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Cheap thermometer calibration technique?

Discussion in 'Electronic Equipment' started by DaveC, May 8, 2007.

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  1. DaveC

    DaveC Guest

    I know this subject of "cheap" and "calibrate" used in the same sentence may
    well be anathema to some of you but I need to verify that either my IR temp
    gun is accurate or my DMM/thermocouple is, or neither. Accuracy to 2 or 3
    degrees F is fine.

    I'm looking for suggestions for a simple way to provide some kind of common
    temperature "standard" (I use the term loosely, here) I can compare these
    against.

    Thanks,
     
  2. MooseFET

    MooseFET Guest

    The freezing/melting point of ice is quite exact. Boiling water is
    good if you correct for your local air pressure.
     
  3. John Larkin

    John Larkin Guest

    Freezing and boiling water are obvious points.

    Tapwater stirred with a lot of crushed ice will get within 10-15 mK of
    0 deg C. Boiling water, corrected for barometric pressure, will get
    within your error budget.

    You can buy a thinfilm platinum RTD, for a few dollars, that is
    accurate to a fraction of a deg C, if you measure it with a good
    ohmmeter. Some of the semiconductor temp sensors are better than 1 deg
    C.

    Fever thermometers are very good close to body temp. That and ice
    point is a combination that's hard to beat.

    John
     
  4. GregS

    GregS Guest

    I don't think i have boiled any water or iced a thermometer lately. I try to
    check out an unknown by a known in the shop. Sometimes I may have to use a third
    or fourth device to be sure. That is a problem to contend with, its very difficult to place more
    than one probe in one spot. I try to test using a beaker of water being stirred very rapidly,
    otherwise temperature differential will kill you. Measuring IR can be trickey, as I found out
    at home measuring aluminum foil. The heat of the reflection is what you will measure.
    Since I have a few thermometers on the wall at home. viewing the IR probe vs the readout
    gives a feel for the accuracies involved.

    greg
     
  5. Chuck

    Chuck Guest

    Don't forget that with your IR gun, the
    emissivity of the source is important in
    calibration. I suspect water and ice are
    not friendly calibration sources. Maybe
    a black anodized heat sink that has been
    heated to some calibrated temperature
    would be a good source. Does your gun
    have an emissivity adjustment?

    Chuck
     
  6. John Larkin

    John Larkin Guest

    If you IR measure shiny copper or brass, what you're measuring is your
    own reflection.

    I dab things with black whiteboard marker to improve the emissivity.
    Apparently Scotch Magic tape has high emissivity, too.

    John
     
  7. Ice mixed with distilled (or for your purposes, tap will be fine) water and
    allowed to rest for a few minutes will give you 32°F within millidegrees.
    Be sure to stir it every half minute or so to get the thermoclines well
    distributed.

    Boiling water at sea level will give you 212°F fairly accurately, less 1°F
    (for your stated accuracy) for every thousand feet of altitude you are above
    sea level up to a few thousand feet.

    Jim
     
  8. Not sure what you mean by "cheap", but you can get a YSI thermilinear
    network, which consists of a composite of three thermistors and some
    precision resistors for about $27. The one that goes from 0-100C
    claims +/- 0.15C accuracy (+/- 0.27F).
    That might be expensive for one calibration, but you might use it in
    the future.
    Then, you could get a black anodized heat sink with one side smooth,
    and drill little holes in the side, close to the flat surface, and
    insert the YSI thermistor and your thermocouple, maybe with some heat
    sink glop. Then point the IR detector at it. Let it go through some
    temperature changes slow enough that the whole thing has time to
    equlibrate, and collect some calibration data for both devices.
     
  9. At what temperature? You have to consider emissivity with IR
    measurements. If your DMM/thermocouple reads okay with the input
    shorted (terminal block temperature) chances are it's accurate enough
    at typical heat sink temperatures. Chances are your IR is not.


