I would have thought that under loss of water conditions, as soon as the water level drops below the pump that the head will be starved of water and overheat. If you want to look after your head, then I would have thought the best place for your probe would be into the metal of the head itself. There are other better ways of detecting low or no water such as a flow switch in the return (top) hose for example or even use your temperature sensor there and look out for lower water temp as none would be circulating (this might be difficult to set an alarm for as it would sound until the engine warmed up). The answers is, I don't know if what you are trying to achieve will work by putting the sensor half way down. Part way down, on the actual head may do it but I would have thought into the metal rather than the water. At that point you can see head temp rising when water temp doesn't. I'm no expert on heads I'm afraid.