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Thread: Thermocouple questions

  1. #1
    kuee is offline New User
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    Default Thermocouple questions

    What is the proper way to connect a thermocouple?

    I have the following all bought from Auberins.

    1 x K Type Thermocouple w/ 6" (150 mm) Probe (WRNK-171)
    1 x K Type Thermocouple w/ alligator clip tip. (TC-K6A)
    1 x 1/16 DIN PID Temperature Controller w/ Timer (SWA-24X1)

    My plan was and is to connect both thermocouples using a switch to swap between them.

    At room temp both apear to function normally. The temp of each is with a few degrees of each other.

    I used a small length of 1/2" conduit to create a feedthrough to pass the wires through the walls of the oven.

    The first time I turned it on, the oven came to temp (400F) quickly. Maybe too quickly looking back at it. I tried to do the autotune and it seemed to complete but then shortly after the PID alarm1 went off and the PV screen showed HH. It seems that this indicates that no thermocouple is connected. I checked all my connections and tried again. It seemed to autotune ok but after it was finished the PID seemed to have trouble regulating the temp. The temp droped by more than 10F and it would not maintain it. I then got the HH error again.

    I read through some of the instructions for the PID and it suggested to ground the negative input to the body of the oven, so I tried that.

    This time the temperature rose very slowly never making it to 400F. Using an infrared thermometer the temperature was actually quite higher than indicating so I shut it back down. The temp was probably more than 100F off.

    In the above description I was using the 6" probe.

    Noticing that the temp seemed to be way off, I switched to the aligator type TC and it alarmed on me right away, I have the alarm set to 475F.

    I am not sure where I am going wrong.

    I will check the thermocouple using some ice water but I am relatively certain they are ok.

    Some notes on how I connected both TC's. I cut off the connectors of both of them and soldered on a servo connector to make it easier to run the wires and to remove them if needed. I then connected the negative side to the PID. The positive side of each TC goes to one pole each of a DPDT switch and the center of the switch goes to the positive of the PID. I used a DPDT because I wired in some LED's to show me which TC is active.

    I read some website that said you can not use just any connector on a TC. I am wondering if the servo connectors and the switch I am using is causing the problem?

    Should I ground the body of the oven to the negative side of the PID? Should I try grounding the shield of the TC to the negative of the PID?

    Do I need to try and isolate the shield of the TC from the body of the oven?

    BTW, my oven is about 3'x3'x4' using two 3k elements.

    Sorry for the long post but I hope someone has some insight.

  2. #2
    SCOTTRODS's Avatar
    SCOTTRODS is offline Metal Finishing Guru
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    Default Re: Thermocouple questions

    Quote Originally Posted by kuee View Post
    What is the proper way to connect a thermocouple?

    I have the following all bought from Auberins.

    1 x K Type Thermocouple w/ 6" (150 mm) Probe (WRNK-171)
    1 x K Type Thermocouple w/ alligator clip tip. (TC-K6A)
    1 x 1/16 DIN PID Temperature Controller w/ Timer (SWA-24X1)

    My plan was and is to connect both thermocouples using a switch to swap between them.

    At room temp both apear to function normally. The temp of each is with a few degrees of each other.

    I used a small length of 1/2" conduit to create a feedthrough to pass the wires through the walls of the oven.

    The first time I turned it on, the oven came to temp (400F) quickly. Maybe too quickly looking back at it. I tried to do the autotune and it seemed to complete but then shortly after the PID alarm1 went off and the PV screen showed HH. It seems that this indicates that no thermocouple is connected. I checked all my connections and tried again. It seemed to autotune ok but after it was finished the PID seemed to have trouble regulating the temp. The temp droped by more than 10F and it would not maintain it. I then got the HH error again.

    I read through some of the instructions for the PID and it suggested to ground the negative input to the body of the oven, so I tried that.

    This time the temperature rose very slowly never making it to 400F. Using an infrared thermometer the temperature was actually quite higher than indicating so I shut it back down. The temp was probably more than 100F off.

    In the above description I was using the 6" probe.

    Noticing that the temp seemed to be way off, I switched to the aligator type TC and it alarmed on me right away, I have the alarm set to 475F.

    I am not sure where I am going wrong.

    I will check the thermocouple using some ice water but I am relatively certain they are ok.

    Some notes on how I connected both TC's. I cut off the connectors of both of them and soldered on a servo connector to make it easier to run the wires and to remove them if needed. I then connected the negative side to the PID. The positive side of each TC goes to one pole each of a DPDT switch and the center of the switch goes to the positive of the PID. I used a DPDT because I wired in some LED's to show me which TC is active.

    I read some website that said you can not use just any connector on a TC. I am wondering if the servo connectors and the switch I am using is causing the problem?

    Should I ground the body of the oven to the negative side of the PID? Should I try grounding the shield of the TC to the negative of the PID?

    Do I need to try and isolate the shield of the TC from the body of the oven?

    BTW, my oven is about 3'x3'x4' using two 3k elements.

