Thermocouple Readings Incorrect in BGE

I've done 4 cooks so far with my Heatermeter with the Rotodamper. So far, everything seems to work great, and my egg stays with +/- 3 degrees of the set point. However, I realized everything was getting done a lot faster than before, and I found an issue with my Thermocouple readings.....

My Thermocouple is reading 30-65 degrees lower than it should when it is mounted in my Big Green Egg, when I check it vs other probes. This causes my real temps to be higher than they should be.

I have the exact same Auber Washer Thermocouple, and have it mounted the exact same way as Byran in this thread:

http://tvwbb.com/showthread.php?54009-Washer-Thermocouple-Mounting-to-Big-Green-Egg&highlight=thermocouple


Here is what I've checked:

1.) When I remove the Thermocouple from the egg, it shows the correct ambient temp.
2.) I boiled some water, and all 3 of my probes and Thermocouple show within +/- 1 degree of 212.8.
3.) I set our kitchen oven to 200, 250, 300, 350, and the Thermocouple reads correctly within a few degrees in all cases.
4.) I removed the analog temp gauge from the dome of my Egg, and stuck my wife's Thermoworks Chef Alarm probe through the dome, with the Thermocouple clipped to the probe inside.The Chef Alarm shows 295F while the Thermocouple on the Heatermeter shows 225F.

I'm not sure exactly why this is happening??

The only thing that I can think of that may be some type of weird grounding issue, since I'm using a 100' power extension cord out to my Egg. I was thinking about buying some ceramic bolts / standoffs to keep the Thermocouple away from all of the other metal in the egg to see if it makes a difference, or trying another 12v adapter.

If anyone has any ideas, I would appreciate it.

I have not experienced this, but I recall Bryan talking about having the TC readings go bad when he touched it while barefoot (I think only on his initial prototype TC circuit though, what HM board are you running?)
TC's come in grounded and isolated varieties, it sounds like you might have two grounded type TC's? I have two TC's, one expensive unit from ThermoWorks, and another cheapo from Amazon, both work perfect, but I confirmed they were both isolated type TC's before I bought them.
Also, do you have the TC plugged directly into the HM or somehow running it through the CAT5 cable?

I'm running a new 4.2.4 board.

The Thermocouple is from Auber Instruments. It's the 6' K-type with a washer probe. I have it plugged directly into the Heatermeter. No Cat 5 cable.

http://www.auberins.com/index.php?main_page=product_info&cPath=20_3&products_id=307

OK, no CAT5 issues, and the 4.2.4 should be good too... The Auberins page doesn't mention whether that TC is grounded or isolated, so you might shoot them an email and ask. These sort of issues that crop up when the TC touches things generally means its a grounded TC, I would guess with an isolated TC you wouldn't have these problems, I don't but I made sure the TC's I bought were isolated. If that's the case the ceramic isolators may help, and you might try and old school (heavy) wall wart that has a transformer rather than the new light weight switching mode power supply.

It's a washer probe so it's not isolated at all. Yeah it is probably some sort of grounding issue but I'm at a loss as to how to fix it. You could try replacing the 1k SMD resistor that's by the TC- pin with a larger or smaller value. If you remove it you can use a regular through-hole resistor between TC- and GND.

Are you reading the temps with your other probes at the same place as the placement of the washer thermocouple? Temperatures are not the same at every point in a smoker the grill temp will be very different then the temp with a washer that is above the cooking grate. On my UDS the temperature varies 50 degrees within 10 inches of dead center and one inch below the grate.

Thanks for all of the information guys. I really appreciate it.

When I purchased the Thermocouple, I didn't even realize that there were isolated vs grounded types.

When I was testing yesterday, I had the other probes right next to (and in one test attached to) the end of the Thermocouple. They were still 40-60 degrees apart.

I did some additional quick testing this afternoon. The Thermocouple reads correctly if I put it in the egg a couple of inches, (just touching the gasket.) and don't let it touch any metal. As soon as it touches the metal band around the handle, the grill, or anything else metal, the temp starts drop until its way off.

I'm going to purchase an isolated Thermocouple, and try it out. If anyone has any recommendations, let me know. I looked at the Auber and Thermoworks website, and they don't really list which units are grounded vs isolated. I was going to contact customer service and ask them.

Thanks!