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TITLE 46 - SHIPPINGCHAPTER I - COAST GUARD, DEPARTMENT OF HOMELAND SECURITYSUBCHAPTER V - MARINE OCCUPATIONAL SAFETY AND HEALTH STANDARDSPART 197 - GENERAL PROVISIONSsubpart b - COMMERCIAL DIVING OPERATIONS197.318 - Gages and timekeeping devices. (a) A gage indicating diver depth must be at each dive location for surface-supplied dives. (b) A timekeeping device must be at each dive location.

TITLE 46 - SHIPPINGCHAPTER I - COAST GUARD, DEPARTMENT OF HOMELAND SECURITYSUBCHAPTER V - MARINE OCCUPATIONAL SAFETY AND HEALTH STANDARDSPART 197 - GENERAL PROVISIONSsubpart b - COMMERCIAL DIVING OPERATIONS197.458 - Gages and timekeeping devices.The diving supervisor shall insure that (a) Each depth gage and timekeeping device is tested or calibrated against a master reference gage or time-keeping device every 6 months; (b) A depth gage is tested when a discrepancy exists in a depth gage reading greater than 2 percent of full scale between any two gages of similar range and calibration; (c) A timekeeping device is tested when a discrepancy exists in a timekeeping device reading greater than one-quarter of a minute in a 4-hour period between any two timekeeping devices; and (d) Each depth gage and timekeeping device is inspected before diving operations are begun.

For thousands of years, devices have been used to measure and keep track of time. The current sexagesimal system of time measurement dates to approximately 2000, BC, in Sumer. The Ancient Egyptians divided the day into two 12-hour periods, and used large obelisks to track the movement of the Sun. They also developed water clocks, which were probably first used in the Precinct of Amun-Re, and later outside Egypt as well; they were employed frequently by the Ancient Greeks, who called them clepsydrae. The Shang Dynasty is believed to have used the outflow water clock around the same time, devices which were introduced from Mesopotamia as early as 2000  BC. Other ancient timekeeping devices include the candle clock, used in China, Japan, England and Iraq; the timestick, widely used in India and Tibet, as well as some parts of Europe; and the hourglass, which functioned similarly to a water clock.

A variation on this theme were oil-lamp clocks. These early timekeeping devices consisted of a graduated glass reservoir to hold oil — usually whale oil, which burned cleanly and evenly — supplying the fuel for a built-in lamp. As the level in the reservoir dropped, it provided a rough measure of the passage of time.

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