Do you measure metal temperature and how ?

Discussion in 'Foundry tools and flasks' started by metallab, Oct 31, 2020.

  1. dennis

    dennis Silver Banner Member

    Good to know my supposition has some truth to it, i.e. better to measure instead of guessing regarding what's in the pot.
     
  2. Jason

    Jason Gold

    I heated it to around 300c in the exhaust. Maybe not hot enough to keep the metal from sticking. I should have sprayed it with boron nitride.
     
  3. HT1

    HT1 Silver Banner Member

    I just gently rub it off on the vent hole of the furnace , I take the Temop through the vent hole , but yes preheat

    V/r HT1
     
  4. rocco

    rocco Silver

    K-type thermocouples with an inconel sheath are available on ebay for less than $50, any thoughts on whether or not these would be usable and durable without additional protection?
     
  5. dennis

    dennis Silver Banner Member

    I recall someone speaking of using electrical conduit to hold the probe. I'm wondering how long it - the probe-holder - should be?
     
  6. HT1

    HT1 Silver Banner Member

    far enough you dont burn your piddies :)

    measure the leads on the probe, that is how long the conduit needs to be . I might have trimmed a little off

    Here is the video I made


     
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  7. rocco

    rocco Silver

    Here's my set-up, I used a piece of 1/2" conduit to extend the probe and form a handle.
    Thermocouple 2.jpg
     
    Last edited: Dec 17, 2020
  8. Melterskelter

    Melterskelter Gold Banner Member

    I am making a new tip for my pyrometer. I learned a couple things about accidentally including a thermocouple due to poor circuit design in my pyrometer. This was due to my misconception as to how thermocouples actually work.

    http://www.advindsys.com/ApNotes/tcfundamentals.pdf

    It is NOT the actual join point of the two wires that generates voltage but rather the lengths of the wires leading to the join. Anyway, my circuit, after an "upgrade" of my pyro probe tip, included an assymetric circuit of copper to the positive side of the couple on one side and stailess steel (the support tube) to the negative side. When all was cool prior to testing metal temperature the pyro zeroed out perfectly and simply heating the tip resulted in a voltage rise and return to zero on cooling just as hoped and expected. But, when actually inserting the probe into a hot furnace the voltage rose and then started to FALL!!! WTF? Well, I was heating the tye S thermocouple initialy, as intended. But then the pyro tube started to heat and generate a counter voltage---falling voltage reading. Then it zeroed out to a negative voltage! Confusing. Until I realized on the upgrade I went from copper wire to pyro probe contact and from pyro probe contact via copper wire to mV meter to a configuration of copper to positive and SS to negative. In other words, there were TWO thermocouples in the ciruit.

    Anyway, here is my evolving tip:
    The ID is .375 and makes contact with the negative side of the TC. The lead is 3/32 brass soldered into a recess cut into the wall of the brass tube. Looks messy, but gets files and cleaned up later.
    New tip socket2.JPG

    The brownish red collumn is phenolic sheet washers stacked and glued to form the insulator and support fot the socket and positive contact. New tip socket3.JPG New tip socket4.JPG New tip socket5.JPG

    Here is a trash tip woitih the paper tube cut off so you cna see the contacts that will connect with the outer 3/8 socket and a 1/8" center conductor also brass. Pyro2.JPG

    More later.
    Denis
     
    Last edited: Dec 26, 2020
    dennis likes this.
  9. Jason

    Jason Gold

    That is correct, it comes down to the length. Anyone that saw that nightmare of mine with the chromal/alumal pins 2years ago would see the importance of wire length. They have to be really close in length and if there is a computer on one end, I only could shorten my cable by quarter of an INCH! It's only creating the reading based on resistance between the two wires. If you recall, the real bitch is what can be used to make connections. With no tech data and the factory experts all dead now, the answer was safety silv 45, flux and braze with a tiny torch. If I would have had to change that cable, it would have cost over 20k and god knows how many man hours! I'm still reading egt perfectly some 500hrs later. Given the choice of looks or brains, I'll take ugly anyday!
     
  10. Melterskelter

    Melterskelter Gold Banner Member

    Here are a few more pics of the the tip I am building. I wish I had taken more pics as I went along, but sometimes I just get caught up in the machine work and forget to document steps.

