Open heart

From Chinese Watch Industry Wiki

The term open-heart specifically refers to a watch in which the occillation of the balance wheel may be observed through an opening in the dial. This is distinct from a skeleton watch in which all the movement plates have been filleted to reveal far more of the workings.

An early example of open-heart was the Swiss Hebdomas 8-day watch, which had a full-width mainspring and thus the balance was relocated to the dial side to allow adjustment. More recent examples of open-heart, made in Switzerland, Russia and Japan feature a simple port-hole cut in the dial to observe the balance in its usual location.

Chinese open-heart watches

Chinese open-heart watches are distinctive in that they return to the Hebdomas pattern of shifting the balance to the dial side. In this case however it is simply for aesthetic reasons. The balance is secured by a straight bridge that is often decorated. Usually the balance is at the 6 o'clock position, although those based on the Standard movement place it at 9 o'clock. Cheaper open-heart calibres are based on existing conventional calibres that have been reworked to relocate the balance. More advanced calibres are clean-sheet designs. The highest grade Chinese open-heart currently is the Sea-Gull ST25. Chinese open-hearts are popular with many internet watch brands such as those assembled by Million Smart Enterprises.

Open-hearts are often falsely marketed as 'tourbillon' or 'tourbillon-style'. This is a misnomer caused by a superficial similarity in appearance between Chinese open-hearts and some well-known tourbillons, where the balance-wheel is similarly exposed. Some open-heart designs play up to this similarity by feeding the seconds train into a rotating cage surrounding the balance-wheel to enhance the tourbillon illusion.