Erratic readings from thermocouple amplifier (MAX6675)

[RosscoS]

Hi everyone,
I am working on a project for a PID temperature controller for a home espresso machine but am having some problems with erratic temperature readings. The K-type thermocouple washer has been attached to the espresso machine boiler and while the espresso machine is NOT plugged in to the power socket the readings are stable and accurate, (This makes me think that my problem is an electrical one as opposed to a coding problem).
However as soon as the machine is plugged in to the power socket (not yet turned on at the machines power switch), the readings become very erratic.

I have tried using mica washers to electrically isolate the thermocouple (and thermal grease to help the heat conduct), but this has made no difference.

The power supply of the Arduino which is in turn is powering the MAX6675 breakout board is a non grounded walwart,

Would I be correct to assume that this is causing a ground loop? If that is the case would I need to ground both the thermocouple washer as well as connecting the negative side of the thermocouple at the screw terminal on the MAX6675 breakout board to the espresso machines ground (chassis) ?

Any help would be greatly appreciated.

[RosscoS]

Ok, I have just discovered that when I unplug the power to the espresso machine the erratic readings are gone after a few seconds. I also noticed that after I disconnect the power to the espresso machine that the readings are about 10-15 degrees F higher than when the power is connected and the espresso machine is on.

This leads me to think that inductive noise is causing both the erratic readings & lower readings.
I think I need a shielded thermocouple, however I do not know which end of the thermocouple shield should be connected to ground.

Should I connect the shield to the GND pin on the MAX6675 breakout board, or to the negative thermocouple input of the MAX6675 breakout board, or should I connect the shield to the thermocouple sensing tip? Or some other method?

Any suggestions would be appreciated!

[john444]

Hi RosscoS,

This leads me to think that inductive noise is causing both the erratic readings & lower readings.

I suspect you are correct about noise causing your erratic readings. However, it may be capacitive instead of inductively coupled.
If isolating the temp sensing circuit is not working then my suggestion is to connect the breakout GND to the mechanical ground of the espresso. Do not ground the thermocouple wires. If your thermocouple has a shield then also connect it to the GND on the breakout but not the thermocouple tip. Electrically isolate the thermocouple wires from the espresso's mechanical ground (green wire) as well as the neutral (white) and especially the hot (black) wire.
How are you powering the temp senser?

[RosscoS]

If isolating the temp sensing circuit is not working then my suggestion is to connect the breakout GND to the mechanical ground of the espresso. Do not ground the thermocouple wires. If your thermocouple has a shield then also connect it to the GND on the breakout but not the thermocouple tip. Electrically isolate the thermocouple wires from the espresso's mechanical ground (green wire) as well as the neutral (white) and especially the hot (black) wire.
How are you powering the temp senser?

Hi John, thanks for your reply!

The MAX6675 breakout board is being powered by the Arduino it is connected to which in turn is being powered by a walwart (which has no earth pin).

So let me see if I am understanding this correctly, connect the GND of the MAX6675 breakout board to the GND (chassis?) of the espresso machine?

[adafruit_support_bill]

So let me see if I am understanding this correctly, connect the GND of the MAX6675 breakout board to the GND (chassis?) of the espresso machine?

You will want to connect the Arduino ground to the chassis ground also.

[john444]

Do not connect the Arduino with a seprate wire to the espresso's ground in addition to the GND on the break-out.
There should be only 1 wire to the espresso's ground. You do not want to create a loop.
You want to have 1 GND wire from the Arduino to the break-out then 1 wire from the break-out to the espresso's ground.

[RosscoS]

After a lengthy delay, I have had the machine & PID controller shipped to me. I connected GND between the PID unit and GND on the Silvia Espresso maker which solved the problem. I tested this by removing the GND connection and observed the PID Process Value reading jumping around (+/- 20°F), reconnected it and the readings are rock solid.

Happy days!

Thanks all for your help!

[adafruit_support_bill]

Great! Enjoy the espresso.