Level
The use of a float switch can be an extremely cheap way of finding problems.
If the water level of a tank drops, it may mean that the pump filling the tank has stopped working, or that there is a tube fractured leading from the tank. Either way, receiving an SMS within seconds of the float switch being tripped could save considerable time, effort and expense later.
Conversely, if a float switch is tripped to indicate a retaining bund is about to overflow, the Plant Manager might be grateful to know about this by SMS immediately.