You have the concept. Remember, Clarke is charging a refrigerator to work in 10-20 deg space. No airflow over the evaporator.Am I understanding this things purpose correctly? Going back to Clark's 3rd video at 16:25 and he's talking about how much to fill based on the moving of the frost line up his evaporator. Where the frost line ends is where the fluorocarbon is fully evaporated (no liquid left). He uses amount added to dial that in, but also comments how things have to reach steady state and takes hours. This gizmo's sensor part is placed at the end of the evaporator tubing and detects that frost line approaching it by its temperature. As it gets colder, it starts closing off the valve so the frost line (and more importantly the liquid) doesn't pass the sensor and into the compressor??? Since it's variable, it doesn't rely on Clark's "steady state" and it can deal with the wild swings of the room's environment???
Charging an air conditioning system doesn't require that long to reach a fairly steady state condition - regardless of the expansion device used.
The TXV will compensate for a variety of conditions, which is why you need to watch the head pressure while you are charging. The system may start cooling well before the charge is correct. You have to keep charging until the head pressure makes sense.
The TXV or other expansion device, goes right in front of the evaporator. It is the pressure differential created by the expansion device, that makes the system work. The reduction in pressure in the evaporator causes the refrigerant to boil (when you reduce the pressure of a liquid, the boiling point temperature drops). When the liquid starts boiling, it takes in heat. Thus, the cooling effect.