Need everyones help on a PID problem

[Arcs_n_Sparks]

MRH,

I'm with pensrock on this. Unless you really understand the grounding arrangement, a bare TC that comes in contact with metal will be a problem. You are dealing with millivolt signals, and ground loops will kill you. Bare TCs are used for the fastest temperature response, but nothing in the BS moves very quickly, and you are better off with an insulated TC.

Arcs_n_Sparks

[Mr Walleye]

I use the Omega TC and Auber controller. I have never seem any problems with mine and I did create a mounting bracket out of stainless steel that the TC snaps into. I would assume this would ground the TC because the bracket I made is screwed to the back wall.

Mike

[Arcs_n_Sparks]

Mr. Walleye,

Is the TC a grounded TC?

[Mr Walleye]

I'm not sure. It was a custom order. I'll see if I can find the numbers for it. Would the order number tell the storey?

Mike

[Mr Walleye]

Here's the number

Omega T/C Assembly W/Trans Joint Model TJ96-CASS-316U-3-SB-SMPW-M

I don't know if there is any way to tell or not.

Mike

[pensrock]

Mike, Arcs,
  From what I can tell from Mike's part number it is an ungrounded type K T/C. I'll show a link to the info from the manufacture.
http://www.omega.com/toc_asp/frameset.html?book=Temperature&file=TJ36-ICIN

At the very bottome of the info page it shows an example ant it appears the 'U' in the part number means 'ungrounded'. At least that is how I read it. But since it is a sheathed T/C I would assume the sheath touching metal would have no effect because the tip of the T/C should be isolated from the sheath itself. Does this make any sence?

Is there a reason everyone seems to be using type 'K' T/C's? I do not see why a type 'J' would not work since this is such a low temp application. We use type K for temp over 800 and up to 2100 F.

[Mr Walleye]

I just checked out the Omega site and it indicates the U (in "316U") in the model number signifies Ungrounded tip....

Ok.... So what does that mean?

Mike

[Mr Walleye]

Sorry Pensrock, I think we must have posted at the same time.

So, if I'm reading this correctly, a person would want an ungrounded tip on whatever TC you use?

Mike

[pensrock]

Mike,
I would think that would depend on the application and the thermocouple. At work our unsheathed T/C's are mostly open ungrounded, that means the welded tip of the two metals that make up the T/C are exposed but seperated along the legnth by a ceramic barrier. This type is also inside a ceramic protection tube, if the end of the ceramic tube breaks and the T/C touches metal it can cause interferance or noise on the signal to the controller, it is a millivolt signal so its does not take much to mess it up.
Our sheathed T/C's are for the most part also ungrounded, the two wires are much smaller and are placed inside the stainless steel or inconel sheath with ceramic power which is to keep the T/C wires seperated from the sheath. So in this case if the sheath touches metal, it would not matter because the ceramic powder is seperating the sheath from the T/C wires inside.
There are applications with certian controllers where they do use grounded tip T/C's but I am not that familiar with them so I cannot give you a good example.

[pensrock]

Here is a very good page explaning the types of thermocouples and the grounded, ungrounded or exposed tips of the T/C's.
http://www.omega.com/thermocouples.html

[Mr Walleye]

Thanks for the link Pensrock. From what I read, in our application in the Bradley I don't see why you would want a grounded tip. It may be quicker but I think the risk of touching metal in an all metal smoker would be greater for sure.

Mike

[pensrock]

I agree, why would we care if the ungrounded tip took another five seconds to get a reading?

[Wildcat]

WildCat

How did your butt turn out? Did you temps continue to hold?

Mike

Butt was wonderful.  Took a total of 20 hours.  It as a 9.2 pounder bone in.  Took it to 185 internal.  It stayed a long time in the 160 range.  The box temp started getting to a +/- 5 degree on a setting of 203.  Did another autotune and it eventually got to a +/- 2 degrees.  Next time I will not set the t to 0.  Had the best results before I did that with a +/- 1 degree.

[pensrock]

Wildcat, At work when we use a relay output controller, we normally have the 'cycles' setting between 20 and 32. We can keep a furnace to +-1 degree at 2100 degrees. Since you are using a SSR output, which is basiclly the same as a triac or relay output controller execpt it is putting out a DC signal, you do not need to worry about contactors snapping in and out all the time or parts on the SSR wearing out so you should be able to keep the cycle setting lower than I do but I do not feel it would be necessary to go any lower than 8-12 cycles. If you can get good control at a higher cycle setting by all means, leave it alone. When I finally get my PID controller put together, I plan to start out with a cycle setting of 24-32, I feel very comfortable to being able to tune the instrument to control within +- 2 degrees.

[Wildcat]

Thanks pensrock.  When I do an autotune, it always sets the cycle back to its original configuration.  If I remember correctly it was 26.  I posted the number earlier on this thread somewhere.  I plan to leave it alone unless I just have to.  I plan to eventually get another probe (thermocouple) to get rid of the grounding issue though.