#1  
Old 01-25-2007, 03:55 AM
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Join Date: Oct 2006
Location: Adelaide, Australia
Posts: 257
Default Thermocouple Queries

I shall be installing up to 6 type K thermocouples in my Pompeii oven, mainly because the switching unit available here comes as a 6-button device, which then connects to a digital temperature display. Switch and display are expensive, while the T/Cs are relatively cheap around 7% of the total (each).

For the T/Cs in the hearth (ie cooking floor), I plan to run the wiring in a metal conduit, from a junction box mounted on the bottom of (and inside) the support slab directly under the T/C probe, to the switch unit. Access to the junction box will be possible from under the slab. The conduit will run from each junction box horizontally on the bottom of the slab to the perimeter of the oven, and then vertically up to the switch and display.

The slab will be 5 thick, and on top of this there will be 2 of Cal Sil board, then the firebrick hearth (bricks laid on the flat). So the horizontal leads will be some 6-7 below the underside of the hearth with 2 of insulating board and 4-5 of reinforced concrete between. The vertical run of conduit will be about 8 from the outside of the oven dome at its closest point, the material between comprising 1 Insulfrax on the dome and the remainder loose fill vermiculite.

Questions:
1. How hot will it get? Im wondering what type of wire insulation to select for the T/C leads, as they range from low temperature (PVC) to fibreglass insulated wire which is good to 950F and ceramic fibre to 2200F!
2. Ditto with the conduit would metal really be necessary, or would PVC plastic be sufficient? Just in case replacement is necessary one day, I dont want to go to this degree of trouble and find that theres a molten plastic mess around the wiring if I need to run another cable!
3. How are the T/C probes best fixed in the hearth, and at what depth below the top of the brick surface should they be positioned? Im proposing to fix a metal sheath (ie thermowell - see attached image) vertically to the top face of the junction box, so each 7-8 long T/C probe can attach easily to it and end up at the desired height.

Id welcome members input on these issues, as I hope to pour the structural support slab soon, and much needs to be done first!

Cheers, Paul.
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Last edited by Hendo; 01-29-2007 at 03:43 PM. Reason: Thermowell clarification & image
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  #2  
Old 01-29-2007, 08:13 AM
CanuckJim's Avatar
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Location: Prince Albert, Ontario, Canada
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Default Re: Thermocouple Queries

Paul,

Finally found your post, so here goes. It'll probably be most helpful if I describe what I did, first. I installed a six location IMC readout meter to the right of the oven door (see pic in the Photo Gallery, posted yesterday). During construction, I used four K type, 24 gauge fiberglass wrapped thermocouples (950 degree). Thought six was overkill for my purposes. For the first lead, I drilled a hole in a hearth brick, more or less dead center on the floor, to within one inch of the cooking surface. For the second lead, I drilled up through the vermic layer and about two inches into the poured slab. Both were secured using furnace cement from a caulking gun. These two were held in place by metal staples (loosely) to the block stand. I drilled a hole through a mortar joint and ran them up to where the box would be in the facade brick. The third lead was placed before the cladding was poured; I drilled a hole in the first dome brick of the arch, the first one above the soldier course, about halfway back from the front of the oven. Again, to within an inch of the oven interior. Once the cladding was set, I drilled a hole in it for the wire, at about half the thickness of the cladding. This arrangement allows me to monitor surface (more or less) temps, plus the degree to which the mass of the oven has been saturated with heat for multiple bakes. The less they're saturated, say 400 F each, the fewer bakes I'll get (6). The more saturated (600 each), I'll get about 8-10 bakes, depending.

I didn't cover the thermocouples in anything, no conduit. They're pretty tough, really, and you might just be complicating the issue. Once the oven was in use for several months, I added insulation board to the bottom of the vermic layer, and just press fit it in, sandwiching the thermcouple wires. I've had zero problems with this arrangement. If you want to keep an extensive log of oven performance, you might add another lead in the vermic or insulation layer at the bottom and one more at the top of your dome insulation.

