Kirk M. from Lynnwood, Washington, USA, asks:
Why do so many Diver's watches have the first 15 or 20 minutes on the bezel a different color? My Seiko SKX009 has red for the first 20 minutes and blue for the remainder. Even a luxury watch like the IWC Aquatimer Deep 2 has red for the first 15 minutes. Is this a standard decompression stop? Or is it just an opportunity to add some color to a no-frills tool watch?
That's a good question that I'm sure many people have considered in the past and will continue to wonder in the future. So, why are the first 15-to-20 minutes on many dive watch bezels highlighted with a color separate from the rest of the timing scale? The irony is that many people who wear dive watches will not be able to appreciate the reason for these sometimes colored sections of rotating bezels on dive watches, because they do not dive. While many of us are among the ranks of "desk divers" who never take our dive watches underwater, it is a good idea to understand the functions of how most dive watches are meant to work.
In short, often times, such coloration on a diver bezel is decorative but based on a tradition of using the rotating bezel to measure the timing of mandatory decompression stops as divers slowly resurface from deeper dives. In order to prevent the "bends" divers can get when resurfacing too quickly, they sometimes need to stop for up to 20 minutes at a time. Those stops can be less with shallower dives. While many diver's today rely on computers and other electronic equipment to measure these stops, it is handy to have a back up dive watch to do so - and they look cool as well.
The reason we say that such coloration is mainly decorative, especially the color red, is because many colors fade as you dive under the water and light wave frequencies stop at various levels of depth. The aBlogtoWatch team - which includes a few divers - notes that the color red disappears at about 20 feet, thus rendering it all but invisible unless you have a light with you. Other colors that fade as you go deeper are orange and then yellow. Thus, unless you have a light underwater, pretty much all the colors on your watch look blue-green. So while the practice of coloring a portion of a diving bezel is rooted in utility, its continued application today is mainly decorative. Having said that - diving bezels in and of themselves are still pretty useful.