Editing Depth gauge (section)

{{Short description|Instrument that indicates depth below a reference surface}}
{{For|engineering instruments|Gauge (instrument)}}
{{More citations needed|date=August 2018}}
{{Infobox diving equipment
|name         = Depth gauge 
|image        = USMC-17563.jpg
|alt          = A diver's left wrist, wearing a diving watch and mechanical depth gauge with a needle indicator
|caption      = US Marine diver with a diving watch and an analog depth gauge
|acronym      = 
|other_names  = 
|uses         = 
|inventor     = 
|manufacturer = 
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[[File:Depth gauge.jpg|right|thumb|A digital depth gauge combined with a timer and temperature display, also referred to as a "Bottom timer"]]

A '''depth gauge''' is an instrument for measuring [[depth (coordinate)|depth]] below a [[vertical reference]] surface. They include depth gauges for underwater diving and similar applications.
A diving depth gauge is a [[pressure gauge]] that displays the equivalent depth below the free surface in water. The relationship between depth and pressure is linear and accurate enough for most practical purposes, and for many purposes, such as diving, it is actually the pressure that is important. It is a piece of [[diving equipment]] used by [[Underwater diving|underwater divers]], [[submarine]]s and [[submersible]]s.

Most modern diving depth gauges have an [[electronics|electronic]] mechanism and [[Digital data|digital]] display. Earlier types used a [[machine|mechanical]] mechanism and [[analog circuit|analogue]] display. Digital depth gauges used by divers 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 combination instrument is also known as a [[bottom timer]]. An electronic depth gauge is an essential component of a [[dive computer]].

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 [[density|densities]] of fresh water and seawater due to salinity and temperature variations.

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 [[Metre sea water|metres of seawater]] or feet of seawater.

Other types of depth gauge use a physical probe to measure the vertical distance from the reference surface to the bottom or other relevant point, such as a [[dipstick]], [[sounding pole]] or [[sounding line]], or use light or sound emitted from a known distance from the surface and reflected by the bottom to calculate depth based on elapsed time of travel. This includes [[echo sounding]] and [[lidar]].

A [[level sensor]] is related technology which measures offset of actual surface from a reference surface, bur does not directly measure depth.