IWC Schaffhausen SPITFIRE MARK XVI Mode D'emploi page 23

Water-resistance
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The indicated quantity in metres does not correspond to
the dive depth, but rather to the test pressure to which the
watch was subjected in the course of a standard testing
procedure. The water-resistance of your watch can be
adversely affected by the sudden and repeated differen-
ces in water pressure that can arise during swimming,
for example, and by sudden changes in the water tempe-
rature.
In order to ensure its perfect function, you must have your
watch inspected by an IWC service point at least once a
year. An inspection must also be carried out after the
watch has suffered abnormal stresses (shock). If these
inspections are not carried out as stipulated, or if the
watch is opened by unauthorized persons, IWC will
decline any guarantee or liability claims.
Recommendation: Your authorized IWC Official Agent
must perform a new water-resistance test every time your
IWC watch is opened and serviced.