Thermocouple Queries

[CanuckJim]

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

[CanuckJim]

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

[Hendo]

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? I?m 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 950?F and ceramic fibre to 2200?F!
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 don?t want to go to this degree of trouble and find that there?s 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? I?m 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.

I?d 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.