Fast K type Thermocouple Reading

Discussion in 'The Projects Forum' started by FardinBiper, Sep 27, 2012.

  1. FardinBiper

    Thread Starter New Member

    Aug 21, 2012
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    Hi,

    I need help designing a circuit that reads K type Thermocouples at 1KHz frequency. I looked at the MAX31855 chip, however the temperature conversion rate is 100 ms. I need something 100 times faster.
    Also I need something that has Cold Junction Compensation as well.
    Maybe something with Cold Junction Compensation and a real time analog readout of thermocouple. Is there anything like this?

    Thanks
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    http://sine.ni.com/nips/cds/view/p/lang/en/nid/207150

    gives 1.4K samples per second and has 16 ch input. If you go cheaper and get the 9311 model, you get 15 samples per second. Which is plenty considering the thermal mass of the thermocouple wire and fusion weld point will prevent any meaningful temperture change in timespans under 50 ms for all but aerospace applications
     
  3. mlog

    Member

    Feb 11, 2012
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    What are you measuring? Why do you need such a fast conversion? How quickly is your temperature changing? My experience with thermocouples is that the temperature sensing rate is a function of mass fluid flow and the diameter of the thermocouple wire. Basically you need a high fluid velocity and a very thin thermocouple wire at the sensing tip.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    You are asking for a thermocouple with response times of less that 1ms.
    I am not aware of any temperature sensor with that kind of response.
    The thermal capacity of connecting wires themselves to the sensor would determine the time constant.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    It could be that OP want to oversample. And the average the data down to a more realistic datarate as part of a noise reduction scheme. That could make some sence.
     
  6. BMorse

    Senior Member

    Sep 26, 2009
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    I am currently using the MAX31855KASA Thermocouple to Digital converter IC, I am actually using 2 of them and also sampling AD values from 2 ADC channels connected to NTC thermistors, and I can read data from the MAX31855 pretty fast (Running a Pic16F887 @ 20Mhz) even using a bit-banged SPI routine. And the MAX31855KASA also has built in cold junction compensation which you can also sample the data from that simultaneously with the thermocouple data....plus it gives you errors for thermistor circuit open, thermistor shorted to Vcc, thermistor shorted to Vss....
     
  7. FardinBiper

    Thread Starter New Member

    Aug 21, 2012
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    Hi all,

    Thank you for your replies.
    The reason I need a fast temperature sensor is that I have a component that changes temperature from room temp to 100 Deg C and back down to room temp in under 300 ms. I need the temp sensor to measure this change. The point of having a temp sensor is to NOT let this component go over 110 deg C. Sort of a safety thing.
    The national instruments device is very good for this application in terms of functionality, but economically.... Not a chance...
    I have tested the MAX31855 thermo amplifier, and it is a really good IC. Unfortunately, the response time is 100 ms (from datasheet) and this is too long for me. The main reason it take this long is because of the onboard analog to serial converter. This feature is very good and you dont need calibration, however its not good for fast changing temp environment... like mine...
    As for a fast acting temp sensor, I have been playing with K type thermocouples and they are very quick to respond. I have been measuring it with a handheld temp readout device.
    I have been looking at the Analog Devices AD8495 temp amplifier that has cold junction compensation and has analog output.
    http://www.analog.com/static/imported-files/data_sheets/AD8494_8495_8496_8497.pdf
    This amplifier has a response time of 40 microseconds... Bought it from digikey yesterday, should be here today and I will be testing it... lets see how it goes...

    Thanks
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    Ok, I think you're looking at the wrong solutions.
    If you wish an over-temperature sensor, forget the amplifier or ADC response.
    You can feed the temperature sensor output to an analog comparator circuit set to trip at your max temp limit. Or you can use a thermal over-temperature switch.
     
  9. praondevou

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    Jul 9, 2011
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  10. t06afre

    AAC Fanatic!

    May 11, 2009
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    What about limeting the curent to the device? That should also protect it
     
  11. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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