In 1983, the US Navy Experimental Diving Unit evaluated several digital watches for use by US Navy divers. In 1996, the International Organization for Standardization (ISO) introduced the standards and features for diving watches regulated by the "ISO 6425 - Divers' watches" international standard. Many contemporary sports watches owe their design to diving watches. The vast majority of divers now use electronic, wrist-worn dive computers. A dive computer or decompression meter is a device used by a scuba diver to measure the time and depth of a dive so that a safe ascent profile can be calculated and displayed so that the diver can avoid decompression sickness. Diving watches and depth gauges are however still commonly used by divers as backup instruments for overcoming dive computer malfunctions. Many companies offer highly functional diving watches. Whilst diving watches are primarily tool watches, some companies offer models that can in addition to this be regarded by some as jewellery or fine mechanical devices. Diving watches can be analog or digital. Besides pure analog and digital models some diving watch models combine digital and analog elements. The standards and features for diver's watches are regulated by the International Organization for Standardization in the ISO 6425 standard; German Industrial Norm DIN 8306 is an equivalent standard. Besides water resistance standards to a minimum of 100 m depth rating ISO 6425 also provides minimum requirements for mechanical diver's watches (quartz and digital watches have slightly differing readability requirements) such as:  Testing diving watches for ISO 6425 compliance is voluntary and involves costs, so not every manufacturer present their watches for certification according to this standard. The watch cases of diving watches must be adequately water (pressure) resistant and be able to endure the galvanic corrosiveness of seawater, so the cases are generally made out of materials like grade 316L or 904L austenitic stainless steel and other steel alloys with higher Pitting Resistance Equivalent factors (PRE-factors), titanium, ceramics and synthetic resins or plastics. If metal bracelets are used they should be made of the same metal alloy as the watch case to prevent corrosion of the metal with the lower PRE-factor as it will act as a sacrificial anode. The case must also provide an adequate degree of protection against external magnetic influences and shocks, though diver's watches do not have to be able to endure strong magnetic fields and shocks. To make mechanical watch movements themselves shock resistant various shock protection systems can be used. The cases of diving watches have to be constructed more stoutly than typical dress watches, because of the requirements necessary to withstand a seawater environment at depth.