two-thermocoupl e model, so they are not the best help). The controller and pyrometer thermocouples are in essential aggreement, being only about twenty degrees apart. However, when monitoring with an external Omega meter with a new thermocouple, the temperatures diverge more as the temp increases. That is, when the controller and meter say 200, the Omega reads about 280. When the furnace reads 1000, the Omega reads 1140. I have checked the Omega vs icewater bath (32.4F) and boiling water (212F) and tend to trust the Omega probes from industrial experience.

My question after this longwinded preamble is, does anyone know if there can exist a mismatch between controller/meter/ new style probe? Meaning, can the newer probes have a different electrical response than the old style, and thus have an ever-increasing potential difference as the temp increases versus my old controller and pyrometer?

Any suggestions in this regard would be appreciated, as I have had a difficult time getting Paragon to let me past the "front desk" where they don't seem to know a whole bunch vis-a-vis this difficulty.

Thanks, folks!

 

[insanity]

I'm mainly trying to fully understand this for myself so this might not be very helpfull. Anyway.. from my understanding of pyrometers (which is what I am assuming is basically what you have) the type (metals used) and the length of the wires will affect the readings. So if your controler is precalibrated that could be a problem. It sounds to me like you need to find the person who has been working for the company long enough to remember the model you have and actually knows what you need instead of those who think they might know.

WS

 

[Mike Hull]

Mike, if the order desk guys are playing "Mr. Important", ask for the manager, and tell him your problem, and ask him to connect you to one of the engineers. I bet it gets handled then.
Wish I could be of more help.

 

[fitzo]

Thx, Mike. yeah, that's what I'm gonna have to do on Monday. Posted here to try and get as much insight as possible before I speak to them. P.O. Bill told me the owner's name is John, and a nice guy. I'll ask for him if need be.

Thx.

 

[Mike Hull]

I just had a thought. Why not call Paul Bos, he knows a bit about that, and he was certainly doing HT in 1986. Maybe he could shed some light on the problem.
I will email you his home #.

 

[fitzo]

thx, guys, for the input. good advice, i appreciate it.

 

[rlinger]

Thermocouples are pretty straight forward. A Type K is a Type K , Type T is a Type T regardless, unless there is something I don't know yet about them. They are just two dissimilar metals for each lead. For example chromel-alumel for one lead and copper-constentine for another (except my spelling is probably way off). Lead length may have a difference because of lead resistance causing voltage drop and the oven electronics may need re-calibration.

You need to talk only to an engineer there (as suggested above). Find out what type thermocouple your oven uses. I think, without looking it up, Type K is rated to something around 2000F. or better. They also have a low end.

Roger

 

[stevomiller]

I'm with rlinger, "K" type is a "K" type, "J" type is a "J" type, etc... Even among a given type there will be a specified temperature range due to the limits of the insulation on the probe/thermocouple. Many industrial controllers (and hand models such as your Omega) can be set to function with J, K, T or E thermocouples. BUT you must SET them to work with that type. If you set your controller to work with a K type, but use a J type, you will get a erroneous reading. It will falsely translate the feedback from the thermocouple. Not saying that this is your problem, but it is the first thing that I would check. I work with Eurotherm, Omron, and Omega controllers at work at least a couple of times a week, and this is the #1 problem I have found. Hope this helps.

 

[fitzo]

Thanks alot, guys. It was an interesting conversation I had with Paragon, today. One of their engineers called me back, thankfully.

Unfortunately, the news was less than exciting, and sorta downright disappointing. Basically, what I was told is tough doodoo, sorry , etc... First off, I was told that my furnace is too old for them to have any surviving information on; apparently no one has worked there long enough to remember the first KM14 furnaces they sold (I bought mine new in '86) and they have no wiring diagrams, specs, etc. That is sorta disappointing, I feel, and poor Customer Service.

Second thing is, I was told that 150 degrees difference from setpoint is not a surprise to him, with an older analog controller. That's sure not what they claimed when they sold me the damned thing!! Sorta makes it hard to do accurate tempering, eh?



I have the "correct" thermocouples from them, but they can offer no explanation whatsoever for why the large difference. (BTW, they are K type, which I suspected) The odd thing is that the controller and pyrometer are pretty close to each other, but WAY different than the external probes, which I know are reasonably accurate. To further aggravate me, the engineer said that until one approaches the upper limit of the furnace, these large differences in set versus actual are to be expected! He said the expensive route was to buy a $535 digital controller. He did agree with me that I could just as well set up two more probes through the peepholes and measure setpoint vs an averaged reading from the external probes and set up a calibration curve to do further work. That is fine by me and the way I will proceed. Paragon has gotten all the $$ out of me they ever will.

The problem here is that I am experienced in the laboratory with really expensive furnaces that do not suffer these expected "deviations" and thus get aggravated when the considerably cheaper home version performs rather disappointingly. Proportional controllers and such sure do spoil one! Yeah, I know, anal retentive. But after 30 years it's not an easy habit to break when trained that precision is everything.

I guess the moral of this story I am passing on here is that one should not necessarily trust the furnace to provide the temp you think it's set to, especially if you have a manual controller. If something seems odd about the HT, then suspect the furnace setting! Man, am I glad I checked! I bought that digi-thermometer and probe to adjust the forge, and just for grins checked the furnace one winter day stuck inside!

Anyway, sorry for the rant, but it's a hard pill to swallow when they tell you their equuipment isn't quite what it's suggested to be, 17 years after the fact.

Thanks again for the info and help. Very grateful we have so many here always willing to help, with such a huge knowledge base from all walks of life!