*** Update July 2019: ***
I've now owned the Breville PolyScience Control Freak for a year. I've written a review on it for my developing website. It's essentially a summary of what I posted in this thread. If you are interested, you can read it here:
*** Update March 2019: ***
Temperature and Time Database (work in progress): https://sizzleandsear.com/precisioncooking/
i emailed vollrath about their 4-series temp control (when not using a probe), and this is what i learned:
they do sense cookware temperature and support closed-loop temp control in this manner when not using a probe. how does it work?
it seems there is a contact-based temp sensor below the glass, so it relies on heat being conducted from the cookware, through the glass, and into the sensor. this is a cookware-air-glass-sensor interface, so there's some appreciable thermal resis