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Thermocouple Queries

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  • 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? 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.
    Attached Files
    Last edited by Hendo; 01-29-2007, 03:43 PM. Reason: Thermowell clarification & image

  • #2
    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
    "Made are tools, and born are hands"--William Blake, 1757-1827

    Comment


    • #3
      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
      "Made are tools, and born are hands"--William Blake, 1757-1827

      Comment


      • #4
        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, it’s 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, I’ll 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 o’clock position when looking from the front and 9 o’clock 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)? I’m 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. I’m having difficulty though finding something small enough. What type of enclosure did you end up with?

        Cheers, Paul.

        Comment


        • #5
          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
          "Made are tools, and born are hands"--William Blake, 1757-1827

          Comment


          • #6
            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.
            Last edited by paulages; 05-03-2007, 10:56 PM.
            -paul
            overdo it or don't do it at all!

            Comment


            • #7
              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
              Pizza Ovens
              Outdoor Fireplaces

              Comment


              • #8
                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.
                -paul
                overdo it or don't do it at all!

                Comment


                • #9
                  Re: Thermocouple Queries

                  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
                  Oven Progress
                  Bread Photos
                  Oven Stand Thread

                  Comment


                  • #10
                    Re: Thermocouple Queries

                    Anyone????
                    Oven Progress
                    Bread Photos
                    Oven Stand Thread

                    Comment


                    • #11
                      Re: Thermocouple Queries

                      Ron,

                      These are the same thing I used, though I didn't get them from this source. As I said, they were 24 gauge, glass wrapped, K type. I've had zero problems with them: they're holding up just fine and reading consistently. Just be sure you get them long enough for your situation. The wires are quite thin, and they cut easily with diagonal cutters. Be steady and sure when you strip the covering to plug into your readout. Also, read the instructions. I didn't at first, only to find out later that red is negative and green positive, but this was only with the meter I used.

                      Jim
                      "Made are tools, and born are hands"--William Blake, 1757-1827

                      Comment


                      • #12
                        Re: Thermocouple Queries

                        Jim

                        Thanks for the advice! I just wanted to get the right ones ordered so that I'd know how big to make the holes in the hearth slab. Any particular reason you went with 24 gauge?

                        Ron
                        Oven Progress
                        Bread Photos
                        Oven Stand Thread

                        Comment


                        • #13
                          Re: Thermocouple Queries

                          Hi crew,
                          I mentioned some information on my thermocouples on:

                          http://www.fornobravo.com/forum/21/n...-a-2119-2.html

                          (permalink #19) and as yet have not done any more about it. Guesss it is not overly important as I also have and use an infra red thermometer for instant temp measurements within the oven.
                          I wasn't overly fussed with putting numerous temp measuring points within the hearth and dome as once you get used to your oven and 'experienced', knowing the actual temp is not as important.
                          I purchased through ebay, a 10 station thermocouple switch from the UK and temporarily coupled it up to my readout unit. I pulled the switch to pieces to check it out, determine where the individual wires went to (as the loom was simply chopped off around 400mm from the switch), and cleaned the contacts. They were not corroded or marked, merely a little dirty with what looked like a lubricant.
                          I purchased 6 metres of new K type wire with fibreglass covering, welded the ends and pushed them down the copper tubes hopefully to their ends in the hearth and dome bricks.
                          I have 2 in the hearth, one dead centre and 10mm from the cooking surface with the other to the right around 200mm in from the edge. This one measures the bottom or soak of the cooking bricks.
                          The dome has one deep within the top centre 'key' brick and another just on the outside between the superwool thermal blanket and firebricks to guage the soak of the dome.
                          The ends were joined temporarily in an electrical junction stick? and the 2 wires from the output sent to the readout. The problem is that I can't get the same or anywhere near the same measurement as the infra red device. Guess I will pull out the wires and feed some heavier guage steel wire through the tubes to their end, mark off their length and check that the thermocouple wire is completely home when re-inserted. I will also put a little 'instrument oil' on the contacts of the meter to aid in transfer of the fine voltage that is generated from the wires at their joint.
                          I am not planning on using the oven until 12th August, so I still have plenty of time to get that done.
                          The pic shows the switch which will be mounted behind the dacia and to the rh side of the vent with the read out on the face bricks. This will be done once i get the new pergola done and waterproof the area.


                          Neill
                          Attached Files
                          Prevention is better than cure, - do it right the first time!

                          The more I learn, the more I realise how little I know


                          Neill’s Pompeiii #1
                          http://www.fornobravo.com/forum/f8/n...-1-a-2005.html
                          Neill’s kitchen underway
                          http://www.fornobravo.com/forum/f35/...rway-4591.html

                          Comment


                          • #14
                            Re: Thermocouple Queries

                            My main concern with temperature is for cooking bread. I'm not "too" worried about it for pizza since I should be able to keep an eye on it during cooking as well as get a temperature estimate with standard hand held devices if need be. Plus I'm a big nerd and would like to study the thermodynamics of my oven...lol. Below is a picture of my temperature monitor in its future resting place. I'll have to make a form for it to fit properly in the hole, as well as buy a weather proof cover for the front. It has 12 inputs but I'll likely only use 5 since thats how many come in a pack, and I doubt I'd need more than that anyway. Thanks guys!
                            Attached Files
                            Oven Progress
                            Bread Photos
                            Oven Stand Thread

                            Comment


                            • #15
                              Re: Thermocouple Queries

                              I bought these;
                              Omega part number:
                              KMQSS-125U-6. 1/8 Stainless sheath 6" K-type with miniature connectors
                              TECK10-9 3 each 10ft extensions.

                              The extension wires were cheaper than a spool and connectors seperately.

                              I work with TC's every day, so I knew I wanted stainless for longevity. The ones you pictured are ceramic sheaths that you feed the wire/bead into. That will leave the bead exposed which will corrode over time exposed to atmosphere. So it may fail in 2-3 yrs. If that is not a problem with you then go ahead with that one. Make sure you get the K-type TC and wire.
                              Wade Lively

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