Patek Philippe CALIBER CH 28-520 QA 24H, Manual

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15
TRADITION AND
INNOVATION
IN PERFECT HARMONY
The CH 28-520 IRM QA 24H chronograph
caliber
is a self-winding mechanical move-
ment with a flyback column-wheel chronograph,
power-reserve indicator, Annual Calendar, and
day/night display. The CH 28-520 QA 24H caliber
has the same features, with the exception of the
power-reserve indicator. Both movements beat
at a frequency of 28,800 semi-oscillations per
hour (4 Hz), assuring an extremely constant bal-
ance amplitude and high rate accuracy. The wind-
ing power is delivered by a heavy, unidirectional
21K gold rotor. And both chronograph calibers are
based on the venerable column-wheel principle
for controlling the start/stop commands. However,
it differs from classic Patek Philippe column-wheel
chronographs by virtue of a remarkable innova-
tion: the clamps do not control the engagement
of a lever-mounted clutch wheel with the chrono-
graph train but instead act on a disk clutch that
transmits (chronograph running) or interrupts
(chronograph stopped) the power between the
fourth wheel and the chronograph hand. This is
a highly reliable solution that makes it possible
to use the chronograph hand as a continuously
running seconds hand without affecting the rate
accuracy of the watch. For this reason, the sub-
sidiary seconds dial customarily found in many
chronographs is no longer needed.