So I had an opportunity to sit down, eat lunch and talk with a process control engineer. He is highly revered and often sought after when dealing with Oil and Gas process control. He is also a fellow home brewer. Being a tech guy, I was able to pull up my system on my phone and show him what I was doing with BrewPi. I will say he was highly impressed with what the BrewPi team has put together. After telling him what it cost his jaw hit the floor. You guys might get another customer out of the deal.
While geeking out over all this he brought up the point that, in his experience, he recommends setting the derivative as small as possible. In the Oil and Gas industry for example, he will always set D to 0.0001 and work up from there as needed. I will mention that this application is quite a bit different than what he normally deals with. At any rate, his first recommendation was to lower my derivative value even further. He had also pointed out a flaw in my initial assumption. Initially I had thought the thermostat on my freezer would adjust the intensity of my cooling process as well as lower the temperature. This was not true. It is merely a temperature control. With that he recommended that I lower the proportional value by another 10 degrees and see where that takes my system. As he gets more familiar with BrewPi and understand the control philosophy he might have more recommendations. But I was impressed with his quick analysis and recommendations, even more so without him having knowledge of the control matrix.
In summary, here are my new settings and results. We will see how it works throughout tomorrow. Elco, please let me know if you have a different perspective. With your knowledge I would trust your judgement over his with this setup.
Kp: -37.78. Ti: 6h. Td: 1m.