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. A depth gauge and an oxygen analyser/oxygen sensor can be used to measure the partial pressure of oxygen of the breathing gas, which is necessary to avoid oxygen toxicity.
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.
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.
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 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 side. It has no moving parts. While diving, the water goes into the tube and compresses an air bubble inside depending on the depth. The edge of the bubble shows on a scale the depth. For a depth up to 10 m, this depth gauge is quite accurate, because in this range, the pressure doubles from 1 bar to 2 bar, and so it uses half of the scale. At greater depths, it becomes inaccurate. The maximum depth cannot be recorded with this depth gauge.
The Bourdon tube depth gauge consists of a curved tube made of elastic metal, the Bourdon tube. On the tube, the water presses depending on the design from the inside or the outside. When the pressure increases, the tube stretches; when it decreases the tube recovers to the original curvature. This movement is transferred to a pointer, which can mark the maximum depth reached with a trailing pointer. Its accuracy is good.
At the membrane depth gauge, the water presses onto a flexible metal can, which is compressed with increasing pressure. It also has a membrane, whose movement is also transferred to a pointer and possibly a trailing pointer. It is very accurate.
Dive computers have an integrated depth gauge, whose measured values are digitized. They can calculate the current decompression time needed. The dive depth is displayed along with other values on the display and recorded by the computer for continuous simulation of the decompression model. Most dive computers contain a piezoelectric pressure sensor. Rarely, capacitive or inductive pressure sensors are used.
A depth gauge can also be based on light: The brightness decreases with depth, but depends on the weather (e.g. whether it is sunny or cloudy) and the time of the day. Also the color depends on the water depth.
In water, light attenuates for each wavelength, differently. The UV, violet (> 420 nm), and red (< 500 nm) wavelengths disappear before blue light (470 nm), which penetrates clear water the deepest. The wavelength composition is constant for each depth and is almost independent of time of the day and the weather. To gauge depth, an animal would need two photopigments sensitive to different wavelengths to compare different ranges of the spectrum. Such pigments may be expressed in different structures.
Such different structures are found in the polychaete Torrea candida. Its eyes have a main and two accessory retinae. The accessory retinae sense UV-light (?max = 400 nm) and the main retina senses blue-green light (?max = 560 nm). If the light sensed from all retinae is compared, the depth can be estimated, and so for Torrea candida such a ratio-chromatic depth gauge has been proposed.
A ratio chromatic depth gauge has been found in larvae of the polychaete Platynereis dumerilii. The larvae have two structures: The rhabdomeric photoreceptor cells of the eyes and in the deep brain the ciliary photoreceptor cells. The ciliary photoreceptor cells express a ciliary opsin, which is a photopigment maximally sensitive to UV-light (?max = 383 nm). Thus, the ciliary photoreceptor cells react on UV-light and make the larvae swimming down gravitactically. The gravitaxis here is countered by phototaxis, which makes the larvae swimming up to the light coming from the surface. Phototaxis is mediated by the rhabdomeric eyes. The eyes express at least three opsins (at least in the older larvae), and one of them is maximally sensitive to cyan light (?max = 483 nm) so that the eyes cover a broad wavelength range with phototaxis. When phototaxis and gravitaxis have leveled out, the larvae have found their preferred depth.