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Fast response air temperature measurement

Discussion in 'Electronic Design' started by John Devereux, Oct 4, 2009.

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  1. Hi,

    I am looking for a small temperature probe with a fast response in still
    air, ideally 10ms or better.

    ~0-100 degrees C is fine, accuracy not that critical say +/- 1

    I am not having much success with google etc...

    Any suggestions appreciated.

  2. Do you really want to measure air temperature or something else?

    If air, perhaps a very fine Pt RTD or thermocouple.

    You could also consider IR or fiber optic measurement of something
    thin and low heat capacity.

    Best regards,
    Spehro Pefhany
  3. Jon Kirwan

    Jon Kirwan Guest

    Possibly, rare earth phosphor thermometry.

    Technically, if you can arrange the optics or use an appropriately
    thin fiber optic cable (less than 10 micron?) you may be able to
    achieve that as a physical sensor matter. I routinely do 100ms
    measurements with rather thicker fibers (200 micron.) Obviously, your
    source characteristics are also important in deciding just what is
    good enough and what isn't. Repeatability using phosphors depends on
    the instrument and method but roughly about 30mK. Accuracy can easily
    be had to 500mK over that range. (There are at least four methods I
    know about, each with differing specs to them.) One nice thing is
    that you can literally _paint_ the surface you want to observe so that
    even the fiber doesn't touch the surface and affect the temperature.
    If the paint has minimal mass, it will track VERY FAST. The method is
    good enough that it can be applied (and is, in fact) to IC wafers and
    used to monitor their temperatures, in situ. (And you know that IC
    manufacturers cannot tolerate much variation over the wafer and
    certainly nothing touching it that might make a cool spot. Lamp
    heating is used and the opposite side can be used to avoid changing
    the emissivity of the heated side.) Usable temp ranges from maybe
    -200C to maybe +500C for two or three of the methods used. Your range
    is well inside this and, in fact, is just about the perfect range for
    it since most everything is 'easy' there. 100 samples per second is
    not impossible.

    Do a search on these methods. You should see at least two companies
    involved, readily. Fibers can be pretty thin. I've used 10 micron,
    but I gather down to 2 micron is available.. maybe less. No real
    impact on the method, so those should work. I stay active in these
    areas, but not with those companies now. You may contact me
    privately, if you want. I can say more, that way.

  4. Jamie

    Jamie Guest

    Have you looked at Ir types?
  5. Jon Kirwan

    Jon Kirwan Guest

    Varies with wavelength, pressure, gas constituents (altitude affects
    H20 significantly), temperature, ... well, it varies.

    Not to mention that 0-100C is where most everything around is also

  6. What's the emissivity of air?

    Best regards,
    Spehro Pefhany
  7. Hi Spehro,

    Yes, it is for actual air. The air will likely not be *completely*
    still, but I don't really know what movements there will be.
    That is the sort of thing I have been looking for, but all I could find
    commercially (RS and Farnell etc) were bead thermocouples/thermistors
    with thermal time constants in the ~1s region.
    That would be hard due to the location, I really need a probe-style
    device of some sort.

  8. You might want to go trolling around for hot wire anemometry probes,
    and run them at a low enough current that they don't heat up too much.

    They can have surprisingly fast response under wind tunnel conditions,
    so they might be fast enough for your application with air almost

    Best regards,
    Spehro Pefhany
  9. Jon Kirwan

    Jon Kirwan Guest

    I'm there. I just failed to notice this was about measuring 'air'
    when I posted. I've never done this, but temperature dependent
    fluorescent properties are probably found in some gases and aerosols.
    The oxygen in the air may quench it, though. Air is a tough target
    for those rates.

    I suppose a good thing is that it is still air. If it had a velocity,
    there'd be a need a way to calibrate out that effect on most sensors
    that were not moving with it at the same speed and direction.
    Yes, I gathered that just after I posted. My mistake. It may yet be
    possible in the situation. But we just don't know enough details (or
    I don't, at the moment of writing this.)

  10. Jon Kirwan

    Jon Kirwan Guest

    If it wasn't clear, your point about heat transfer in still air and
    mine regarding net velocity against the sensor provide a dilemma. To
    get better transfer, circulate lots of air over the sensor implying
    relative velocities; but to get accuracy, don't do that as the
    velocity cannot be discerned vs the temperature by some sensor which
    is affected by the statistics of molecular/atomic bombardment. I
    remember that this presented a problem in measuring exhaust gases,
    some time back, trying to discern which part was which part in what
    the sensor 'observed.'

  11. Hi,

    Yes, one of the reasons for the fast temperature change is due to a
    pressure change, which I am already measuring. Other reasons are the
    temperature of the enclosure and of the incoming air.

    Perhaps I can use the pressure changes to "predict" the temperature
    changes as I think you are suggesting. I still need a fairly fast single
    temperature measurement, less than 1 second. (But better than 10ms in
    this scenario).
  12. Sure, good idea, that might help.

  13. Hi,

    Yes it does sound do-able. Maybe a couple of transducers facing each
    other across a fixed gap, track the phase changes?
  14. Hi Bill,

    I did think of that, also bouncing it off one wall. Unfortunately I
    don't trust the volume to be sufficiently consistent. Although if I can
    use something like that to actually *measure* the volume that would be
    interesting in its own right, will look it up.

  15. Actually it *is* a closed container, but not a very rigid or accurately
    defined one. (Sorry to dribble out these scraps of information, don't
    mean to!)
    That was the sort of thing I had in mind (but you have thought it
    through better!)
  16. I would look at using a single self-resonant piezo bender element
    mounted in a Helmholtz cavity with a small opening to the atmosphere,
    then just read the frequency off with a counter. This could be really,
    really inexpensive, in fact it should work using an ordinary piezo
  17. Ah, I see where you got that 100kHz from now! I was assuming looking at
    the phase all along - there will probably be less than a wavelength
    between Tx and Rx anyway.
    Should be pretty strong with the transducers facing each other. Gap
    would really need to be <~10mm anyway for the application.
    It's A few litres, but a bit variable and floppy unfortunately.
  18. This thread should be titled, "Fast air response temperature" measurement,
    i.e., "FART" meaasurement. ;-P

  19. Steve

    Steve Guest

    Omage sells some non-insultated thermocouples that are 36 gauge wire
    fro about $5 each.
  20. Thanks Steve. I looked at their site again and found some that I didn't
    find before.

    There is a 0.025mm one with 50ms response in still air. That may be fast
    enough for what I want (since there should be *some* air movement).

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