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Thread: How I wired a PID to control temperature

  1. #41
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    1. Yes. TCs are directional. If the temp goes down when heated, it is backwards.
    2. No
    3. Yes.

  2. #42
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    The jumper wire is for RTD only. Type "K" TCs are not RTDs
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  3. #43
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    Quote Originally Posted by 6bg6ga View Post
    I do have a question. I ordered a thermocouple that is a type K and it has two terminals. The picture that I found on wiring is showing a three terminal connection. Can anyone help me on this?
    You're using this to control a lube-sizer or candle melter?
    The 'K' type is overkill. A cheap (although they aren't as cheap as a K in any case) RTD is easier to use. It doesn't require special wire or have a polarity. It's also more accurate, but that's not likely to matter in this application. The RTD is more limited in it's temperature range.

    <It Might be better if only those that know what they are talking about answered these questions. Not that that has ever bothered anyone on the misinformation super highway before.>

  4. #44
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    I'm using it to control the heater on my Star sizer. I had intended to have my thermocouple serve double duty in both the base of my sizer and as a temperature monitor in a smoker that I plan to buy when it warms up.

    I am NOT using it for candle making. In a prior post I commented that I would advise that poster purchase an additional thermocouple for their intended usage.

    Polarity won't be a concern because I can flop the wires if I put it on incorrectly the first time around. With respect to shielded wire... I have a number of different gauges and types of shielded wire at my disposal.

  5. #45
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    “an armed society is a polite society.”
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  6. #46
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    Quote Originally Posted by 6bg6ga View Post
    With respect to shielded wire... I have a number of different gauges and types of shielded wire at my disposal.
    You can't (easily) mix wire types in a K type thermocouple; You need to use the wire provided.

  7. #47
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    Quote:
    You can't (easily) mix wire types in a K type thermocouple; You need to use the wire provided.

    If the thermocouple has a shield around the pair and or a overall shield it might be necessary to use the same type of wire. If it doesn't then it is of no major concern. I have used unshielded thermocouples in the past with no problems.


    Within reason you are correct. If I need to add I will use a shielded wire and it will work fine. If the thermocouple is not shielded it won't matter.

  8. #48
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    Quote Originally Posted by 6bg6ga View Post
    . If I need to add I will use a shielded wire and it will work fine. If the thermocouple is not shielded it won't matter.
    You Are In Error.

  9. #49
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    If you guys want a good read, try this one.

    http://jumoplus.com/pdf_educational/...easurement.pdf

    I met the guy a few times. He knows EVERYTHING about thermocouples.

  10. #50
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    Talking

    Thank You Lee! I downloaded it.

  11. #51
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    All thermocouples are calibrated with a specific length of a certain wire.
    If you use a longer/shorter or different wire , you will affect the reading.
    The type "K" is popular for high temperature applications for its longevity and accuracy.
    RTDs are required when the wire length cannot be exacted because wire length is less critical. Example: The bottom of a nuclear reactor that has several hundred feet of wire to the control.
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  12. #52
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    Extension wire - Extension grade wires made of the same metals as a higher-grade thermocouple are used to connect it to a measuring instrument some distance away without introducing additional junctions between dissimilar materials which would generate unwanted voltages; the connections to the extension wires, being of like metals, do not generate a voltage. In the case of platinum thermocouples, extension wire is a copper alloy, since it would be prohibitively expensive to use platinum for extension wires. The extension wire is specified to have a very similar thermal coefficient of EMF to the thermocouple, but only over a narrow range of temperatures; this reduces the cost significantly.

    The temperature-measuring instrument must have high input impedance to prevent any significant current draw from the thermocouple, to prevent a resistive voltage drop across the wire. Changes in metallurgy along the length of the thermocouple (such as termination strips or changes in thermocouple type wire) will introduce another thermocouple junction which affects measurement accuracy.

