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Magnetic thermometers

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by mj5001, Nov 19, 2012.

  1. mj5001

    mj5001 Member

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    Do these work? I thought that heat will destroy a magnetic field over time.

    Come to think about it though I have a double wall stovepipe so I guess I would need something with a probe?

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  2. begreen

    begreen Mooderator Staff Member

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    Yes, double-wall pipe requires a probe thermometer.
    corey21 and Jags like this.
  3. Ole Olson

    Ole Olson New Member

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    I have a magnetic Thermometer that I put on the Top of My Stove in front of the Stove Pipe.. It protect's it from the fan air and over time.. Interesting to see hot warm that top plate gets..
  4. Ole Olson

    Ole Olson New Member

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    Hot / Warm ??? Need to work on my typing skill's..
  5. charly

    charly Guest

    I use this on my double wall pipe for both my stoves. Works great, no magnet to worry about , clip mounts to 1/4 hole you drill. Probe slides into bracket. Nice interference fit. Stays put. Gauge can be calibrated by just immersing the probe in boiling water. Comes with instructions, Easy to read from across the room at a quick glance. This gauge is part of an oil furnace test kit. Usually over 100 dollars. Cheapest price I found.

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  6. Sprinter

    Sprinter Minister of Fire

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    Just to answer one of the questions, the magnets used in these stovetop thermometers will not be affected by this kind of heat.

    Even with a flue thermometer, you may want a stovetop one as well.
  7. begreen

    begreen Mooderator Staff Member

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    As long as they are run in the normal range for wood heating that is correct. We have a 33 yr old Sandhill thermometer that still works fine and has good magnetic strength. But take the pipe or stove top up over 1000F and the magnet will slide right off the pipe.
  8. Sprinter

    Sprinter Minister of Fire

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    Interesting. Does it immediately regain the magnetic properties when it cools below that? Sounds like it's reaching the "Curie Point" of the material which is a very specific point for various materials.

    I hope you haven't experienced that too many times, especially on the stove top:)
  9. BobUrban

    BobUrban Minister of Fire

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    Not sure but it may be the steel becoming non-magnetic at the temp and nothing to do with the magnet?? I only say this because I play blacksmith from time to time forging damascus knives and to Heat Treat carbon steel you take it to a temp in which it no longer sticks to a magnet and then quench in oil so I know steel can get hot enough to become non-magnetic.

    Caviat: my forge gets beyond 2000 degrees and you need the steel pretty hot to do this
  10. Billybonfire

    Billybonfire Feeling the Heat

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    Hi folks,
    thought I would contribute, I think the thermometer can fall off when the coiled spring inside expands to far due to extreme heat and pushes it off the pipe.
  11. Sprinter

    Sprinter Minister of Fire

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    That's interesting as well. It sounds like you need around the same temperature for treating as the curie point, which is around 1440 F depending on any other alloy materials in there. Curious: Do you also measure the temp or just go by the magnetic properties? If that is the right temp, it will be very accurate every time.
  12. Sprinter

    Sprinter Minister of Fire

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    I never thought of that possibility... Do you have one that does that? Since iron has to get around 1400F to lose the mag properties, your idea may make more sense. I really doubt if bg would let his temps get that high:)
  13. Billybonfire

    Billybonfire Feeling the Heat

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    Hi Sprinter, never had it happen myself, never been over 800f, but if you look at the back of the thermometer (if it is like mine) you will see it wont go past 900f then it hits the stop and if the steel continues to expand the spring will buckle and push it off the pipe.
  14. Sprinter

    Sprinter Minister of Fire

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    Okay, I'll go with that one...
    Billybonfire likes this.
  15. BobUrban

    BobUrban Minister of Fire

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    Sprinter - I do not know the exact temp - there is some guess work involved in heat treating steel. Once it begins to get orange it goes in the quench tank. I then test the hardened steel with a file before I draw it back in the over to a targeted 58-62r
  16. pen

    pen There are some who call me...mod. Staff Member

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    If that were the case, that would simply be extremely poor design. I've never seen that situation occur.

    However, it is well known that heat causes magnets to lose the orientation of the domains which made them a magnet in the first place.

    Here's a visual: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/imgsol/domain.gif
    [​IMG]

    Under extreme heat, I've seen magnets lose virtually all of their magnetic properties, but gain some back once cooled, however, not as much as they had to start with.

    This is the reason that most magnetic thermometers that are meant to be used on a chimney pipe (and not set flat on a stove top) are meant to either have a screw placed through them or else a piece of wire secured to each side of the thermometer and around the stove pipe. That way, if things were to get too hot, you don't have to worry about the thermometer falling off, landing on your foot, carpet, hardwood floor, cat, dog, or anything else in the house you don't want to have a nice burn mark on.

    pen
  17. Billybonfire

    Billybonfire Feeling the Heat

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    Well it was cheap, probably made in china :)
  18. David Tackett

    David Tackett Member

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    Most normal magnets lose their ability to be a magnet at 800 degrees Fahrenheit.
  19. nellraq

    nellraq Member

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    One of my thermometers has lost all of its magnetic power and now also reads about 100 degrees too low. I'm sure it's from being overheated. The Newmac stove that I had before I got the BK would get very hot very quickly!
    Never had it glowing red, but the thermo went over 850 degrees a few times.

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