Agilent DMM U1252B thermocouple temperature compensation

Discussion in 'General Electronics Chat' started by smilem, May 19, 2015.

  1. smilem

    Thread Starter Active Member

    Jul 23, 2008
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    Hello, I have Agilent DMM U1252B the thermocouple is +3 deg C (I calibrated with mercury thermometer ).

    Now the problem is how to auto compensate this, the DMM does not seem to have service more etc. to enter degrees for compensation, or am I wrong?

    Is it possible to add say a resistor to thermocouple etc?
     
  2. Reloadron

    Active Member

    Jan 15, 2015
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    I am guessing you want to do a CJC (Cold Junction Compensation). Using one of your precision liquid in glass thermometers note the temperature at the junction of where the TC alloy meets the Cu (Copper). Depends on how your TC is set up. That junction is the "cold" junction. Note the cold junction temperature and now look up the millivolt output for the type TC and temperature. This web page covers CJC. You will then add that mV amount to your readings. I don't know if your DMM does CJC or not but people were measuring TC outputs and converting to temperature long before the first DMM was ever invented. :) It is just a matter of understanding CJC. While an ice bath or ice point reference is a nice to have it is hardly a necessary item.

    I just looked at your meter's manual. Can you read page 45? The meter comes with a temperature probe to measure temperature. Looks to be all covered in the manual.

    Ron
     
    Last edited: May 19, 2015
  3. smilem

    Thread Starter Active Member

    Jul 23, 2008
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    Yes I read the manual, there is no compensation features in the DMM. Since this not cheap DMM I wonder why should I DIY some arduino type controller etc. to measure temperature with Agilent K type thermocouple ? Since I don't think there is a knob in the DMM for adjustment.

    The temperature is +3 deg C. I just measured a glass of water with DMM and with mercury lab grade thermometer simultaneously. I did not measure cold junction etc. because calculation tables type of compensation is no desired. I want the DMM to show the compensated (true) temperature by adjusting something inside DMM or using inline controller like arduino.

    If this sounds complicated or stupid then I'm sorry. English is not my native language perhaps I misunderstood the article you linked to.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Subtract 3°C from every reading on the meter.
     
  5. smilem

    Thread Starter Active Member

    Jul 23, 2008
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    The goal is to make DMM show true temperature. I don't think Agilent could not do with all their features etc. Or are they too stupid?
     
  6. smilem

    Thread Starter Active Member

    Jul 23, 2008
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  7. Reloadron

    Active Member

    Jan 15, 2015
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    Back in post #2 I gave you a link to the manual for your meter. No Aglent is not too stupid but apparently you don't have a clue as to how to take a simple measurement. Pretty arrogant on your part for someone using an expensive meter with no manual and not knowing how to find the manual online. This is again a link to the manual, try to read it.

    Ron
     
  8. smilem

    Thread Starter Active Member

    Jul 23, 2008
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    Measuring temperature is listed on pages 45-48.

    Page 45 lists instruction for preparing to make measurements
    Page 46 list feature of relative measurements.
    Page 47 lists relative measurements instructions that has nothing to do with accuracy, it's for measuring how one thing is warmer than the other etc.
    Page 48 has diagram of measurement.

    I'm sorry if it seems arrogant but what has this to do with thermocouple compensation?
    Do you say the proper way by Agilent is to

    1. have a mercury lab grade thermometer to measure ambient temperature,
    2. then to use "zero C" compensation on DMM
    3. measure the object temperature using DMM
    4. add the ambient temperature value to the DMM

    This works, but you need to have mercury lab grade thermometer to measure ambient temperature in the first place.
     
  9. smilem

    Thread Starter Active Member

    Jul 23, 2008
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    Page 10 lists that:

    When rotary switch is at , ETC is ON by default. You may press to disable ETC (Environment Temperature
    Compensation), will appear on display. For pulse and duty cycle measurement, press to switch trigger slope
    to positive or negative. When meter is in peak or dynamic-recording mode, press to restart 1 ms peak hold or
    dynamic recording mode.

    So maybe the ambient temp is adjustable inside the DMM etc that would correct the thermocouple results? Because ambient temperature is not correct also.
     
  10. Reloadron

    Active Member

    Jan 15, 2015
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    If you want the thing to read correctly then do a calibration at 0 C and 100 C also covered in the manual. Using the sensor you have. You should know at 0 C the allowable error is +/- 3 degrees C and at 100 C the allowable error is 3.3 degrees C. See how close you get using an ice bath and boiling pure distilled water depending on your elevation for the boiling water. The manual covers calibration.

    Now if by TC Compensation you mean offset I don't see that offered on this meter. Offset would not be any good because it would not take into consideration span.

    Actually that 3 degrees C is within rated tolerance and I believe what you want to say is you checked the accuracy because you did not calibrate the instrument.

    Ron
     
  11. smilem

    Thread Starter Active Member

    Jul 23, 2008
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    Page 148 lists calibration for 0 deg C only, where did you find 100 deg C?
     
  12. Reloadron

    Active Member

    Jan 15, 2015
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    Page 133 the list shows 0C and 100C. Something you need to be real careful with is getting to temperature, just doing temperature and getting out of temperature mode. My guess is you can just scroll to temperature, do it at those two cardinal points and scroll out without doing calibration on any other modes.You may be able to just do 0 but I don't know.

    Ron
     
  13. smilem

    Thread Starter Active Member

    Jul 23, 2008
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    Thank you for you help. The thing that bothers me the page 1 shows 3 degrees accuracy, so it seems I can't get better reading even if I do the calibration. But perhaps if I calibrate on the thermocouple that I use I would get better readings, strange. If I swap another thermocouple I think I would need to do it all over again.

    Then the "zero C" option with mercury thermometer compensation for ambient seems very good approach.
     
  14. Reloadron

    Active Member

    Jan 15, 2015
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    You pretty much called it. Doing a calibration, assuming you have accurate enough standards, would likely help. Among the problems are each and every thermocouple has limits of error. For example let's look at a Type K TC:

    Those are allowable standard deviation limits of error. Remember, this is just the actual thermocouple and does not even take into consideration the rest of the measurement plane. There is the CJC, the readout A to D conversion and a good number of things which all contribute error. There are work around and ways to improve the accuracy but they require some specialized equipment or calibration standards equipment. I think if you do a simple calibration along the lines of what I suggested you will improve but yes, as you mention, a change of thermocouple and all bets are off. Meaning if you change thermocouple the calibration would change.

    Frequently actual thermocouples are calibrated using temperature baths (low temperature) and furnaces (high temperature) for a specific range of temperatures. For example I can calibrate a Type K specific TC over a range of 0 to 1,000 degrees F or C at cardinal points of 0, 100, 200, 300 .... 1,000 and at each temperature list the TC mV output and deviation from the true temperatures. The test TC is bundled with a standard TC or PRT in a common thermal environment and at each cardinal point readings taken. While this can get involved and expensive it is one way to know what you actually have.

    Ron
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    Agilent is not stupid. Did you read the spec?
    3°C is within the error of the instrument.
     
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