    Best regards,
    Spehro Pefhany
     
  10. DaveC

    DaveC Guest

    At what temperature?

    ~130F, max. I am frequently asked to measure the temp of a stack of press
    output (big 6-color sheet-fed presses) to determine if the IR dryer lamps are
    functioning properly. Sometimes using an IR gun is fine, but some press
    operators prefer the traditional method of a probe inserted halfway down the
    stack. It would be nice if my tools' readings corresponded.

    Re. the ice "calibration": as someone pointed out, ice & h2o isn't very
    IR-emissive, it it? Would it help to mix in a little black ink? (Lots lying
    around a print shop...)

    Thanks,
     
  11. It's possible to get a couple of degrees F error (high) from boiling
    water, particularly if you use a smooth vessel, such as a glass
    beaker, to boil the water. Maybe put something like an airstone in the
    bottom of the vessel.


    Best regards,
    Spehro Pefhany
     
  12. Rich Grise

    Rich Grise Guest

    I once used cigarette ashes and spit on a piece of aluminum. :)

    Cheers!
    Rich
     
  13. Rich Grise

    Rich Grise Guest

    Can you take the probe and the IR out to the line simultaneously and check
    a pile of paper with both of them simultaneously? You'd need the
    cooperation of the foreman and the workers, of course.

    Good Luck!
    Rich
     
  14. DaveC

    DaveC Guest

    All that IR from the lamps might be reflecting around and giving you
    Measurement is taken after printing is done and lamps are cooled (they aren't
    in direct "sight" of the delivery stack on the press, anyway).
    We've got lots of (large format) inkjet printers, too! Would h2o-soluble ink
    in the ice water increase the emissivity?
     
  15. DaveC

    DaveC Guest

    Can you take the probe and the IR out to the line simultaneously and check
    So where's the "standard" in this experiment?
     
  16. All that IR from the lamps might be reflecting around and giving you
    problems too.
    It won't mix in (it's oily), that's kinda the point with offset
    printing. ;-)

    Best regards,
    Spehro Pefhany
     
  17. John Larkin

    John Larkin Guest

    If you use a stainless thermos or Starbucks coffee mug thing, and fill
    it with stirred crushed ice and water, and fire an ir sensor down into
    that, it should be pretty good.

    I've got an ice cube on a paper plate on my desk, just starting to
    melt. Our FLIR thermal imager shows it at about +0.5C, and a cheap
    Extech IR thermometer claims -2C.

    The emissivity of the ice cube seems close to 1.0. As I image it and
    wave my hand around nearby, there's no hint of thermal reflections or
    temperature change, so it's almost totally black in the thermal IR.

    Oh my red plastic Presidential Water Cup is on my desk, too, with
    about an inch of water in the bottom. It reads 6.8C with a
    thermocouple, 7.0 on the FLIR, and 4C on the Extech. Again, no hint of
    reflections. So water also looks black at thermal wavelengths.

    John
     
  18. Tim Wescott

    Tim Wescott Guest

    Is your FLIR imager one of the cryo-cooled ones, or do you have a newer
    one (less than a decade old) with the uncooled detector?

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  19. Talk about measuring with a micrometer, marking with chalk, and cutting with
    an axe!!!

    The fellow is looking for a couple of °F accuracy.

    Jim
     
  20. John Larkin

    John Larkin Guest

    It's an E45, about a year old, uncooled. About every 30 seconds, it
    clicks and seems to close an internal shutter for a couple of seconds,
    which I assume is some sort of auto-cal of the sensor array.

    It's pretty slick, except for the USB interface which is bizarre. It's
    a network device, not memory like other cameras, and you have to
    install their strange software to talk to it.

    We can focus so close that we can image the hot-spot temp on an 0603
    resistor! Imaging an operating pcb can lead to all sorts of
    revelations.

    We looked at the Fluke, but nobody at Fluke seemed to understand it.
    Not a good sign.

    John
     
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