Depth gauge  A depth gauge is a pressure gauge that displays the equivalent depth in water. It is a piece of diving equipment used by underwater divers, submarines and submersibles. Most modern diving depth gauges have an electronic mechanism and digital display. Earlier types used a mechanical mechanism and analogue display. Digital depth gauges commonly also include a timer showing the interval of time that the diver has been submerged. Some show the diver's rate of ascent and descent, which can be is useful for avoiding barotrauma. This is also known as a bottom timer. A diver uses a depth gauge with decompression tables and a watch to avoid decompression sickness. A common alternative to the depth gauge, watch and decompression tables is a dive computer, which has an integral depth gauge, and displays the current depth as a standard function. As the gauge only measures water pressure, there is an inherent inaccuracy in the depth displayed by gauges that are used in both fresh water and seawater due to the difference in the densities of fresh water and seawater. A depth gauge that measures the pressure of air bubbling out of an open ended hose to the diver is called a pneumofathometer. They are usually calibrated in metres of seawater or feet of seawater. Experiments in 1659 by Robert Boyle of the Royal Society were made using a barometer underwater, and led to Boyle's Law. The French physicist, mathematician and inventor Denis Papin published "Recuiel de diverses Pieces touchant quelques novelles Machines" in 1695, where he proposed a depth gauge for a submarine. A "sea-gage" for measuring ocean depth was described in "Philosophia Britannica" in 1747. But it wasn't until 1775 and the development of a depth gauge by the inventor, scientific instrument, and clock maker Isaac Doolittle of New Haven, Connecticut, for David Bushnell's submarine the "Turtle", that one was deployed in an underwater craft. By the early nineteenth century, "the depth gauge was a standard feature on diving bells". With water depth, the ambient pressure increases 1 bar for every 10 m. Therefore, the exact depth can be determined by measuring the pressure and comparing it to the pressure at the surface. The "Boyle-Mariottesche depth gauge" consists of a circular curved glass tube open at one end. It has no moving parts. While diving, the water goes into the tube and compresses an air bubble inside proportionally to the depth.