My BBQ probe is saying that my dome is 90 degrees hotter than my built in thermometer

Check both of your thermometers in boiling water.  The one in your dome is adjustable.

They all nailed it.  Verify cal on probes and gauge with boiling water.

Also make sure the platesetter leg shields the temp gauge in your dome.  I use a pizza stone so I have no legs.  I use a fire brick to shield the temp gauge.  Also make sure your digital probes are not subject to direct radiant heat from the fire.

My grate temp and dome temp run 50 to 100 deg F delta for a few hours depending on temp.  They will eventually match.

@RRP- definitely Greek at a minimum (Edit):  Make sure your instruments are calibrated and then go with whatever you like.  There are thermo gradients within the BGE.  For me-better to have one thermo I can trust than two that read differently.  You can get too much info...FWIW-

lousubcap said:

@RRP- definitely Greek at a minimum  

Wouldn't surprise me that Ozzie was a member of Delta Tau Chi

DoubleEgger said:

Delta T pays my bills....

Ditto ...

While it could be some of the things mentioned, I found huge differences too using thermometers that were calibrated correctly. About a year ago I had trouble making Holiday baked goods in November and December of 2014. The identical items had come out fine in the same period in 2013. In 2014 was getting burned bottoms and under cooked tops. The same items were perfect in 2013. I tried everything I could think of, platesetter legs up, legs down, different pans. What I finally discovered was that the weather was quite a bit colder than the year before. in 2013 it was mid to high 20's and in 2014 it was single digits.  So it was taking the dome thermometer more time to get within the same ballpark as the grate level. This meant the grate temps were too hot. I discovered this by putting a calibrated thermometer at the grate level and there was often a very large difference. The two do balance out over time and get within 20-25 degrees of each other, but it takes a long time in the cold weather and/or if you open the lid a lot.

I never understood why on the BGE everyone tended to use the dome thermo. I had always been taught that you measure the cooking temp at the level you're are cooking at...the grate level. Since there were other things you do differently with the Egg, I went with the flow and used the dome thermo. When I started having issues and discovered the huge temperature discrepancies, I switched to putting a temperature probe right at my grate level. It solved all of the problems I was having at that time and I haven't had a single problem since. I know conventional wisdom with the Egg is you use the dome thermo, but I don't care. I have a 100 percent success rate. Just make sure your grate thermometer is accurate. The only time I might use the dome thermo is if I am using the AR with the rig extender to cook high up in the dome at the same level as the dome thermo. Your mileage may vary.

Jim

With regard to cooking temp-it is just a reference to what is happening within the BGE.  As long as you know your BGE performance from whatever temp measured from whatever device placed wherever then it really doesn't matter the temperature input source.  Just an opinion... 

@grege345 how did you find that pick  

@jfm0830- Gotta admit I have not worried about the off-sets that you describe, although I do know they exist.  The largest concern is with a low&slow cook and I generally give the BGE at least an hour to stabilize.  As you note-go with whatever works.  That said, the only thing I "bake" is pizza so the temperature variations don't have such an impact.  But as long as you are driving by one speedo and not multiple then you are home-free.

@lousubcap If what you do is working for you then that is great. The bottom line is how the food turns out. I can work with a temperature differential but the problem is that differential varies wildly in the coldest weather.

I don't know what the low temps are in the Winter in Louisville  but I only ran into the really severe offsets last Winter when the air temps were holding in the single digits. The winter before was milder and on some of the coldest days I was using the wok. Just an sidebar comment: There is something a bit surreal about standing outside at night when it is -2 degrees and you are having trouble keeping the temps down at your wok. So if you don't get down that low where you are, you may not have seen the problem.

When cooking indirect I usually let my Egg run for at least 30 minutes once it hits my desired temp and in real cold weather up to an hour. Also a piece of meat cooked indirect is a lot more forgiving than baked goods.The other reason I have come to like measuring the grate temp is repeatability If something takes 2 hours with a grate temp of 375, then it should take that amount of time winter spring summer or fall. Yes I know assuming a similar sized piece of meat with similar fat content etc. But I don't need to even think about the temperature difference between the dome and the grate. For me the dome thermometer has become irrelevant.