Home Forums General Thermocouple grounding

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  • deejay1272
    Participant
    Post count: 20

    Hi Everyone,

    I’m using the osPID to control the temperature of a boiler in my home espresso machine. In order to do this, I have to bolt a thermocouple to a solid brass boiler to get the best measurement accuracy. My challenge, however, is that by bolting my “grounded” thermocouple to the brass boiler, I’m effectively grounding it at another point and, as I have learned, introducing a potential voltage difference between it and the osPID system. This potential voltage difference is causing instabilities of sorts (when this happens, the display reads “ERROR”).

    Here’s my question: how do I ground my osPID? I see a GND pin hole in the input card, so should I solder a ground wire to run from inside the osPID box to a suitable common ground between the thermocouple/osPID system?

    Thanks for your help!

    -Dan

    deejay1272
    Participant
    Post count: 20

    Further to the above post, I’m also experiencing considerable noise in my thermocouple measurement (I am using this TC: http://www.omega.com/pptst/WT.html). Any input regarding filtering techniques would also be appreciated.

    Thanks,
    Dan

    qglass
    Participant
    Post count: 1

    For noise, Adafruit suggests using a 0.01uF cap across the thermocouple leads in noisy environments. They note that the MAX31855 is very sensitive… Check http://learn.adafruit.com/thermocouple/f-dot-a-q.

    Kevin

    deejay1272
    Participant
    Post count: 20

    Thanks for the help, Kevin. I believe this noise issue has been resolved in a bug fix released by the developers (http://www.ospid.com/docs/index.php?title=Temperature_Input_Card_v1.2_BugFix)

    All other readers: Note that I’ve edited my original post to clarify my question a bit. Please have a read above and let me know if anyone else has experience with grounding the osPID.

    rocketscream
    Keymaster
    Post count: 65

    Hi Dan,

    The MAX31855KASA+ chip doesn’t allow any grounded leads on the thermocouple. If you look at the MAX31855KASA library I wrote, you’ll notice that I would look for those error conditions to happen (shorted to grounded, shorted to VCC, and open). In the osPID main firmware, when these error are detected, the error will pop out. The MAX31855KASA+ chip uses a differential amplifier without grounding any of the thermocouple lead in order to measure below 0 Degree C.

    deejay1272
    Participant
    Post count: 20

    Thanks for the input. I have switched over to an isolated thermocouple to measure temperature. I’m having success with this approach now.

    -Dan

    hairykiwi
    Participant
    Post count: 7

    I ran into this exact same problem late last year. I discovered that powering the osPID through its front mini-USB socket using a USB recharge wall-wart effectively isolated the osPID power supply from mains earth. It’s not ideal, sure, but at least it allowed me to make some way better coffee.

    For reference, I used the following Type K thermocouple drilled out (very carefully – the copper washer is so thin it needs to be punched rather than drilled) and mounted by one of the existing 3 mm tapped holes in the boiler: http://uk.rs-online.com/web/p/thermocouples/6212287/

    deejay1272 – which isolated thermocouple did you use and where did you buy it?

    deejay1272
    Participant
    Post count: 20

    @hairykiwi – I’m afraid I can’t claim to be using a particularly smart solution. I’m simply using a standard bead K-type thermocouple (the absolute cheapest, simplest K-type thermocouple). In order to isolate the metal bead at the tip of the TC, I’m simply wrapping the tip with tape! To measure the temp of the boiler of my espresso machine, I’m simply using a zip tie and some neoprene insulation to hold the TC firmly to the boiler wall.

    atommat
    Participant
    Post count: 9

    just to keep this thread alive, because its important….

    Ive had significant problems with the osPID. ive been using a kiln thermocouple that wasnt grounded and the readings have been zeroing out erratically when i hit ~900c. Ive been informed again and again that grounding is the issue. Ive no real idea what this means, im no electrical engineer but I switched to a thermocouple that was shielded (the shielding was then earthed) and BINGO…. back in business.

    My other PID from Auberins is reportedly compatible with grounded and ungrounded thermocouples. I can therefore deduce that the osPID is only compatible with grounded thermocouples. Unless someone can explain how to make the osPID compatible with UNgrounded thermocouples then you should stay clear of them.

    heres the proof….

    [attachment=0:q7vofqjh]Screen Shot 2014-02-03 at 18.45.51.png[/attachment:q7vofqjh]

    mat

    Cracked
    Participant
    Post count: 44

    Are you sure the thermocouple itself is grounded, and not just the shielding (which I suspect has no electrical connection to the shielding).

    I use an unshielded k-type bead thermocouple. I get errors if it makes electrical contact with something grounded.

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