Page 609 - Instrumentation Reference Book 3E

Underwater non-destructive testing 591

Wincl ne

safety
cut-outs
Sea level

Parallel conductor

center
line

I

Hand-held ultraviolet
with ink dispenser
Figure 24.38 Magnetic particle inspection in deep water.

material, and are calibrated for steel although potential to reverse the electrolytic action. In
alternative calibration is possible. Ideally the unit order to check the operation of the impressed
therefore has no diver controls and can display a potential or the corrosion liability of the sub-
range of steel thickness from 1 to 300mm on a merged structure, surface voltage is measured
digital readout. Where detailed examination of a with reference to silver/silver chloride cell (Figure
weld is required, either a conventional surface 24.39). Hand-held potential readers containing a
A-scan set housed in a waterproof container with reference cell, contact tip, and digital reading volt-
external extensions to the controls is used, or the meter along with internal power supply will allow
diver operates the probes of a surface-located set a survey to be completed by a diver which may be
under the direction of the surface non-destructive remotely monitored from the surface.
testing engineer. In either case, there is a facility
for monitoring of the master set by a slave set. impressed current
The diver, for instance, may be assisted by an
audio feedback from the surface set which triggers
on a threshold gate open to defect signals. The
diver's hand movements will be monitored by a
helmet video to allow monitoring and adjustment
by the surface operator.

24.6.61 Corrosion protection
Because seawater behaves as an electrolytic fluid, anode
corrosion of the steel structures may be inhibited
by providing either sacrificial zinc anodes or
impressing on the structure a constant electrical Figure 24.39 Measuring cathodic protection.