Drawings of new pits come through my desk nearly every month and most have the MultiTrode Level Sensing Probe already specified, but from time to time I see a ball float right at the top to indicate high level and an alarm. The following figures show the backup level sensing configuration with a ball float and how that configuration matches with the backup Fail Safe Probe sensor design.
 

Proposed Tank Level With Ball Float

Proposed Tank Level With Fail Safe

It seems no one ever thought of changing the ball float to a new and more advanced Fail Safe Probe and install the Fail Safe Relay as well. Ball floats have their place; but when mission critical high level flow’s need a mission critical alarm, a Fail Safe Probe is the best bet together with a Fail Safe Relay.

Those who are familiar with the probe would know that the 200mm single sensor probe has just one wire, but the Fail Safe has two. The Fail Safe Controller will “integrate the loop” to check for continuity of the circuit; and if there is a break in the cable it will close a relay that will warn that, the integrity of the probe to do its job has been limited or has failed. Handy if you have a sewerage pumping station near a creek or river where people swim a lot.