Mine got to the -10 to -20 C range. You cpu will probably run at 0 C or above depending on the heat output and your water pump setup.
The temp probe is a capilliary thermo expansion device design to operate a mechanical electrical switch that you adjust with the knob. Of course you leave the Knob with the electrical switch outside. the probe end you can put into the fluid. It is better to let it run automically to maintain a cold supply of antifreeze at all times if you have good insulation, it does not use much power at all. If you plan to overclock with it and set a high FSB and you use a manuel switch as you paln to do . You will have to wait an hour or more to precool your solution otherwise your cpu will flake out or manuelly reset bios all the times. Remember that you will have to have 3 or 4 gallon if you want any reserve heat capacity to your system. These compressor are not designed to run all the time and the intergral design of the temp probe is design to shut off compressor once the max cooling temperature of the surround material is reached . Beyond a certain tempurature the phase change of of your refrigerant will no long take place ,determine by the design of your system, so you're just wasting your energy if you run all the time.
The only way your going to have instant on and off running is to convert it to direct die phase change cooling instead of water cooling
