(yes another) Thermocouple question

[saury]

Sorry . . . I did look at about 25 thermocouple questions for answers but have decided to confirm some things here.

I'm building a sort of experimental oven, the details of which I won't bother you with right now, so I do need some quick feedback on temperature as I am using gas jets in a sort of unusual way. Some of my need for the TC's is a learning experience to see if my heat transfer calculations are reasonable.

This is a very small oven which I am casting parts of myself. The deck will be made of a standard density 2800 degree F refractory cement and the dome will be an insulating, light weight commercial 2300 F refractory cement. I need quick temp feedback on the deck temp at perhaps 3 places, center, middle, outer (side to side, not top to bottom). I also want 3 locations to measure the dome temp, like center, mid and lateral.

I have extensive electrical experience with sensors and a little with TCs, so the electrical aspects shouldn't be a problem. I have read in other TC forum responses about different ways of placing the TCs in the material:

• embed a probe in the material
• drill a hole after casting then place TC
• place a dowel or something to later remove to leave a tunnel for the TC
• Place a copper or other conduit into the castable material at the time of casting

I'm thinking that a copper conduit (1/4" pipe for ex) might be a good option. But that leads to a related question of what form of TC assembly to use? Here are some varieties:

• ceramic insulated wires terminating with the bare TC junction
• TC ungrounded probes of varying lengths (look like something you would stick in a turkey) some pointed, some flat. This includes about a 2" probe with a right angle that is used to measure exhaust gas temp up to 2300F

This page has some of the wire ones.

ThermoWorks Thermocouple Wire Probes

I sort of like the HIGH TEMP FLEXIBLE CERAMIC FIBER-INSULATED PROBE but it is relatively expensive. I guess that a metal conduit provides the best option for TC placement. That is, I could slide either a probe or wire pair down the conduit whereas with a drilled hole or bare hole I might have trouble threading the wire down the hole against rough cement. A conduit, particularly copper, could distort the temp but probably not by much.

One last question - are TC's ever just placed in the open space of the oven, say a couple inches from the dome (drilled through). Might be interesting to get a reading there as well, although open flame near by might distort the reading or damage the probe.

Regards . . .

[david s]

Re: (yes another) Thermocouple question

Kiln probes usually come with a porcelain sheath. I used a copper pipe for my mobile oven, but now use stainless steel pipe. Whatever you use it is probably better than just a hole in the refractory as there will be some wear as the thing moves with expansion all the time. You want to be able to replace it easily. Thermocouples always eventually fail and require replacement.

A note on the cast floor, if you are casting it in one piece expect it to crack. Any large piece of refractory that is going to be heated unevenly is subject to stress and therefore cracking. The larger the piece the more likely it is to crack, particularly the floor. This is why most ovens have loosely laid floor bricks. If you want to cast the thing then at least use plenty (2% by weight min) of stainless steel needles in the mix.

[kbartman]

Re: (yes another) Thermocouple question

saury,
I'm with David on not using cooper, after his suggestion I gathered up some scrap stainless and used that. I have eleven TC total on my oven. I have less then $100 total in my temperature monitoring system. Below is a post that may be of some help to you. Let me know if I have any question.

Hope this helps, follow the post back to see the pictures (click little blue arrow).

I was able to get my temperature monitoring system up and running over the weekend. Attached are a few pics to show some detail. The temperature controller is mounted in a swing out configuration that can be neatly tuck away to preserve ambiance and satisfy a few critics. I still need to finish the aux jacks and put on the permanent switch location labels and face plate. My final plan is to use the output of the controller to some how control oven temp. If anyone would like some further detail or has any questions ask I will try to help.


Here are my locations:

1 Enclosure top outside insulation air space
2 Top of dome inside brick, 1" from face
3 Top of dome on outside brick face
4 Right side mid dome inside 1" from brick face
5 Right side mid dome on outside brick face
6 Floor right on outside brick face
7 Floor center on outside brick face
8 Left side mid dome inside 1" from brick face
9 Left side mid dome on outside brick face
10 Aux 1 temperature jack
11 Aux 2 temperature jack

[saury]

Re: (yes another) Thermocouple question

Yes, the steel wires are a good idea. Found some on ebay,

Refractory Reinforcing 1" Stainless Steel Needles 10 | eBay

These are the first ones that came up in a search. 10# is more than plenty for my prototype. Maybe I should put some in the dome as well?

KB, what sort of TC's did you use? Probes? Ceramic wires? etc. And where did you get them if you recall.

Regards,

[kbartman]

Re: (yes another) Thermocouple question

KB, what sort of TC's did you use? Probes? Ceramic wires? etc. And where did you get them if you recall.

Regards,

I fashioned them out of inexpensive thermocouple extension wire. This allowed me multiple configurations and lengths. I purchased a 100 ft roll for about $60. I still have plenty left over for repairs and replacement.

[saury]

Re: (yes another) Thermocouple question

I just looked around a bit re. making thermocouples from the wire. Looks simple enough. What did you use to fuse the junction? As I look around I see fusing with a sort of spot welder, soldering and just twisting together. Soldering and twisting don't sound so good for longevity. If spot welding, what did you use?

What did you use the standard type-K yellow connectors? I see these as well as the type K wire for sale on ebay and they are reasonably priced.

And . . . what insulation did you have on the TC wire?

Thank you.

[kbartman]

Re: (yes another) Thermocouple question

I just twisted the junctions together. I figured that I could pulled them out and retwisted if they fail easily enough. I did not use connectors. The extension wire was attached directly to the controller, which doubles as the temperature read out. They are still working great after many heating cycles.

[kbartman]

Re: (yes another) Thermocouple question

I used a high temperature glass insulated wire