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Thread: Thermocouple inaccurate at higher temperatures

  1. #1
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    Thermocouple inaccurate at higher temperatures

    Hi All,

    I've been having issues with my thermocouple channel (tried 2 different thermocouples) with them being accurate in boiling water, but reading inaccurately at higher temperatures (250-275).

    I noticed my dome temperature was always much lower than what was displayed on the heatermeter, and cooks were taking longer than expected. I finally compared grate temp with my thermoworks dot and HM side by side. I had 270 on the thermoworks, and 350 on the heatermeter.

    Has anyone experienced this? I'm going to take the case apart and inspect the solder joints closely, but figured I'd ask.

    Thanks

  2. #2
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    Quote Originally Posted by Chris Smeraglinolo View Post
    Hi All,

    I've been having issues with my thermocouple channel (tried 2 different thermocouples) with them being accurate in boiling water, but reading inaccurately at higher temperatures (250-275).

    I noticed my dome temperature was always much lower than what was displayed on the heatermeter, and cooks were taking longer than expected. I finally compared grate temp with my thermoworks dot and HM side by side. I had 270 on the thermoworks, and 350 on the heatermeter.

    Has anyone experienced this? I'm going to take the case apart and inspect the solder joints closely, but figured I'd ask.

    Thanks
    Did you calibrate it?

  3. #3
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    Quote Originally Posted by Chris Smeraglinolo View Post
    Hi All,

    I've been having issues with my thermocouple channel (tried 2 different thermocouples) with them being accurate in boiling water, but reading inaccurately at higher temperatures (250-275).

    I noticed my dome temperature was always much lower than what was displayed on the heatermeter, and cooks were taking longer than expected. I finally compared grate temp with my thermoworks dot and HM side by side. I had 270 on the thermoworks, and 350 on the heatermeter.

    Has anyone experienced this? I'm going to take the case apart and inspect the solder joints closely, but figured I'd ask.

    Thanks
    Did you calibrate it? If its right at 212 it should be at higher.
    Grate temps can vary a lot by an inch or two apart

    And....the different mass of different probes heats up at different rates. A heavy meat probe may be several minutes behind a bare wire thermocouple . If temp never stabilizes, they can read widely different, 20+ easy.

    Wire them touching, put in hot oven and leave for 30 min, and see if different.
    Last edited by MartinB; 03-06-2019 at 06:11 PM.

  4. #4
    TVWBB Honor Circle Bryan Mayland's Avatar
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    That's definitely weird. There should be 0 offset in the configuration and then the mv/C should be within a few percent of 5.0 (so like 4.9-5.1) so if you've had to enter some numbers that don't fit that envelope, then that would point to why you're seeing strange temps. You can also measure the 3.3V voltage because HeaterMeter sort of assumes that the 3.3V rail is very close to 3300mV and uses this value to calculate the thermocouple scaling. Finally, you might want to put it into an oven that can get to 500F and watch as the temperature goes from around 410F to 460F which is where it transitions between the low scale and high scale. If there's a giant temperature jump on the way up, then could indicate there's unexpected voltage apart from the 3300mV or an atmega with a really crazy bandgap reference voltage.

    Oh something else I just thought of is that you can measure the thermocouple output voltage too, to see if there's an error coming out of the amplifier or a software error. Measure between the "GND" and "0" on the "PROBE" header over the probe jacks. Take that voltage, say 0.750V, divide by 0.005 and that's the temperature, so that would be 150C (302F).
    I'm that HeaterMeter guy what ruins everybody's free time.

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