I've had the hearth thermocouple reading 1050 degrees with no breakdown in the wire. Especially under the oven, I don't think PVC will stand up over time.

Hope that's a help.

Jim
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  #3  
Old 01-29-2007, 08:15 AM
CanuckJim's Avatar
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Default Re: Thermocouple Queries

Paul,

Should have added that the wire position for the cladding thermocouple is in approximately the same position as the one for the dome.

Jim
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  #4  
Old 01-31-2007, 05:58 PM
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Default Re: Thermocouple Queries

Jim,

Thanks for this info. It will certainly help me in decisions about thermocouple placement etc, even though my oven is a Pompeii style, rather than an Alan Scott design.

While I agree that six T/Cs is an overkill, its still relatively cheap from an overall cost point of view to have the extra ones to fill up all the switching terminals. So I was thinking of three in the hearth in a circle about two thirds from the centre to the dome wall two on the left of the oven where pizze will be cooked and one on the right at a depth of one inch below the cooking floor. When the coals are removed for bread baking, Ill have a good idea when the hearth is uniformly heated.

Alternatively, I could place all three at different depths to gain some insight into the temperature gradient of the hearth, rather than evenness of temperature around the hearth. Or a combination of these three as in the paragraph above and another in the centre at the junction of the Cal Sil board and bottom surface of the floor fire brick.

Then the remaining two probes could be deployed in the dome one just inside the face of a dome brick at around the 10 oclock position when looking from the front and 9 oclock position when looking from above, and the other on the outside of the brick dome just under the insulation blanket. What do you think? Would the first-mentioned dome T/C be better if it were an inch into the brick (measuring brick surface temperature) rather than inside the dome (measuring internal oven air temperature)? Im not sure that I appreciate the relevance of T/Cs in, or on the outside of, the top and/or bottom insulation surely brick and surface temperatures are more important?

I shall go down the metal path for boxes and conduits, rather than use PVC. I have also firmed up on how I shall fix the probes, following some good advice from a local supplier. Thermowells (see pic in previous post) will be fixed to small tins and positioned in the appropriate places on the slab support. After pouring the slab, the T/C probes are easily fixed inside each thermowell from below the slab by means of an integral compression fitting, and their wires pulled through the conduits to the switch and display.

I spotted the readout enclosure on the pic in the Photo Gallery (WOW an absolutely stunning result in beautiful surroundings!), and will require something similar for my switch and display setup. Im having difficulty though finding something small enough. What type of enclosure did you end up with?

Cheers, Paul.
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Old 02-05-2007, 08:01 AM
CanuckJim's Avatar
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Default Re: Thermocouple Queries

Paul,

You're correct, in my opinion, brick and surface temps are the most important.

As far as the arrangement of probes is concerned, you might be better off going for a gradient measurement in the hearth, rather than the circular route, because I don't think the refractory surface will vary that much once it's up to baking heat.

For the dome, I'd go with one one inch from the surface of the brick and one between the dome and insulation. That way you'd know when the bricks have been saturated with heat.

I've already emailed you about the enclosure, but for other members, I built a very sturdy box out of high quality 3/4 inch thick, 8 ply plywood (had enough kicking around in my woodshop, though you can buy quarter sheets at Home Cheapo). Used lock joints, high strength glue and screws to put it together. Added a 1/4 back fitted into a routered slot in the 3/4 ply on which to mount the junction box for power. In front of that, at the right depth, I routed in another 1/4 piece on which to mount the actual gauge. I built it strong so the facade bricks above it would not make it sag. You could also use a 1/4 piece of flat steel as a lintel for the opening, but it my case it wasn't necessary. I faced the plywood edges with white pine trim and made a small fielded panel door on my router table. The look, I think, suits the building, and it works fine. I really didn't want the LEDs showing all the time (too high tech?), hence the door.

