Blancpain Fifty Fathoms Bathyscaphe Chronograph flyback
The rotating bezel, commonly found on sports watches, is a utilitarian tool when used in conjunction with the time shown by the running movement. The dive counter on the rotating bezel is one such example where there is typically a track in one-minute increments up to the first 15 or 20 minutes while the other reference markers are in five-minute intervals.
Dive times depend on many factors such as the depth reached, the capacity of the air tank and the diver’s air consumption rate. It is said that a standard 80-cubic foot tank for a dive down to around 12 metres or 40 feet will last on average between 45 minutes to an hour.
Most dives will generally be around 30 minutes. This is one reason for the minute track up to 15 or 20 minutes. At the start of the dive, the minute hand is aligned with the reference marker – the zero mark. As the minute hand shows the elapsed time of 15 minutes, it is an indication to the diver to end the dive and begin the ascent.
Another reason for the 15- or 20-minute track is for the timing of more accurate decompression stops. Decompression stops are necessary to prevent decompression sickness by allowing the body to expel inert gases such as nitrogen that are absorbed by the tissues during the dive.
The rotating bezel became especially useful in the 1950s on early diver’s watches. In 1953, Blancpain’s Fifty Fathoms became the French Navy’s dive watch of choice featuring not only the rotating bezel but a patented locking mechanism to prevent any accidental turn of the bezel that might compromise an accurate reading of dive time.
Rotating bezels can either be bidirectional or unidirectional with a ratchet spring. Unidirectional bezels revolving in an anti-clockwise direction are preferred as a security feature. Why? Any accidental turn of the bezel will only mean time elapsed during a deep dive will be increased rather than decreased, thereby negating the risk of running out of air while submerged.
The rotating bezel with the dive counter can serve as a 60-minute chronograph on land as well. Timing one’s parking or even how long the food delivery will take is a breeze with just a simple turn of the rotating bezel.