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TMP007 reading

DAN ROSS1
Intellectual 670 points
Other Parts Discussed in Thread: LMT70, TMP75C, TMP103, TMP108

I'm using a TMP007 in an application to measure air temperature of a tire. The sensor is in a aluminum tub, that on one end is a sealed connector for the I2C signals, the other end has a air chuck so that it's connected to a tire, the tube is 'sealed'. The air chuck threaded part ends up .5" from the face of the TPM007 (no window currently in front of the sensor) inside the tube. When I connect air to the air chuck, the temperature reads correct for maybe .5 seconds than displays a temperature way to hi.

The aluminum tube is black anodized on the inside. What would cause this? No window in front of the sensor? Sample rate to fast (every second)?

Please help

Thanks much.

Dan Ross

  • 0
    TI__Mastermind 36940 points
    Hi Dan,

    I need more time for this thread, and get back to you soon.

    Aaron
  • 0 in reply to Aaron Heng
    Intellectual 670 points
    Hi Aron, I meant to add this - I was thinking of adding a small black tube that would fit around the sensor which would limit the FOV of the sensor to that of the end of the air chuck I.D. This should - if my thinking is right - limit the TMP007 to react only to the air temperature. Remember, there is no airflow once the tube is pressurized to that of the incoming air pressure.

    Hope this helps.

    Dan Ross
  • 0 in reply to DAN ROSS1
    TI__Mastermind 43715 points
    Dan,

    Can you send us a drawing of what you're trying to do?

    Keep in mind your black tube has a high emissivity. If you're not trying to measure the temperature of the tube itself, you should use a raw metal with low emissivity.

    The TMP007 is very sensitive to thermal change. Air movement around the device (even just from a case fan) is known to cause settling errors.

    Ren
  • 0 in reply to Ren Schackmann
    Intellectual 670 points

    Hello,

    I'm not trying to measure the tube temperature. Envision this: an aluminum tube with a sealed connector on one end for the I2C signals. One the other end is an air chuck that simply fits onto a tire valve stem. Exactly like when your checking pressure on a tire with a mechanical type pencil gage. When the air chuck is connected to a tire valve, the tube is then sealed and pressurized to that of the tire - no airflow.

    Inside of this tube is a round PCB that has the TMP007 sensor mounted on it. This 'sensor' PCB is trapped in place by a step in the aluminum tube that the backside of the PCB reset against it, and a spacer that mounts to the front of the sensor board and keeps a .5"  distance from the end of the air chuck that threads into the tube to the sensor face. The sensor in in the exact middle of the PCB which also means it's facing the I.D. of an air chuck inside of the tube.

    When I first connect this, the initial result is correct but only for approx. .5 seconds, then the values I'm getting are way high. I'm not getting any errors on the sensor.

    I currently do not have a window in front of the sensor. I'm also thinking about modifying the spacer to add a concave function to it, which would limit the sensors FOV to that of the air chucks id.

    I'm wondering what I can do to get this straightened out? I'm sure the TMP007 does not require airflow so it has to be something in the mechanical portion. I'm trying to get a sketch for you, however it really is a simple setup.

    Looking for your ideas.

    Thanks

    Dan Ross

  • 0 in reply to DAN ROSS1
    TI__Mastermind 43715 points
    Yes, I suspected you were not trying to measure the tube. That's why I suggested that you change your black tube to a raw metal, because you will get more infrared radiation (the efficiency of which is called emissivity) from a black tube than shiny metal. This signal coming from the tube is undesired, and affects your measurement. However, this is probably not your biggest problem.

    By default, the TMP007 only measures once per second. When you say the measurement is correct for 0.5 second, I'm inclined to think you're still looking at the previous measurement. As I mentioned before, air flow, even the brief flow from the pressurization, can have a big effect on device operation. I would expect the device to need a while to recover from this change. Since the air that you seek to measure has little mass of its own, I would expect it's temperature to have equalized with the tube by the time TMP007 is ready to measure it. Additionally, gases have very poor emissivity, and thus don't emit infrared radiation for TMP007 to measure. Finally, our thermopile is protected by a film membrane. It is likely flexing when your tube is pressurized. We have tested to ensure it is not damaged by pressurization, but we have not verified operation under pressurization.

    I think you would be better served using a conventional temperature sensor with a fast response. A WCSP device like TMP007 would have the lowest mass and fastest response, but the sampling speed may not be up to par. Have you considered using something like LMT70?

    Ren
  • 0 in reply to Ren Schackmann
    Intellectual 670 points
    Ren, I looked (briefly) at the LMT70 but dismissed it because first it's analog output and second and most importantly it would require airflow across it which means the tube would not be totally sealed and the tire would lose some air even though small.

    How about placing some very thin material that has very good thermal properties and let the TMP007 'measure' that?
  • 0 in reply to Ren Schackmann
    Intellectual 670 points
    Hi Ren. I looked at the LMT70 in detail this morning and believe it would work - very impressive. I didn't mention previously that I'm also measuring tire pressure, which is why there's the mandate of no air loss so the tube has to be 'sealed' while connected to a tire. I have up to 2 seconds to perform both a temperature and pressure acquisition which is plenty of time for the LMT70. I just don't know if it can sense with accuracy temperature without airflow. I'm thinking I can make a small narrow PCB with the sensor on it that would actually fit into the air chuck inside of the tube, getting it as close as possible to the radiated temperature of the tire air without airflow.

    The pressure sensor is also I2C and I would prefer the temperature sensor to be I2C compatible vs. analog output just for less complexity and ruggedness. With that said, I'm looking at the TMP75C.

    Is my thinking on the right track with these observations?

    Thanks

    Dan Ross
  • 0 in reply to DAN ROSS1
    TI__Mastermind 43715 points
    TMP103 is a digital sensor with I2C in the same package as LMT70. Accuracy is only +/-2C.
    TMP108 is a digital sensor with I2C in a rectangular package similar to LMT70. It is longer than LMT70 in one dimension because it has 6 balls instead of 4. Accuracy is +/-0.75C.

    Ren