The first design stage of this project was to find a solution that was capable of the required thermal performance. During this stage the sous vide cooker solution was found. The focus of this next design stage is minimizing the number and severity of operator and patient safety hazards as well as minimizing opportunities for human error in the operating procedure.

The wiring diagram above depicts the first revisions from this second stage design effort. 

The big changes:

• No more toggle switches. Toggle switches are no longer used because they maintain state if the operator unplugs the device. So if the power switch was in the "on" state and the device was unplugged and plugged back in, the safety involved with the operator being able to anticipate and be confident in their ability to control the device state is compromised. Momentary push buttons are used instead.
• Microcontroller is always powered. The microcontroller and associated safety features are always powered whenever the device is plugged in to the AC supply. Ensures that temperature of the water bath is always being monitored and displayed regardless of whether the sous vide device is on or off. 
• 4 digit, 7 segment display. During normal operation, the water bath temperature will be displayed. When a fault occurs, the display will transition between the water bath temperature and the number and type of fault that is occurring.
• LED rings on the push buttons instead of independent indicator lights. The "Sous Vide Power" and "Temperature Set" push buttons are the only means to control device state with the microcontroller once the device is plugged in. From a operator perspective, I believe it will be more intuitive that the state indicator lights that change in response to button presses are on the buttons themselves. The "Sous Vide Power" button will use a red ring (implying heater/hot) and the "Temperature Set" button (for setting water bath temperature monitoring) will use a blue ring (implying water bath). 

The main design challenge I am working through right now is on how to ensure that the operator submerges the correct length of IV tubing and to ensure that it remains submerged throughout operation. 

Most of the electronic components for this new architecture arrive this week. Hope to have a semi-operational assembly by this weekend.