    Here is the end that plugs into the thermocouple probe.
    In order:
    Outer steel shell .625 diameter
    Phenolic insulator shell
    Brass .375 I.D. contact
    phenolic insulator
    .125" dia brass contact
    Pyro Tip.JPG

    The outer contact conductor courses through the phenolic insulator via a hole drilled about .187" off the center axis. (Not documented in photos unfortunately.)
    Pyro Tip1.JPG

    These are the sockets that will connect to mica wire (high temp) that will course down the tube of the pyrometer to the mV meter. It is important that the leasds be symmetric in construction but equality of length is not important. In this case leads go from brass to copper on both the positive and negative sides.
    Pyro Tip3.JPG

    Pyro Tip4.JPG

    Denis
     
  11. John Homer

    John Homer Silver Banner Member

  12. John Homer

    John Homer Silver Banner Member

    Got my Mifco thermocouple today. Thanks for the information all
     
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  13. Petee716

    Petee716 Gold Banner Member

    I have some serious suspicions about my current pyrometer setup. It just ain’t right. I decided to join the others here and ordered the Mifco probe last night.

    Pete
     
    Jason likes this.
  14. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Denis,

    I was kind of curious about the use of those spade connectors inline with the TC wire. I was under the impression that introducing different metals into the the TC wire change induces error because like the TC wires, those metals have a difference in potential too. Now maybe that can be washed out thrugh calibration but I would think it would create a biased raading if using a say a meter that expected a particular type of TC. Does it?

    Best,
    Kelly
     
  15. Melterskelter

    Melterskelter Gold Banner Member

    After I built my pyrometer, I went down to the foundry south of me and put mine to the test side-by-side their commercially built pyrometer. Mine measured within 25 degrees of theirs. That is as closely as my meter could read voltages.

    It is not a metal junction in thermocouples that generates voltage potential.

    Here is a reference that discusses thermocouples and briefly talks about their function.
    https://www.edn.com/thermocouple-physics-even-jim-williams-got-it-wrong/
    "
    BY MARTIN ROWE

    Early in my editorial career, I wrote an article about thermocouples in Test & Measurement World where I incorrectly said that a voltage proportional to the junction temperature develops across the junction of two dissimilar metals. A reader wrote me a detailed letter (on paper, no e-mail then) explaining the Seebeck effect. Ever since, whenever I see anything about thermocouples using the incorrect description, I realize that yet another author is perpetuating the same old error."

    Here is more and Wikipedia has an article as well. This is an area of physics that is not intuitive as we have no everyday experience with it. https://dducollegedu.ac.in/Datafiles/cms/ecourse content/Applied Physics_Thermoelectric effect.pdf

    Denis
     
  16. John Homer

    John Homer Silver Banner Member

    Thanks for the information. I built as you suggested and it works great. Thanks again
     
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  17. HT1

    HT1 Silver Banner Member

    your very welcome
     
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  18. Petee716

    Petee716 Gold Banner Member

    I second that!

    Pete
     
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  19. 0maha

    0maha Copper

    Gotta say, pretty impressed with both Clay-King and MIFCo.

    Ordered from each on Tuesday. Arrived today (Thursday).

    I got a "Single Channel Digital Pyrometer - UT320A" and a thermocouple connector from Clay-King.

    The connector with the green wire is a thermocouple that came with the pyrometer.

    [​IMG]

    Got the probe ("8 inch replacement tip with leads, part #004041) from MIFCO. Other than arriving wrapped in the New York Times, it looks fine:

    [​IMG]

    Seems like putting this together should be pretty straightfoward.

    And I want to confirm: You guys just pre-heat the probe in the chimney coming out the foundry then plunge it in? That's it? About how long does it take once immersed to give a reading?

    What happens if my melt (bronze) is overheated beyond 2400F? Do I end up smoking the probe, or does it just not work accurately at those temps?

    What's your practice? Do you measure repeatedly, trying to work up to pouring temp, or do you let it get over-temp then cool down to where you want to pour?

    While you're at it, go take a look at my progress thread. Got some more questions over there: http://forums.thehomefoundry.org/index.php?threads/im-back-updates-and-a-question.1942/

    Cheers!

    Jeff
     
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  20. HT1

    HT1 Silver Banner Member

    Go watch my video again, if you just use the thermo0couple as is without protecting the ceramic insulators, you will have a super short lived pyrometer , but yes you want to preheat the very tip of the thermocouple, it only reads the temperature at the tiny nub at the end where the wires are connected , the rest of the metal probe is to allow you to go through thick layers of slag and such... No none of us will ever need that much, but it gets the heat away fron the bare wires inside the ceramic insulators



    V/r HT1
     
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