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Ultrafast Temperature Measurement

Discussion in 'Misc Electronics' started by Gerd Roethig, Jul 16, 2003.

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  1. Gerd Roethig

    Gerd Roethig Guest

    Hello all,

    for medical research purposes we are desperately looking for a good
    method to measure temperature very quickly.
    The most important thing is the time constant of the measuring system;
    it should be as low as possible.
    Microbead thermistors appear to be one possible solution but they have
    some problems, mostly mechanical ones.

    We want to measure temperature in breath air, for example in
    ventilated patients. So there are flow and water vapour that will be
    dangerous for microbead sensors.
    Encapsulated ones are not suitable since their time constant is too
    high due to the encapsulation.

    So there are two questions:

    Is there an alternative to microbead thermistors for an ultrafast
    temperature measurement?

    How can microbeads be protected against mechanical and chemical
    influences without prolonging the time constant by some encapsulation
    or things like that?

    Any hint would be greatly appreciated.

    Thank you very much for your efforts in advance and best regards

  2. Gerd Roethig

    Gerd Roethig Guest


    The time constant should be as low as possible. There are
    Thermometrics microbead sensors that are supposed to have a time
    constant of 10 ms in still air. Seems that this is the lowest possible
    They achieve this time constant by using a very low amount of material
    for the thermistor itself which makes that microbeads somewhat
    sensitive to environmental influences.
    You're partially right - we really want to measure temperature changes
    caused by certain factors. But the system must react very fast to
    these changes - that's the problem.


  3. Zak

    Zak Guest

    Wouldn't a thin platina wire be useful? Corrosion resistant and easy to
    warm up if there is any appreciable flow.

    It is big, but you have the mass to be measured - and it will react
    quickly. How thin platina wire can one get? How wind resistant is it?

  4. [snip]

    In one of the Omega temperature measuring books there is
    a reference to: "The Measurement of Respiratory Air
    Temperature", by P Webb. Rev Sci Instrumentation, Vol 23,
    pp 232-234, May 1952.

    He appears to have constructed a fine copper-constantan
    micro-thermocouple for use in nasal passages. Claimed
    results were 0.1C resolution, with the response time
    being 50% down at 200 milliseconds.

    The Omega book is "Temperature Measurement in Engineering",
    Vol 2. Might be worth obtaining, because it describes
    various resistive, thermoelectric, and velocity of sound
    techniques that have claimed responses down to 1uS.
  5. Ian Stirling

    Ian Stirling Guest

    I suspect it could be done with IR spectroscopy to work out the water/CO2
    content, then measuring the speed of sound.
    You would need also to measure the Oxygen in the incoming air.
    Measuring Argon and Nitrogen would be nice, but you could probably skip
    it, and go with just pressure.

    This could have response times in the microseconds, but may not be
    very accurate.

    The patient can't change temperature in 10ms, or even 1s significantly,
    unless they are on fire.

    Are you trying to measure peak exhalation temperature, or pick up
    faults in the equipment?

    Very fine platinum wires can be obtained, and you can accurately
    measure the resistance of these.
    This is somewhat complicated as you can't have significant current
    flowing through them due to self-heating causing large errors.

    Condensation on the sensing element may also be a problem, depending
    on where it is.

    -- | mailto: | Ian Stirling.
    "The device every conquerer, yes, every altruistic liberator should be required
    to wear on his shield... is a little girl and her kitten, at ground zero"
    - Sir Dominic Flandry in Poul Andersons 'A Knight of Ghosts and Shadows'
  6. Zak

    Zak Guest

    You could use two, and extrapolate the temperature of a currentless one
    from one with I and the other with 2I current.
    Condensation... hmm. If the wire heats up a bit as above...

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