Depth gauge

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Depth gauge
USMC-17563.jpg
US Marine diver with a diving watch and an analog depth gauge
A digital depth gauge combined with a timer and temperature display, also referred to as a "Bottom timer" Depth gauge.jpg
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 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 divers, submarines and submersibles.

Contents

Most modern diving depth gauges have an electronic mechanism and digital display. Earlier types used a mechanical mechanism and 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 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 metres of seawater or feet of seawater.

History

Experiments in 1659 by Robert Boyle of the Royal Society were made using a barometer underwater, and led to Boyle's Law. [1] 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. [2] A "sea-gage" for measuring ocean depth was described in Philosophia Britannica in 1747. [3] 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". [4]

Mode of operation

With water depth, the ambient pressure increases 1 bar for every 10 m in fresh water at 4 °C. Therefore, the depth can be determined by measuring the pressure and comparing it to the pressure at the surface. Atmospheric pressure varies with altitude and weather, and for accuracy the depth gauge should be calibrated to correct for local atmospheric pressure. This can be important for decompression safety at altitude. Water density varies with temperature and salinity, so for an accurate depth measurement by this method, the temperature and salinity profiles must be known. These are easily measured, but must be measured directly.

Types

Boyle-Mariott depth gauge

The Boyle-Mariotte depth gauge consists of a transparent tube open at one end. It has no moving parts, and the tube is commonly part of a circle or a flat spiral to compactly fit onto a support. While diving, water goes into the tube and compresses an air bubble inside proportionally to the depth. The edge of the bubble indicates the depth on a scale. 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. This type of gauge is also known as a capillary gauge. At greater depths, it becomes inaccurate. The maximum depth cannot be recorded with this type of depth gauge, and accuracy is strongly affected by temperature change of the air bubble while immersed.

Bourdon tube depth gauge

Bourdon tube Manometer anim 02.gif
Bourdon tube

The Bourdon tube depth gauge consists of a curved tube made of elastic metal, known as a Bourdon tube. Water pressure on the tube may be on the inside or the outside depending on the design. When the pressure increases, the tube stretches, and when it decreases the tube recovers to the original curvature. This movement is transferred to a pointer by a system of gears or levers, and the pointer may have an auxiliary trailing pointer which is pushed along but does not automatically return with the main pointer, which can mark the maximum depth reached. Accuracy can be good. When carried by the diver, these gauges measure the pressure difference directly between the ambient water and the sealed internal air space of the gauge, and therefore can be influenced by temperature changes.

Membrane depth gauge

In a membrane depth gauge, the water presses onto a metal canister with a flexible end, which is deflected proportionally to external pressure. Deflection of the membrane is amplified by a lever and gear mechanism and transferred to an indicator pointer like in an aneroid barometer. The pointer may push a trailing pointer which does not return by itself, and indicates the maximum. This type of gauge can be quite accurate when corrected for temperature variations.

Strain gauges may be used to convert the pressure on a membrane to electrical resistance, which can be converted to an analog signal by a Wheatstone bridge This signal can be processed to provide a signal proportional to pressure, which may be digitised for further processing and display.

Piezoresistive pressure sensors

Dive computer showing depth display Tauchcomputer Suunto Vyper Air.JPG
Dive computer showing depth display

Piezoresistive pressure sensors use the variation of resistivity of silicon with stress. A piezoresistive sensor consists of a silicon diaphragm on which silicon resistors are diffused during the manufacturing process. The diaphragm is bonded to a silicon wafer. The signal must be corrected for temperature variations. [5] These pressure sensors are commonly used in dive computers. [6]

Pneumofathometer

Surface supplied diving gas panel for one diver:
.mw-parser-output .plainlist ol,.mw-parser-output .plainlist ul{line-height:inherit;list-style:none;margin:0;padding:0}.mw-parser-output .plainlist ol li,.mw-parser-output .plainlist ul li{margin-bottom:0}
PG: pneumofathometer gauge
OPV: overpressure valve
PS: pneumo snubber
PSV: pneumo supply valve
DSV: diver supply valve
MP: manifold pressure
RSV: reserve supply valve
RP: reserve pressure
MSV: main supply valve
SP: supply pressure
RGS: reserve gas supply
MGS: main gas supply
UP: umbilical pneumo hose
UB: umbilical breathing gas hose
DP: depth measured by pneumofathometer Gas panel 1.png
Surface supplied diving gas panel for one diver:
  • PG: pneumofathometer gauge
  • OPV: overpressure valve
  • PS: pneumo snubber
  • PSV: pneumo supply valve
  • DSV: diver supply valve
  • MP: manifold pressure
  • RSV: reserve supply valve
  • RP: reserve pressure
  • MSV: main supply valve
  • SP: supply pressure
  • RGS: reserve gas supply
  • MGS: main gas supply
  • UP: umbilical pneumo hose
  • UB: umbilical breathing gas hose
  • DP: depth measured by pneumofathometer
Pressure gauge on Siebe Gorman manual diver's pump, indicating delivered pressure in pounds per square inch (black) and feet sea water (red) Pressure gauge on Siebe Gorman manual diver's pump P3220126.jpg
Pressure gauge on Siebe Gorman manual diver's pump, indicating delivered pressure in pounds per square inch (black) and feet sea water (red)
Surface supply air panel with supply pressure gauges (small) and pneumofathometer gauges (large diameter). Three of the four "pneumo lines" are blue. Surface supply air panel for 4 divers P3053737.jpg
Surface supply air panel with supply pressure gauges (small) and pneumofathometer gauges (large diameter). Three of the four "pneumo lines" are blue.

