Fishing vessel fitted with a Sentry corrosion monitor.

The Sentry provides an entry level corrosion monitor for measuring the cathodic protection level on a single zone such as a ship's hull. 

Pressing the 'test' button lights up a pair of LED's. Green shows the vessel is cathodically protected. Red LED indication points to probable remedial action being required, such as replacing the hull anodes.

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The whalewatch vessel, Te Ao Marama, New Zealand.  One Sentry multizone and 3 reference electrodes (hull, port and starboard sea chests) monitor  cathodic protection levels.

The Sentrymultizone has the same features as the Sentrylog but without the capability to log data.

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Graph showing corrosion events on a New York ferry Click to enlarge

The Sentrylog is a multi-zone corrosion monitor with data logging capability.  Used as a monitor, an LCD display and LED’s provide indication of the vessel or structure's level of cathodic protection. Used as a data logger, the corrosion history can be recorded and examined.  

Each zone requires a reference electrode to be installed. Up to 3 zones can be monitored with one Sentrylog

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For more information on zones and corrosion in general, see corrosion basics.