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 US Marine diver with a diving watch and an analog depth gauge |
Adepth gauge is an instrument for measuringdepth below avertical datum or other reference surface. They include depth gauges for underwater diving and similar applications.A diving depth gauge is apressure 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 ofdiving equipment used byunderwater divers,submarines andsubmersibles.
Most modern diving depth gauges have anelectronic mechanism anddigital display. Earlier types used amechanical mechanism andanalogue 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 avoidingbarotrauma. This combination instrument is also known as abottom timer. An electronic depth gauge is an essential component of adive computer.
As the gauge only measures water pressure, there is an inherent inaccuracy in the depth displayed by gauges that are used in bothfresh water andseawater due to the difference in thedensities 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 apneumofathometer. They are usually calibrated inmetres 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 adipstick,sounding pole orsounding 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 includesecho sounding andlidar.
Alevel sensor is related technology which measures offset of actual surface from a reference surface, bur does not directly measure depth.
Experiments in 1659 byRobert Boyle of theRoyal Society were made using a barometer underwater, and led toBoyle's law.[1] The French physicist, mathematician and inventorDenis Papin publishedRecuiel de diverses Pieces touchant quelques novelles Machines in 1695, where he proposed a depth gauge for asubmarine.[2] A "sea-gage" for measuring ocean depth was described inPhilosophia Britannica in 1747.[3] But it wasn't until 1775 and the development of a depth gauge by the inventor, scientific instrument, and clock makerIsaac Doolittle ofNew Haven, Connecticut, forDavid Bushnell's submarine theTurtle, that one was deployed in an underwater craft. By the early nineteenth century, "the depth gauge was a standard feature ondiving bells".[4]
With water depth, the ambient pressure increases 1bar 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.
TheBoyle-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 ascale. 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.
The Bourdon tube depth gauge consists of a curved tube made of elastic metal, known as aBourdon 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 apointer 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.
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 themembrane is amplified by a lever and gear mechanism and transferred to an indicator pointer like in ananeroid 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 aWheatstone 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 exploit the stress-dependent resistivity of silicon. A micromachined silicon diaphragm carries diffused/implanted resistors near high-stress regions and is wired as a Wheatstone bridge, so applied pressure produces a differential output roughly proportional to diaphragm stress.[5] Bridge offset and span vary strongly with temperature; commercial modules therefore include an on-die temperature sensor and factory calibration, and the raw bridge signal is digitally compensated over the operating range. Gel-isolated piezoresistive modules are widely used for water-depth measurement in dive computers, where compensated pressure is converted to depth.[6]
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 crankeddiver's air pump used to providebreathing air to a diver wearingstandard diving dress, with a free-flow air supply, in which there was not much back-pressure other than the hydrostatic pressure of depth. Asnon-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. The type of high precision gauge used is also known as acaisson gauge. Precision is typically 1% to 0.25% of full scale.
Dive computers have an integrated depth gauge, withdigitized output which is used in the calculation of the currentdecompression status of the diver. The dive depth is displayed along with other values on thedisplay and recorded by the computer for continuous simulation of thedecompression model. Most dive computers contain apiezoresistive pressure sensor. Rarely, capacitive or inductive pressure sensors are used.[citation needed]
A diver uses a depth gauge withdecompression tables and awatch to avoiddecompression sickness. A common alternative to the depth gauge, watch and decompression tables is adive computer, which has an integral depth gauge, and displays the current depth as a standard function.
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