On an immersed structure, the efficiency of cathodic protection is checked by potential measurements compared to a reference electrode immersed in the vicinity. It can be :
- a reference electrode in free immersion, moved by a diver or moving on a fixed guide,
- a waterproof “gun” which contains the reference electrode and the millivoltmeter ,
- a reference electrodes with a fixed system (Ag/AgCl) or with a Zn electrode (the potential of zinc in sea water is rather stable, close to –0,78 V/ENH). The measurement is transmitted by cable or by acoustic transmission.
These measurements are taken manually approximately every five years, or more frequently if the measurement is automated.