A pneumofathometer is a depth gauge which indicates the depth of a surface supplied diver by measuring the pressure of air supplied to the diver. Originally there were pressure gaues mounted on the hand cranked diver's air pump used to provide breathing air to a diver wearing standard diving dress, with a free-flow air supply, in which there was not much back-pressure other than the hydrostatic pressure of depth. As non-return valves were added to the system for safety, they increased back pressure, which also increased when demand helmets were introduced, so an additional small diameter hose was added to the diver's umbilical which has no added restrictions and when a low flow rate of gas is passed through it to produce bubbles at the diver, it gives an accurate, reliable and rugged system for measuring diver depth, which is still used as the standard depth monitoring equipment for surface supplied divers. The pneumofathometer gauges are mounted on the diver's breathing gas supply panel, and are activated by a valve. The "pneumo line", as it is generally called by divers, can be used as an emergency breathing air supply, by tucking the open end into the bottom of the helmet or full face mask and opening up the valve to provide free flow air. A "gauge snubber" needle valve or orifice is fitted between the pneumo line and the gauge to reduce shock loads on the delicate mechanism, and an overpressure valve protects the gauge from pressures beyond its operating range.

Dive computer

Dive computers have an integrated depth gauge, with digitized output which is used in the calculation of the current decompression status of the diver. 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 piezoresistive pressure sensor. Rarely, capacitive or inductive pressure sensors are used.[ citation needed ]

Uses

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.

Light based depth gauges in Biology

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. [7] [8]

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. [9] [10] 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. [7] [8] 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 (λmax = 400 nm) and the main retina senses blue-green light (λmax = 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. [11]

A ratio chromatic depth gauge has been found in larvae of the polychaete Platynereis dumerilii . [12] The larvae have two structures: The rhabdomeric photoreceptor cells of the eyes [13] and in the deep brain the ciliary photoreceptor cells. The ciliary photoreceptor cells express a ciliary opsin, [14] which is a photopigment maximally sensitive to UV-light (λmax = 383 nm). [15] 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. [10] Phototaxis is mediated by the rhabdomeric eyes. [16] [17] [12] The eyes express at least three opsins (at least in the older larvae), [18] and one of them is maximally sensitive to cyan light (λmax = 483 nm) so that the eyes cover a broad wavelength range with phototaxis. [10] When phototaxis and gravitaxis have leveled out, the larvae have found their preferred depth. [12]

See also

Related Research Articles

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An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system.

<span class="mw-page-title-main">Altitude diving</span> Underwater diving at altitudes above 300 m

Altitude diving is underwater diving using scuba or surface supplied diving equipment where the surface is 300 metres (980 ft) or more above sea level. Altitude is significant in diving because it affects the decompression requirement for a dive, so that the stop depths and decompression times used for dives at altitude are different from those used for the same dive profile at sea level. The U.S. Navy tables recommend that no alteration be made for dives at altitudes lower than 91 metres (299 ft) and for dives between 91 and 300 meters correction is required for dives deeper than 44 metres (144 ft) of sea water. Most recently manufactured decompression computers can automatically compensate for altitude.

<span class="mw-page-title-main">Dive computer</span> Instrument to calculate decompression status in real time

A dive computer, personal decompression computer or decompression meter is a device used by an underwater diver to measure the elapsed time and depth during a dive and use this data to calculate and display an ascent profile which, according to the programmed decompression algorithm, will give a low risk of decompression sickness.

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<span class="mw-page-title-main">Saturation diving</span> Diving decompression technique

Saturation diving is diving for periods long enough to bring all tissues into equilibrium with the partial pressures of the inert components of the breathing gas used. It is a diving mode that reduces the number of decompressions divers working at great depths must undergo by only decompressing divers once at the end of the diving operation, which may last days to weeks, having them remain under pressure for the whole period. A diver breathing pressurized gas accumulates dissolved inert gas used in the breathing mixture to dilute the oxygen to a non-toxic level in the tissues, which can cause decompression sickness if permitted to come out of solution within the body tissues; hence, returning to the surface safely requires lengthy decompression so that the inert gases can be eliminated via the lungs. Once the dissolved gases in a diver's tissues reach the saturation point, however, decompression time does not increase with further exposure, as no more inert gas is accumulated.

<span class="mw-page-title-main">Scuba diving</span> Swimming underwater, breathing gas carried by the diver

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<span class="mw-page-title-main">Opsin</span> Class of light-sensitive proteins

Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most prominently, they are found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in vision. Humans have in total nine opsins. Beside vision and light perception, opsins may also sense temperature, sound, or chemicals.

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<span class="mw-page-title-main">Phototaxis</span>

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<i>Platynereis dumerilii</i> Species of annelid worm

Platynereis dumerilii is a species of annelid polychaete worm. It was originally placed into the genus Nereis and later reassigned to the genus Platynereis. Platynereis dumerilii lives in coastal marine waters from temperate to tropical zones. It can be found in a wide range from the Azores, the Mediterranean, in the North Sea, the English Channel, and the Atlantic down to the Cape of Good Hope, in the Black Sea, the Red Sea, the Persian Gulf, the Sea of Japan, the Pacific, and the Kerguelen Islands. Platynereis dumerilii is today an important lab animal, it is considered as a living fossil, and it is used in many phylogenetic studies as a model organism.

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<span class="mw-page-title-main">Vertebrate visual opsin</span>

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Articles [usurped] on depth gauges hosted by the Rubicon Foundation