    Sorry for the long post but I hope someone has some insight.
    I have heard that switching between the two TC's while the PID is on, will give erroneous readings, with the Auber PID. If using a Watlow the switch works better and the temp readings are next to "instant", where the Auber controller takes a while to "catch up" to correct.

    The fun thing about the TC wiring is it's very specific in resistance..... you may be causing some errors as well, using the switch and LED's.... obviously not a lot of resistance is needed to throw bad readings. Not that I'm certain it would work, but ry deleting the LED's at least, to see if it works without them.... The shield of the TC is meant to be un-grounded normally. When you ground the TC housing it's a shield... when you "ground" the other end (open wires end) you are now making a circuit.... no more shielding (or at least a lesser Shield).

    I would recommend a separate PID to read the second TC..... it's relatively inexpensive in the grand scheme of things.


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  3. #3
    ed_denu is offline Metal Finishing Guru
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    Default Re: Thermocouple questions

    I pretty much agree with Scott. You have introduced a few variables into the TC circuit that are probably throwing off the voltage. I've used a switch to control 2 tc's a couple of times and the temps are always the same. But the only variable I introduced was the switch as I cut off the ends of the tc's, connected them directly to the switch and used the cut-off piece to run to the pid. You've add the servo connectors which probably includes some length of different type wire. Since the neg is bypassing the switch, there will be a difference there also in length of the conductors and resistance of the switch. I'm assuming that the led's are powered from a unique source and not draining voltage from the TC. Simple it down to only adding the switch to control both conductors of the TC, remove the servo wire and I'm betting it will work fine.
    Last edited by ed_denu; 03-19-2012 at 11:00 AM.

  4. #4
    kuee is offline New User
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    Default Re: Thermocouple questions

    I have spent the day reading about thermocouples. It is quite interesting at least to me.

    I assume I can just cut off the ends as you say at any point right? Strip off some of the metal sheathing and insulation and connect directly to the PID?

    Is there any sort of calibration to be done with the PID? It would seem to me that the PID needs to know the reference temperature.

    Lastly, should I minimize the length of wire that is running inside the oven? There is about 18" or so of wire that is running inside the oven.

    To answer some question above, the LED's are being switched from a separate 120V source. If I want to continue to use them and switch both of the wires I will find a 3PDT switch.

    I will simplify my setup tonight to make sure it is working correctly before trying to add the switch back in.

    Thanks.

  5. #5
    ed_denu is offline Metal Finishing Guru
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    Default Re: Thermocouple questions

    Quote Originally Posted by kuee View Post
    I have spent the day reading about thermocouples. It is quite interesting at least to me.

    I assume I can just cut off the ends as you say at any point right? Yes Strip off some of the metal sheathing and insulation and connect directly to the PID?If you are still using the switch, connect to the switch and then to the pid with the cut-off length of wire.

    Is there any sort of calibration to be done with the PID? No It would seem to me that the PID needs to know the reference temperature. As you've read, the different type wires in the TC produce a voltage proportional to a temperature difference, between either end of the two conductors. My understanding of the pid controllers is that they are capable of doing what is referred to as cold junction compensation, versus actually having the cold end at 0 degrees. Therefore the pid is able to read the voltage signal from the TC and compute the temperature within the oven at the hot end of the TC.

    Lastly, should I minimize the length of wire that is running inside the oven? There is about 18" or so of wire that is running inside the oven. I don't believe that is necessary. I've left about that amount free in my oven so I can maneuver the TC around based on the objects within the oven.

    To answer some question above, the LED's are being switched from a separate 120V source. If I want to continue to use them and switch both of the wires I will find a 3PDT switch. That's a nice addition to the oven controls(using the multiple pole switch to turn on a reference light).

    I will simplify my setup tonight to make sure it is working correctly before trying to add the switch back in.

    Thanks.
    Sounds like you are close to getting everything working.



  6. #6
    kuee is offline New User
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    Default Re: Thermocouple questions

    Ed
    I am doing a cold temp cal now. I have a handheld TC and it shows 32.6

    My TC from the oven is showing about 72. Same as room temp. This does not sound right to me. Is something broken here?

  7. #7
    ed_denu is offline Metal Finishing Guru
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    Default Re: Thermocouple questions

    If the TC within the oven is reading ambient then that sounds right. I'm not sure what you are describing for the hand held at 32.6?

  8. #8
    kuee is offline New User
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    Default Re: Thermocouple questions

    Both TC were immersed in an ice bath. The TC with the aligator clip seems to be bad. Reads the same temp all the time.

    I installed the probe one again and the oven seems to work fine.

    I ran the auto tune and it seemed to be able to hold the temp ok verified by my handheld setup.

    Do you have any suggestions on how to optimize the PID settings? It seems to take it a while to get back to temp after I open the door and it looses heat.

    How long should I expect it to take to stabilize again?

  9. #9
    ed_denu is offline Metal Finishing Guru
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    Default Re: Thermocouple questions

    Recovery from opening the door is probably most dependent on the wattage available. How long does the oven take to get from ambient to 400? Depending on to total temperature drop it could take a couple minutes to recover. As long as the pid is calling for heat then the recovery is totally dependent on the wattage available.

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