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Temperature Measurement System Design

Discussion in 'Electronic Design' started by Davidson, Apr 27, 2004.

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  1. Davidson

    Davidson Guest

    Hi, everyone. I have a project concerned with a temperature
    measurement system design. My proposal is to do a body thermometer
    that display the result on a PC(therefore I set the temperature
    measuring range to about 25 ~ 45). One project requirement is to
    design a "measurement system" - we can't use an off-shelf sensor
    package (e.g. LM35)- we must use a temperature transducer together
    with linearization and signal conditioning circuit. The result must be
    outputed to a computer.

    My problem is, are there any good (simple, accuracy around 0.5~1
    centigrade is fine) circuit design that I can use? I thought I would
    use a thermistor, but somebody said that a thermistor is not used
    unless I want to design a boiler?! Could I use a thermistor to
    construct a simple circuit? Or are there any other better transducer I
    can use?

    My project is only paper-design.

    Thanks in advance.
    Davidson
     
  2. Robert Baer

    Robert Baer Guest

    Thermistors are fine, and would seem to "fit" the requirement of
    "necessary" linearization.
    Most thermistors have specs at room temp anyway.
     
  3. I'm selling temperature controllers that have a stability
    approaching 0.01 degrees Celsius. They are based on NTC
    sensors and need calibration for absolute values.


    Rene
     
  4. legg

    legg Guest

    A thermometer is a 'patient applied part' in medical product safety
    classification.

    You might get browney points by taking this into account, in your
    design, and allowing for suitable isolation in interfacing to the
    'PC'.

    RL
     
  5. Fred Bloggs

    Fred Bloggs Guest

    Where on/in the body is the measurement to be made? Skin, tongue, anus?
     
  6. Bill Sloman

    Bill Sloman Guest

    You can buy glass encapsulated "interchangeable" thermistors that have
    stabilities of the order of 0.001C per year below 45C, and initial
    accuracies of +/-0.05C

    http://www.ysi.com/extranet/TEMPKL....34e470aaf6efd3d885256ac400478e95!OpenDocument

    Newark stock some of the range, which means that you can buy small
    quantities via Farnell.

    Similar parts have been used in "microdegree" temperature controllers
    stable to down to +/-3.5uC. E-mail me - my e-mail address is real - if
    you want more details.
     
  7. Davidson

    Davidson Guest

    Firstly, thanks for all the replies above.

    This project is meant to test my ability to design a measurement
    system (i am a 4th year university student), therefore we are not
    allowed to use any prebuild, linearized sensor package (e.g. LM35 is
    not allowed). Also, this is a paper design project, we don't need to
    build the instrument, but we need to do all the other tasks (e.g.
    ananlysis, simulations, etc.)

    Davidson
     
  8. Guest

    You may be able to answer a long standing uncertainty:
    Platinum resistance, NTC thermistor of the best type.
    Quartz crystal resonator specially cut for temperature measurement.
    Which yields the best absolute accuracy initially if cabrated ie thermal
    signal to noise.
    Best long term drift.
    Ive generally used NTC thermistors for crystal oven temperature control
    the absolute temperature is fairly unimportant as the temperature is set
    for each crystal.
    Hammond of HP designed a crystal with a linear tc and 1Hz? / deg K
    that was used in a meter that indicated udeg, A linear correspondence of
    freq/temp is no longer valuable and the crystal is not made anymore (nor
    is the instrument)
    I also remember that the mechanical trimming process to take out
    thermistor manufacturing tolerance degrades the aging performance of
    some thermistors.

    Note dont reply to demon emailmail

    Doug.
     
  9. Bill Sloman

    Bill Sloman Guest

    Standards laboratories use platinum resistors as temperature sensors.
    Some thermistors are pretty good, if you don't dissipate too much
    power in them, nor use them at too high a temperature - one of my
    ex-colleagues found that his went slightly but definitely unstable
    above 55C.

    Quartz crystal resonators for temperature measurement appear to be a
    minority taste.
    The main advantage of themistors is that their resistance varies ten
    times as fast with temperature (-4%/C) as does the reistance of
    platinum (closer to +0.4% per degree C).
     
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