Pretty much, yes -- when you use thermocouples, there end up being three dissimilar-metal junctions of interest:

• The thermocouple itself
• The "left" wire of the thermocouple to the connector to the PCB (copper trace, eventually)
• The "right" wire of the thermocouple to the connector to the PCB

If you don't compensate for the two thermocouples you've made at the PCB, you'll have a hard time reading the "real" one. Measuring the ambient temperature around the PCB is a good start. Some of the more sophisticated designs make sure that the PCB connections and the thermal sensor are, as much as possible, tied together thermally, often with a big block of aluminum (or copper, if you are feeling wealthy).

There's a pretty good schematic of the "cold junction" on Wikipedia's Thermocouple article

Jeff wrote:Pretty much, yes -- when you use thermocouples, there end up being three dissimilar-metal junctions of interest:

• The thermocouple itself
• The "left" wire of the thermocouple to the connector to the PCB (copper trace, eventually)
• The "right" wire of the thermocouple to the connector to the PCB

If you don't compensate for the two thermocouples you've made at the PCB, you'll have a hard time reading the "real" one. Measuring the ambient temperature around the PCB is a good start. Some of the more sophisticated designs make sure that the PCB connections and the thermal sensor are, as much as possible, tied together thermally, often with a big block of aluminum (or copper, if you are feeling wealthy).

There's a pretty good schematic of the "cold junction" on Wikipedia's Thermocouple article

OK, reading the wikipedia stuff, you want to measure the temp at the TC input to the TC4.

http://code.google.com/p/tc4-shield/

Looking at the pic, the 9800 is the tiny IC next to the 4 TC connectors, which on the left of the board in the board pic. The IC is between the 2 resistors and the TC connectors.

And, the nice thing about an arduino based system is it's easy to add a compensation factor, if you need/want to. I have seen a meter that lets you do this, maybe the better meters have it though, never really looked either.

Randy

I have one of these:
http://www.sparkfun.com/products/306

they claim "+-3 degrees accuracy"; looking at the datasheet, this part has 3 degrees "calibration accuracy", and +-0.05% "Stability vs. Temperature". I originally got it long before the TC4 came out, when I was going to build a roast controller using my Arduino (from scratch). I lucked out and found a programmable PID controller for less than half the cost of an arduino, so that's what I use

Jeff wrote:Some of the more sophisticated designs make sure that the PCB connections and the thermal sensor are, as much as possible, tied together thermally, often with a big block of aluminum (or copper, if you are feeling wealthy).

There is a provision in the TC4 design (not documented) that would allow you to do this. The idea would be to epoxy a brass (or copper, aluminum, etc.) bar to the backs of the green wiring terminals.

Instead of reflowing it to the main PCB, the MCP9800 then gets mounted on a small breakout board. There are already 4 extra vias added to the PCB for the flywires needed to connect to the breakout board.

The breakout board with the 9800 then gets epoxied to the back side of brass bar. This arrangement would provide a nice isothermal block for the cold junctions of the 4 thermocouples.

I haven't found this refinement to be necessary, though. Apparently the combination of a nearly solid copper ground plane on the back side of the TC4, plus the low mass of the green connectors, is enough to maintain a nearly constant (and small) offset between the junctions and the ambient temperature sensor.

Jim