Using a spare zinc to test a bonding system
Nigel Calder has a nice procedure on Pg 149 of the Boat Owners Mechanical and Electrical Manual to test the bonding of a boat that would be nice to do periodically. He recommends dipping a silver-silver chloride half cell over board and measuring the voltage between it and any of the underwater bonded metal parts. The voltage of all the underwater pieces of metal when compared to the half cell he says should be the same and greater than 0.2volts.
Why can't I just use a spare zinc, which we all have on board, as the half cell and drop it in the water close to the metal to be tested. The voltage between it and any piece of underwater metal connected to the bonding system should be 0 volts - since the test zinc should be at the same voltage as the zinc connected to the bonding system and protecting the boat. Incidentally if you use a spare zinc as the test cell this procedure will also tell you when its time to change the boat zinc - i.e. whenever the voltage rises above 0.1 volts.
I believe Nigels procedure should be modified at any rate to place the half cell close to the metal to be tested. I believe also that his 0.2 volts is in error and should be modified to -0.98 volts which is the voltage of zinc relative to silver-silver Chloride since all the bonded metal parts should be at boat zinc potential.
Along with the bonding system test Nigel also includes a test for DC leakage (on Pg 149) which could save your life some day but I think it would be more convenient to do this test with your spare zinc if it would work.