Jim
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Old 05-03-2007, 09:55 PM
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Default Re: Thermocouple Queries

just got your PM asking for my input-- sorry, i haven't been on this board in forever. i use an infrared thermometer to check my oven's temperature, and the only two spots i even bother with any more are the floor right where the pizza will cook, and the dome on the cooking side. it varies depending on where it's pointed, but when it comes down to it, all i really ever pay attention to is if the floor has dropped below 700F. usually if it has, the dome will also be too cool.

that said, i'd put one dead center in the top, either flush with the brick or just below it; a couple in the floor, on the cook side; a couple in the walls of the dome, also on the cook side; and one on the back side of the brick somewhere to let me know how much heat is being driven and stored into the entire brick. for that matter, one under the floor would be interesting to see as well. these seem like they would be helpful for figuring out firing times before cooking. when it comes down to it, i don't think you need any of them for anything but experimentation, personally.
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Last edited by paulages; 05-03-2007 at 11:56 PM.
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Old 05-04-2007, 02:31 AM
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Default Re: Thermocouple Queries

Hey Paul,
Nice to hear from you. How and you and the oven doing?

You should drop in from time to time and let us know what you've been cooking!
James
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Old 05-04-2007, 12:10 PM
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Default Re: Thermocouple Queries

hey james-- i've been very well. thanks for asking. always busy with other projects, but my oven still gets plenty of love and use. yesterday, i made 25 pizzas, a mix of marinera, margherita, and my favorite--caramelized onions and olive oil. afterward i baked a couple loaves of bread and roasted a bunch of eggplant and bell peppers. yum.
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Old 07-30-2007, 04:39 PM
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Default Re: Thermocouple Queries

Quote:
Originally Posted by CanuckJim View Post
Paul,

Finally found your post, so here goes. It'll probably be most helpful if I describe what I did, first. I installed a six location IMC readout meter to the right of the oven door (see pic in the Photo Gallery, posted yesterday). During construction, I used four K type, 24 gauge fiberglass wrapped thermocouples (950 degree). Thought six was overkill for my purposes. For the first lead, I drilled a hole in a hearth brick, more or less dead center on the floor, to within one inch of the cooking surface. For the second lead, I drilled up through the vermic layer and about two inches into the poured slab. Both were secured using furnace cement from a caulking gun. These two were held in place by metal staples (loosely) to the block stand. I drilled a hole through a mortar joint and ran them up to where the box would be in the facade brick. The third lead was placed before the cladding was poured; I drilled a hole in the first dome brick of the arch, the first one above the soldier course, about halfway back from the front of the oven. Again, to within an inch of the oven interior. Once the cladding was set, I drilled a hole in it for the wire, at about half the thickness of the cladding. This arrangement allows me to monitor surface (more or less) temps, plus the degree to which the mass of the oven has been saturated with heat for multiple bakes. The less they're saturated, say 400 F each, the fewer bakes I'll get (6). The more saturated (600 each), I'll get about 8-10 bakes, depending.

I didn't cover the thermocouples in anything, no conduit. They're pretty tough, really, and you might just be complicating the issue. Once the oven was in use for several months, I added insulation board to the bottom of the vermic layer, and just press fit it in, sandwiching the thermcouple wires. I've had zero problems with this arrangement. If you want to keep an extensive log of oven performance, you might add another lead in the vermic or insulation layer at the bottom and one more at the top of your dome insulation.

I've had the hearth thermocouple reading 1050 degrees with no breakdown in the wire. Especially under the oven, I don't think PVC will stand up over time.

Hope that's a help.

Jim

Jim

What specific thermocouples did you use, and are they still holding up well for you? Was it these ( Ready-Made Insulated Thermocouples, Models 5TC-GG, 5TC-TT ) or something totally different? Anyone else who's put thermocouples in their oven, feel free to chime in as well. Thanks!

Ron
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Old 07-31-2007, 03:24 PM
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Default Re: Thermocouple Queries

Anyone????
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