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Airflow??

Benedict Hartmann
Prodigy 30 points
Other Parts Discussed in Thread: TMP006, TMP102

Hey,

I am from Germany and got a problem with the TMP006EVM accurancy. I want to measure the temperature of a rotating system by using the TMP006EVM. The distance between the TMP006EVM and the object is 2-3mm. My problem is, that the system is not all the time rotating. It stops, accelerates, breaks. Everytime, when it stops the temperature falls within a few seconds and rises within a few seconds, when the system starts again. Even, if I just puff a bit, this thermal behaviour starts. When I puff with warm air, the temperature falls, with cold air, it raises.

I also got a problem with calculating the object temperature. When I use the given parameters (object voltage, local temperature), my calculated object temperature is always too low. At the given example my calculated temperature is: 75,51 °C

What is the physical property? And how can I solve the problem?

 

  • 0
    Genius 5430 points

    Benedict;

    The reason that your readings are consistently low may be due to the low emissivity of the object's surface that you are measuring. Black surfaces have a higher emissivity than white or shiny surfaces-- but this is tricky because we are talking about emissivity in the infrared portion of the spectrum, not in the visible. If you have ever left a shiny chrome-plated wrench lying in the Arizona desert sun for a while and then picked it up, you learn a painful lesson. The chrome surface is shiny (emissive) in the visible but not in the infrared-- the chrome surface does not radiate away the absorbed heat well so it gets HOT.

    You might try painting the surface with a very thin coat of flat black paint to see if the measured accuracy improves.

  • 0 in reply to Neil Albaugh
    Prodigy 30 points

    Thanks for this quick answer. I already painted the surface with a black enamel coating. So the surface is already prepared. The emission coefficient is around 0.85-0.9.

    What do you mean with our low readings? Is in your opinion the Sensor Voltage too low? Does the sensor have problems with airflow or rotating systems? I guess this is our main problem

  • 0 in reply to Benedict Hartmann
    Genius 5430 points

    Benedict;

    If you have corrected your reading for the target emissivity, then I will have to defer an answer to others.

  • 0
    TI__Mastermind 27715 points

    Hello Benedict,

    The behavior you're observing with the TMP006EVM has also been seen in our own tests when temperature transients are introduced into the system, since the IR thermopile in the TMP006 is sensitive to conducted and radiated IR energy from below the sensor as well as the radiated IR energy coming in from above.

    What happens is that these transient events, such as a puff of air, cause the PCB temperature and the TMP006 die temperature to drift apart from each other due to the thermal time constant of the thermopile. This difference in temperatures causes a heat transfer between the IR sensor and the PCB to occur. Because of the small distance between the PCB and the bottom of the sensor, this heat energy will be conducted (as opposed to radiated) through the thin layer of air between the IR sensor and the PCB below it, which causes offsets in the IR sensor voltage readings and ultimately leads to temperature calculation errors.

    I've attached a presentation showing an test we performed comparing the object temperature of the TMP006 to its local temperature sensor value and the temperature reported by the TMP102. You can clearly see that when the local temperature increases quickly, the measured object temperature will drop significantly until the system stabilizes. Similarly, when the local temperature decreases quickly the measured object temperature will spike before settling to the proper value.

    These overshoots and undershoots in measured object temperature can be suppressed in software by using a transient correction algorithm. The algorithm essentially monitors the local temperature over a 4-second interval and uses the data to calculate a local temperature slope. This slope, as well as the known thermal resistance and capacitance of the TMP006 thermopile, are then used to correct the sensor voltage reading and therefore the target object temperature.

    I'm in the process of updating the TMP006EVM software and user's guide to include this transient correction functionality. If you like, you can test the new version of the software by downloading it at this link: ftp://ftp.ti.com/pub/linear_apps/TMP006/TMP006EVM%20Installer.zip

    Best regards,

    Ian Williams
    Linear Applications Engineer
    High Performance Linear and Sensing Products 

    tmp006 transient correction_Sept 8th.pdf
  • 0 in reply to Ian Williams
    Prodigy 30 points

    Thank you really much for this answer. I will test the new software and give you a feedback