Depth gauge
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A depth gauge is an instrument for measuring
Most modern diving depth gauges have an
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
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
Bourdon tube depth gauge
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
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
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Pneumofathometer
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
Dive computer
Dive computers have an integrated depth gauge, with
Uses
A diver uses a depth gauge with
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
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
- Altimeter – Instrument used to determine the height of an object above a certain point
- Bathometer – A scientific instrument for measuring water depth
- Bathymetry – Study of underwater depth of lake or ocean floors
- Depth sounding – Measuring the depths of a body of water
References
- ^ Jowthhorp, John (editor), The Philosophical Transactions and Collections to the end of the Year MDCC: Abridged, And Disposed Under General Heads, W. INNYS, 1749, Volume 2, p. 3
- ISBN 978-1-59416-105-6. OCLC 369779489, 2010, pp. 37, 121
- ^ Martin, Benjamin, Philosophia Britannica: Or, A New & Comprehensive System of the Newtonian Philosophy, C. Micklewright & Company, 1747, p. 25
- ^ Marstan and Frese, p. 123
- ^ "Pressure sensor". www.omega.com. 17 April 2019. Retrieved 9 December 2019.
- ^ "How to measure absolute pressure using piezoresistive sensing elements" (PDF). www.amsys.info. Retrieved 9 December 2019.
- ^ PMID 19720648.
- ^ PMID 23578808.
- ISBN 978-1-4612-8317-1.
- ^ PMID 26255845.
- S2CID 21808560.
- ^ PMID 29809157.
- S2CID 46930876.
- S2CID 2583520.
- PMID 28623234.
- PMID 24867217.
- PMID 19020621.
- PMID 23667045.
External links
Articles[usurped] on depth gauges hosted by the Rubicon Foundation