![Depth gauge](https://upload.wikimedia.org/wikipedia/commons/d/dd/Depth_gauge.jpg)
A depth gauge is an instrument for measuring depth below a reference surface. They include depth gauges for underwater diving and similar applications, and engineering instruments used to measure the depth of holes and indentations from a reference surface.
A diving depth gauge is a
pressure gauge
Pressure measurement is the measurement of an applied force by a fluid ( liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pres ...
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
Diving equipment is equipment used by underwater divers to make diving activities possible, easier, safer and/or more comfortable. This may be equipment primarily intended for this purpose, or equipment intended for other purposes which is found ...
used by
underwater divers
This is a list of underwater divers whose exploits have made them notable.
Underwater divers are people who take part in underwater diving activities – Underwater diving is practiced as part of an occupation, or for recreation, where t ...
,
submarine
A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...
s and
submersible
A submersible is a small watercraft designed to operate underwater. The term "submersible" is often used to differentiate from other underwater vessels known as submarines, in that a submarine is a fully self-sufficient craft, capable of ind ...
s.
Most modern diving depth gauges have an
electronic
Electronic may refer to:
*Electronics, the science of how to control electric energy in semiconductor
* ''Electronics'' (magazine), a defunct American trade journal
*Electronic storage, the storage of data using an electronic device
*Electronic co ...
mechanism and
digital display. Earlier types used a
mechanical
Mechanical may refer to:
Machine
* Machine (mechanical), a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement
* Mechanical calculator, a device used to perform the basic operations of ...
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
Barotrauma is physical damage to body tissues caused by a difference in pressure between a gas space inside, or contact with, the body and the surrounding gas or liquid. The initial damage is usually due to over-stretching the tissues in tens ...
. This combination instrument is also known as a
bottom timer
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 th ...
. An electronic depth gauge is an essential component of a
dive computer
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 t ...
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
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...
and
seawater
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
due to the difference in the
densities
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek language, Greek letter Rho (letter), rho), although the Latin letter ''D'' ca ...
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
Surface-supplied diving is diving using equipment supplied with breathing gas using a diver's umbilical from the surface, either from the shore or from a diving support vessel, sometimes indirectly via a diving bell. This is different from sc ...
. They are usually calibrated in
metres of seawater or feet of seawater.
History
Experiments in 1659 by
Robert Boyle
Robert Boyle (; 25 January 1627 – 31 December 1691) was an Anglo-Irish natural philosopher, chemist, physicist, alchemist and inventor. Boyle is largely regarded today as the first modern chemist, and therefore one of the founders of ...
of the
Royal Society
The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...
were made using a barometer underwater, and led to
Boyle's Law
Boyle's law, also referred to as the Boyle–Mariotte law, or Mariotte's law (especially in France), is an experimental gas law that describes the relationship between pressure and volume of a confined gas. Boyle's law has been stated as:
The ...
. The French physicist, mathematician and inventor
Denis Papin
Denis Papin FRS (; 22 August 1647 – 26 August 1713) was a French physicist, mathematician and inventor, best known for his pioneering invention of the steam digester, the forerunner of the pressure cooker and of the steam engine.
Early lif ...
published ''Recuiel de diverses Pieces touchant quelques novelles Machines'' in 1695, where he proposed a depth gauge for a
submarine
A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...
.
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
Isaac Doolittle (August 3, 1721 – February 13, 1800) was an early American clockmaker, inventor, engineer, manufacturer, militia officer, entrepreneur, printer, politician, and brass, iron, and silver artisan. Doolittle was a watchmaker and c ...
of
New Haven, Connecticut
New Haven is a city in the U.S. state of Connecticut. It is located on New Haven Harbor on the northern shore of Long Island Sound in New Haven County, Connecticut and is part of the New York City metropolitan area. With a population of 134,02 ...
, for
David Bushnell
David Bushnell (August 30, 1740 – 1824 or 1826), of Westbrook, Connecticut, was an American inventor, a patriot, one of the first American combat engineers, a teacher, and a medical doctor.
Bushnell invented the first submarine to be used in ...
's submarine the ''
Turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked tu ...
'', that one was deployed in an underwater craft. By the early nineteenth century, "the depth gauge was a standard feature on
diving bells
A diving bell is a rigid chamber used to transport divers from the surface to depth and back in open water, usually for the purpose of performing underwater work. The most common types are the open-bottomed wet bell and the closed bell, which c ...
".
[Marstan and Frese, p. 123]
Mode of operation
With water depth, the ambient pressure increases 1
bar
Bar or BAR may refer to:
Food and drink
* Bar (establishment), selling alcoholic beverages
* Candy bar
* Chocolate bar
Science and technology
* Bar (river morphology), a deposit of sediment
* Bar (tropical cyclone), a layer of cloud
* Bar (u ...
for every 10 m in fresh water at 4°C. Therefore, the exact 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.
