Hi, Dave.
Good points.
I don't know if you've read all of my long-winded posts earlier in this thread or not. I admit they're boring.
You're probably right that the position of the sensor in the MES might see some of the air coming into the system and read too cool. On the other hand, because it's right above the heating element, depending on how the incoming air moves over the heater and out of that area, it might read too high when the element is on. There's probably no perfect location for the cabinet sensor probe in such a system, especially if the heater is cycling on and off.
My main curiosity at this point stems from what I measured testing my MES. In mine, carefully controlling things, the temperature reported by the controller is reasonably far away from the temperature that the
probe is actually experiencing. So I'm not addressing the variations in temperature throughout the cabinet because I know that won't likely be very even. I was just testing the responses of the probes themselves. And what seems odd to me is that the meat probe is quite accurate (within 2 degrees F at every point I tested). But the cabinet probe in mine appears to be about ten times less accurate.
Some of my posts further up in this thread describe what I've tested on mine, how the tests were performed, and some of my observations and questions.
When you say you installed a rheostat to control the temperature, I'm guessing that you mean you've installed a dimmer circuit or maybe a variac (not as likely). If my assumption there is correct, then what you'll get is constant power to the heater.
That has the advantage that the heater will not cycle on and off. So you'll get no temperature cycling. But as we were talking about above, we can only get away with that if we use an auxiliary smoke generator of some kind. Many of us do, though, so we're free to control the heater any way we want.
The disadvantage of a constant power setup is that you'll have to "ride herd" on the temperature more than if you have a true temperature controller. But that may be of little or no consequence depending on your weather and how much you're planning on babysitting the system during the cooking. I'm lazy, and often like to cook overnight while I sleep. The remote control/readout really makes that fun! But you can still use that even if you're using a separate temperature (or power) control. You just can't make adjustments while laying in bed.
As you point out, what's important is the actual temperature of the cabinet in the area where your meat is. If we can't trust the cabinet probe that comes with the MES, then using a separate probe that you do trust makes a lot of sense.
I'm actually not all that worried about my MES's sensor/display/controller being off as long as I know how far and in what direction it's off. So characterizing mine has proven helpful to me because I know what temperature to set to achieve the actual temperature I want. And knowing that the meat probe is quite accurate gives me the confidence to make good use of it. However, I did run into something today that I wasn't aware of.
I need to test to see if what I think I observed was correct or not. I was finishing my picnic butt this afternoon, and it was reading 199 degrees IT. My reading on here told me that I ought to run it up to 205 IT. So I went out for dinner, and when I got back, it was still reading exactly 199. Hmmm. Could it be that the max temp the meat probe can read is 199? I'm sure the instructions would answer that, but you know how that goes. Does anyone really read them?
I'm also pretty certain that I ran into that when I was testing the calibration of the meat probe, but you know how it is getting old. It's been more than a week, and I've slept since then, so who knows?
If it maxes out at 199 and keeps reading 199 instead of reading "OL" or something to tell us that the reading is off scale, then I need to be aware of that! The butt might have been up to 250 IT, and I'd have never known it! I might lower myself to read the instructions. If I have to. I guess.