I've learned so much lurking here for quite a while and feel like I should try to contribute...
I've been brewing for several years, but just switched to all grain BIAB late last year. I was limited to about 3 gallon batches and was looking for an easy way to throw in some 5 gallon batches. The grainfather was appealing, but I didn't want to spend the money, so I ordered my mash and boil last Thursday. As I continued to read this thread the last few days, I became increasingly worried that I made a mistake.
The unit arrived yesterday (the box was a bit beat up, but no damage of any kind) and I immediately set up for some temp tests. I added 4 gallons of hot water and set the temp to 153. It took about 20 mins to get to temp and the heat shut off. It took 25 minutes to drop to 147 - the point that the heat turned back on. The temp was back to 153 in about 2 minutes - the heat relay definitely turned off at 153 although the temperature did briefly get to 154 (maybe 20 seconds). Again, it took 28 minutes for the heat to turn back on at 147.
After the 2 minutes to get back to 153, I quickly wrapped it with two layers of reflectix (very sloppy, with no cutouts for the handles or ball valve). This time it took 38 minutes for the temp to drop down to 147. I feel like with proper insulation, it may not even drop 6 degrees during a 60 minute mash.
I feel a bit better now after my tests - I know it may be different with grains instead of all water. At least in my case, the hysteresis in only in one direction - maybe they updated the firmware?
I've also read here that the majority of conversion takes place within the first 15 minutes of the mash. Is that true? If that's the case, I'm even less concerned given that the temperature only dropped 1-2 degrees in that time with my poor insulation.
Hopefully I'll be brewing this weekend for a real test.