    I found the above information and while it does state that a change would take place I personally view an addition of 6-12" that I had planned when using my thermocouple on a smoker would have no large consequences in accuracy. If we really want to open this to interpretation then the people using a terminal strip should take a good hard look at the consequences of doing this.

    Now if we really want to get this on a gnats butt then when I get my PID, thermocouple, and SSR I would be more than happy to document the differences in accuracy between the stock length of wire on the thermocouple and the addition of say 6",12" 2 feet, 3 feet and so forth.

    Now I really appreciate those with the Your Wrong comment. Your absolutely correct. There is a difference in the addition of extra wire. How much? I'm sure that no one here is qualified to tell me because no one has documented what the error will be. Any impedance difference or resistance difference would be very small if the same gauge and type of wire was used. The proposed length that I had planned might be 1-2 degrees in accuracy. Now this spread over several hundred degrees or even 80 degrees isn't anything to get upset over in my humble opinion. I'm sure that I could design a compensating circuit to bring thermocouple back into 100% accuracy.

    .

  13. #53
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    I'm sure that I could design a compensating circuit to bring thermocouple back into 100% accuracy.
    Just switch to RTD and don't worry about it.
    Just because something can be done does not mean that it is worth while.
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  14. #54
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    Get an RTD and all your troubles will be gone. A three wire RTD is best. The controller will compensate for the lead wire resistance and ambient temperature fluctuations.

    A class B pt100 has an accuracy of 0.3 C at 0C.

  15. #55
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    I would like to see a circuit to tare a temp meter. We can use just barely boiling water to zero the gaget. Why? Just for shix and grins. One dial for gross, and another for fine tune. ... felix
    felix

  16. #56
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    Quote Originally Posted by 6bg6ga View Post
    Now I really appreciate those with the Your Wrong comment. Your absolutely correct. There is a difference in the addition of extra wire. How much? I'm sure that no one here is qualified to tell me because no one has documented what the error will be. Any impedance difference or resistance difference would be very small if the same gauge and type of wire was used. The proposed length that I had planned might be 1-2 degrees in accuracy. Now this spread over several hundred degrees or even 80 degrees isn't anything to get upset over in my humble opinion. I'm sure that I could design a compensating circuit to bring thermocouple back into 100% accuracy.

    .
    I can tell you how much when you tell me the wire you'll use. Impedance has nothing (or at least very littel) to do with K types. K TCs generate a voltage at the junction that is a function of temp, called 'dielectric'.

    Since I am an EE, I am entitled to have the Professional Opinion of you're wrong.
    Last edited by Frozone; 01-12-2011 at 03:11 AM.

  17. #57
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    Quote Originally Posted by felix View Post
    I would like to see a circuit to tare a temp meter. We can use just barely boiling water to zero the gaget. felix
    Except water boils at different temps depending on barometric pressure.
    You can't cook noodles on mt. Everest, boiling water won't get hot enough.

  18. #58
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    I know. I have been delegated to make coffee everyday before she wakes. The best coffee taste consistently is made by the hobo method to match the water flow with the granule size for the amount of coffee. It requires several attempts to make a new coffee lot taste ideal, and that typically is very dependent on the time and heat of the water passing through the granules. For example, excessive heat and time kills caffeine, and gives a guttural taste to the coffee. ... felix
    Last edited by felix; 01-12-2011 at 10:11 AM.
    felix

  19. #59
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    The PID and stuff arrived today. I wired it up and I have a PV reading of 76deg and a SV of 71 Can I get these closer?

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    closer to what?
    PV should be 'process value' and is the temperature read by the TC. I'm going to guess it room temp at this point, That will be the actual temp of the melt in operation.
    SV is 'set value' and is the target temperature you wish the PID to maintain.

    Hold the TC in your fingers (or put it in hot water) and see if the PV rises.

    What PID do you have? They all vary a little in how to set. You can't (realisticly) adjust PV you can SV.
    Last edited by Frozone; 01-12-2011 at 10:58 PM.

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