Before we have discussed 5 features of a divers watch and now we will continue with some more features.
Digital and some analog chronograph scuba diving watch - such as the Breitling Avenger Seawolf Chronograph, Doxa T-Graph, IWC Aquatimer Minute Memory, the Omega Seamaster Professional Chrono Diver series and Sinn U1000 - have specially-designed push pieces that can be operated at depth without allowing water to enter the case.
Helium release valve
Some scuba diving watches intended for saturation diving at great depths (typically greater than 100m) are fitted with a helium or mixed breathing gas release or escape valve to prevent the crystal from being blown off by a pressure build up caused by helium that has seeped into the watch in helium enriched environments as the watch and diver adjust to normal atmospheric conditions. Other helium safe/for mixed-gas rated diving watches can withstand the helium used in certain diving situations by using gaskets that simply do not allow helium gas to enter the watch case in a harmful way in the first place.
The dials and markers on the scuba diving watch face and of course the bezel have to be legible under water and in low light conditions. An indication that the watch is running in total darkness also has to be present. For easy legibility most diving watches have high contrasting, none cluttered dials and markers with a large, easily identifiable minute hand. The markers for 3, 6, 9 and 12 o'clock on the scuba diving watch face and the zero marker on the bezel of analogue diver's watches are usually conspicuously styled to prevent disorientation induced read out errors. For low light conditions luminous phosphorescent non-toxic strontium aluminate based lume pigments marketed under brand names like Super-LumiNova or NoctiLumina and tritium based self-powered lighting devices called "gaseous tritium light source" (GTLS) is applied on the dials and markers. As for digital diving watches back lights are used for low light conditions legibility.
Power reserve indicator
A scuba diving watch with an electric battery powered movement must have an End Of Life (EOL) indicator, usually in the form of a jumping second hand or a warning message on a digital display to safeguard against insufficient power reserve during underwater activities. Some electric and mechanical powered movement diving models have power reserve indicators that show the current power status of the watch. Mechanical movements should be wound or in case of automatic movements given enough motion before a dive.
Scuba Diving watches have relatively thick watch crystals face. Sometimes domed crystals are used to enhance the pressure resistance of the scuba diving watch. The typical materials used for crystals are acrylic glass, hardened glass and (synthetic) sapphire which all come with own pros and cons.
Some manufacturers use sapphire/hardened glass laminate crystals, where the scratch resistance sapphire is combined with the better shatter resistance of hardened glass.