Phototransistor for high temperature sensing

Discussion in 'General Electronics Chat' started by lakmalp, Jan 30, 2011.

  1. lakmalp

    Thread Starter New Member

    Oct 27, 2008
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    Hi Folks,

    I am operating a furnace having temperature reaching approx. 1900 °C and use a Thermocouple to get the temperature. But I am having a serious problem with my Thermocouple since it can only read till 1650 °C. :(

    I have not put the Thermocouple in contact with the heating point because of this. I roughly estimate the temperature based on previous calibration results. But I feel what I read is wrong due to unexpected results I get from the heating process.

    I am planning to use a Photo-transistor (View hole can be used to place the Photo-transistor) to fix this and I need some help from you Guys! I have enough knowledge in PICs.

    Basically what I need to know is an interfacing circuit to connect the Photo-transistor to PIC.

    Please let me know how can I build an interface circuit. Any help from you os greatly appreciated!

    Regards,
    /Lakmal
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What type of thermocouple are you using?
    There are special types for high temperatures upto 2300 °C
    http://www.omega.com/techref/thermcolorcodes.html
    (types G C abd D )

    How do you want to measure the temperature with a photo transistor?

    Bertus
     
  3. lakmalp

    Thread Starter New Member

    Oct 27, 2008
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    Hi,

    Yes, there are. I googled for those thermocouples and it seems they are comparatively high cost.

    I want to have a quite low cost solution. Deviation of ± 3 °C from actual temperature is acceptable.

    My idea is to detect emitted light from glowing object in the furnace and make it incident on the photo-transistor and measure the output and express it as Temperature.

    I might be heading in the wrong direction. Please help...

    Regards,
    /Lakmal
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    That is quite a contradiction, you can never have cheap and accurate at the same time. Besides, ±3°C at 2000°C is 0.3% accuracy, which I think even the right thermocouple will struggle with.

    The phototransistor might work, but there is a lot of problems with that.
    First, I have no idea if whatever you place inside the furnace and its shape will change the amount of light registered.
    Second, I am not sure how much is the amount of emmited light proportional to the tempereature, and if it even is possible to detect such small changes.

    I would definitely go with the thermocouple, more so if it is a one off project.
     
  5. bertus

    Administrator

    Apr 5, 2008
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  6. Jaguarjoe

    Active Member

    Apr 7, 2010
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    Last edited: Jan 30, 2011
  7. lakmalp

    Thread Starter New Member

    Oct 27, 2008
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    Hi,

    Thank you all for your time in replying to the thread. Having read the articles attached to the thread thought of going for "http://www.pyrometer.com/Pyro_Optical.html" one.

    Thank you again!

    /Lakmal
     
  8. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Check out stores serving pottery makers. The ovens used for firing reach those temps and higher, and they have something called 'cones'. It might be more what you want for setting equipment calibration points than for everyday use though.
     
  9. Jaguarjoe

    Active Member

    Apr 7, 2010
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    The Pyro website doesn't say much about emissivity except for molten metal. If you are cooking different things, they will have different emissivities which will affect your readings.
     
  10. lakmalp

    Thread Starter New Member

    Oct 27, 2008
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    Hi,

    Thanks!

    This is for a Geuda heat treatment process. So I am cooking only one type of thing. Once it is setup and calibrated, I think I don't have to worry about it.

    I think you mean "Seger cones". I used them. But my heat treatment process takes 3-4 hrs for a single firing. So it is difficult to keep an eye on the Seger cone till it gets melted. Thats why I am looking for an automated process.

    Though I was planning to go for Pyrometer's one, still I am thinking of a low cost one compromising the accuracy.

    Anyway I am planning to experiment with a photo-transistor too.

    Can I use the photo-transistor (with a resister) output directly to input to PIC, or do I have to build any extra circuitry? Please advice.

    Regards,
    /Lakmal
     
  11. Markd77

    Senior Member

    Sep 7, 2009
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    I just looked up the seger cone. Perhaps you could use them in some automated way with a break beam detection system.
    Using an op amp buffer is usually recommended with a resistive divider type sensor and PIC because of the characteristics of the analog input circuit in the PIC.
     
  12. Jaguarjoe

    Active Member

    Apr 7, 2010
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    A PIC ADC needs a low resistance source driving it- like 2.5k or less. It is unlikely that a simple phototransistor + resistor circuit will meet that requirement with out an opamp in between.
     
    lakmalp likes this.
  13. lakmalp

    Thread Starter New Member

    Oct 27, 2008
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    Thank you for giving useful information. I will look in to the OP-Amp. I thought I can directly feed PIC with the transistor-resistor network.
     
  14. lakmalp

    Thread Starter New Member

    Oct 27, 2008
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    I searched the internet and couldn't think of which op-amp configuration to be used. Please give me some hints regarding the op-amp configuration.
     
  15. Markd77

    Senior Member

    Sep 7, 2009
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  16. someonesdad

    Senior Member

    Jul 7, 2009
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    We used tungsten-rhenium thermocouples for high temperature measurements (over 2000 °C) over 30 years ago and it was fairly routine. But our environment was in ultrahigh vacuum systems (molecular beam epitaxy systems), which made things easier. The problem with high temperatures and most metals is the resulting oxidation of the thermocouple material.

    As you've probably discerned, optical/IR pyrometry is probably your best choice. An inexpensive IR photodiode could be a good choice. However, unless you have a calibrated spectroradiometer, you're not going to be able to calibrate your sensor. The spectroradiometer's output can be fit to a black body spectrum and the absolute temperature inferred. A hole in the oven makes a good simulation of a black body, as is well-known. Otherwise, you also have the problem of estimating the emissivity if you're trying to estimate temperature from a particular heated material.

    If I was doing this, I'd want a spectroradiometer to also measure the response of the photodiodes as a function of temperature. Otherwise, if you want to estimate temperature over a range, you'll have to know the bandwidth of the photodiode and do some integrals of the blackbody curve.

    My guess is, since you want cheap, you'll be satisfied with a relative calibration rather than an absolute one. You can use the slumping cones to provide the calibration point and thereafter use the output of the photodiodes. Note you can get Tempilaq materials that measure up to around 1000 deg C, so that could provide another data point.
     
  17. Jaguarjoe

    Active Member

    Apr 7, 2010
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    This is one of dozens of rail to rail i/o single supply op amps that will work. Its about $2.50:

    http://cds.linear.com/docs/Datasheet/16389ff.pdf

    Wire it up as a voltage follower as previously mentioned.
     
    Last edited: Feb 2, 2011
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