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Calibrating high temperature instruments

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  • Calibrating high temperature instruments

    I've been measuring the flue gas temperature of my oil burner furnace lately, and I find that no two thermometers or instruments I have read the same at around 290 degrees C ( 554 degrees F). I have a glass / mercury thermometer , and two type K thermocouple instruments. I've become a bit reluctant to use the mercury filled thermometer any more , since if it breaks off the mercury would fall into my heat exchanger. I can calibrate all of my instruments at 212 degrees F , of course, with boiling water. But how do I calibrate at higher temperatures? I'm not looking for absolute accuracy better than plus or minus 20 degrees F at 500 F , but I do want consistency. A way to calibrate at , say 350 to 400 F would be better than nothing. I've looked at the wood stove thermometers sold, mostly intended for double walled pipe, and they're rather crude and heavily influenced by surface temperature.

  • #2
    The melting temperature of 63/37 lead/tin solder is exactly 361.4 degrees Fahrenheit. 60/40 solder has a small crystallizing range centered at 370F. The range is about 10 degrees either way. Coat the thermocouple with soot and stick it in a pool of solder as it freezes. Then heat the solder to remove it. The soot will prevent the solder from sticking.
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    • #3
      Bill,
      I don't have any links handy, but they make a wide range of temperature sticks that are somewhat like a crayon that are also supposed to be quite accurate and well within your accuracy levels. They melt at the range their rated for. Templaque? might be one brand name. Someone here will know the correct name. Brownell's gunsmithing supply at one time sold them, and may still do that.

      Pete

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      • #4
        Just thinking about this, can the colour of something be "measured" from a digital photograph and thereby find the temperature?

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        • #5
          Not really. There are too many sources of error, especially in the camera. Nearly all camera do automatic white balance which immediately destroys the colour fidelity. Even switched off there are many steps in the chain before it is presented to your eyes where the balance will be altered. It is impossible to present an image as seen by the eye at the same fidelity and colour gamut via any other imaging device. Not difficult but impossible.

          It is possible to exactly characterise every step in the chain so that the sources of error are known and quantified but that does not solve the problem of limited colour gamut.
          Last edited by Evan; 11-28-2012, 04:54 PM.
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          • #6
            You could use the melting point of pure lead.

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            • #7
              Finding some pure lead could be very difficult.
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              • #8
                Pure (for reasonable values of purity) lead's fairly easy to get. I can buy it at my local metal store (metals supermarkets). It's sometimes available at plumbing supply houses. For the OP, since he's in the states, he can just order it at http://www.rotometals.com/product-p/leadingotpure.htm.

                J

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                • #9
                  Lead has a rather higher melting point than the OP asked for at 621.5F. He also probably has some solder at hand.
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                  • #10
                    I'll go out on a limb and say the 2 Type K thermocouples are accurate and it is the means you use to measure the resultant Mv from the Thermocouple you are calibrating.

                    It always is ...

                    I calibrate 100s if not 1000s of Type J, Type K, Type E thermocouples in the course of commissioning a Combined Cycle Power Plant or a Delayed Coker Petrolieum Refining plant and it is always the same - we measure the Thermocouple's result and calibrate the controler's measurement.

                    More often then not the Thermocouple works, or it doesn't. Long lengths of interconnecting thermocouple cable, or special purpose thermocouple terminal spaces or the controller/measurement means might have problems with accuracy - but the thermocouple doesn't. It works or it doesn't.

                    Any Type K Industrial Grade probe you can find off Eboner that passes an "Ice Point" calibration and your boiling point calibration will be plenty accurate for your needs.

                    I would be much more concerned with what I was measuring it with

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                    • #11
                      If the mercury thermometer is industrial quality wont it be more than accurate enough to calibrate your thermocouples?

                      Slide them all inside a steel tube without touching the wall and heat the outside of the tube with a torch to maintain a stable temperature at the level you wish to calibrate.

                      Phil

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                      • #12
                        This is a calibration I did of a type K compared to a very accurate platinum/palladium calibrated thermocouple. Vertical divisions are 50C to 900 degrees C. The offset is based on the same reading at 50C. If the type K is offset up by about 25C the average error will be approximately split. The calibration was done in very controlled conditions in my heat treat oven with both probes within a few millimetres of each other. Data was recorded from both instruments simultaneously by photography.

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                        • #13
                          Slide them all inside a steel tube without touching the wall and heat the outside of the tube with a torch to maintain a stable temperature at the level you wish to calibrate.
                          That will not be accurate. There will be a hot side and a cold side with significant differences in the internal infrared emission. To make it work the sensors will need to be in an insulating jacket such as a ceramic tube within the steel tube.
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                          • #14
                            Depends on how you use the torch. Covering with sand is also a possibility

                            The point was you can use the mercury thermometer to do the calibration.

                            Phil

                            Originally posted by Evan View Post
                            That will not be accurate. There will be a hot side and a cold side with significant differences in the internal infrared emission. To make it work the sensors will need to be in an insulating jacket such as a ceramic tube within the steel tube.

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                            • #15
                              Thanks for the ideas . I agree that good quality type K thermocouples ( mine are industrial made in the USA Thermoelectric ) are probably uniform and accurate enough, but it's my instruments that have questionable calibrations. Going through my " stuff", I've found an additional thermocouple pyrometer, and a new old stock dial type thermometer. I think I'll try them all at the same temperature in the flue when it stabilizes, throw out the low and high reading devices, and use the one that is closest to the average of the remaining three. It will probably be accurate enough, and as long as it's consistent will do well for my uses. I'll keep a record of the readings of the other devices as backups. Oddly enough, it's the glass/mercury laboratory thermometer that reads about 40 degrees F low, and it's supposed to be a high quality made in USA device ( from about 25 years ago).
                              Last edited by Bill736; 11-28-2012, 11:08 PM.

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