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
![Manometer_anim_02](https://upload.wikimedia.org/wikipedia/commons/1/10/Manometer_anim_02.gif)
The Bourdon tube depth gauge consists of a curved tube made of elastic metal, known as a
Bourdon tube
Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressure ...
. 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
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. B ...
is amplified by a lever and gear mechanism and transferred to an indicator pointer like in an
aneroid barometer
A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis ...
. 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 gauge
A strain gauge (also spelled strain gage) is a device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports ...
s 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
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The primary benefit of the circuit is its ability to provid ...
This signal can be processed to provide a signal proportional to pressure, which may be digitised for further processing and display.
Piezoresistive pressure sensors
![Tauchcomputer_Suunto_Vyper_Air](https://upload.wikimedia.org/wikipedia/commons/d/d4/Tauchcomputer_Suunto_Vyper_Air.JPG)
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.
These pressure sensors are commonly used in
dive computer
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 t ...
s.
Pneumofathometer
![Gas panel 1](https://upload.wikimedia.org/wikipedia/commons/b/b0/Gas_panel_1.png)
![Surface supply air panel for 4 divers P3053737](https://upload.wikimedia.org/wikipedia/commons/d/d9/Surface_supply_air_panel_for_4_divers_P3053737.jpg)
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
Standard diving dress, also known as hard-hat or copper hat equipment, deep sea diving suit or heavy gear, is a type of diving suit that was formerly used for all relatively deep underwater work that required more than breath-hold duration, which ...
, with a free-flow air supply, in which there was not much back-pressure other than the hydrostatic pressure of depth. As
non-return valve
A check valve, non-return valve, reflux valve, retention valve, foot valve, or one-way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction.
Check valves are two-port valves, meaning they have ...
s 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
The practice of decompression by divers comprises the planning and monitoring of the profile indicated by the algorithms or tables of the chosen decompression model, to allow asymptomatic and harmless release of excess inert gases dissolved in ...
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
Decompression theory is the study and modelling of the transfer of the inert gas component of breathing gases from the gas in the lungs to the tissues and back during exposure to variations in ambient pressure. In the case of underwater diving a ...
. Most dive computers contain a
piezoresistive pressure sensor
A pressure sensor is a device for pressure measurement of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually act ...
. Rarely, capacitive or inductive pressure sensors are used.
Uses
A diver uses a depth gauge with
decompression tables
There are several categories of decompression equipment used to help divers decompress, which is the process required to allow divers to return to the surface safely after spending time underwater at higher ambient pressures.
Decompression o ...
and a
watch
A watch is a portable timepiece intended to be carried or worn by a person. It is designed to keep a consistent movement despite the motions caused by the person's activities. A wristwatch is designed to be worn around the wrist, attached by ...
to avoid
decompression sickness
Decompression sickness (abbreviated DCS; also called divers' disease, the bends, aerobullosis, and caisson disease) is a medical condition caused by dissolved gases emerging from solution as bubbles inside the body tissues during decompressio ...
. A common alternative to the depth gauge, watch and decompression tables is a
dive computer
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 t ...
, 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
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
: The
brightness
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. The perception is not linear to luminance, ...
decreases with depth, but depends on the
weather
Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...
(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
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
, 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
Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...
. To gauge depth, an animal would need two
photopigment
Photopigments are unstable pigments that undergo a chemical change when they absorb light. The term is generally applied to the non-protein chromophore moiety of photosensitive chromoproteins, such as the pigments involved in photosynthesis and pho ...
s 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
Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made ...
''
Torrea candida''. Its eyes have a main and two accessory
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
e. The accessory retinae sense UV-light (''λ''
max = 400 nm) and the main retina senses blue-green light (''λ''
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
''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 t ...
''.
The larvae have two structures: The rhabdomeric
photoreceptor cell
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiatio ...
s of the eyes
and in the deep
brain
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
the ciliary photoreceptor cells. The ciliary photoreceptor cells express a ciliary
opsin
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 pro ...
,
which is a photopigment maximally sensitive to UV-light (''λ''
max = 383 nm).
Thus, the ciliary photoreceptor cells react on UV-light and make the larvae swimming down gravitactically. The
gravitaxis
Gravitaxis (or ''geotaxis'') is a form of taxis characterized by the directional movement of an organism in response to gravity. Gravitaxis is one of the many forms of taxis. It is characterized by the movement of an organism in response to gravita ...
here is countered by
phototaxis
Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves towards or away from a stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive li ...
, 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 (''λ''
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.
See also
*: A device used in , , and mountain sports to measure terrain elevation.
*
*
References
External links
on depth gauges hosted by the
Rubicon Foundation
Rubicon Foundation, Inc. is a non-profit organization devoted to contributing to the interdependent dynamic between research, exploration, science and education. The foundation, started in 2002, is located in Durham, North Carolina and is primar ...
{{Underwater diving, divequ
Underwater diving safety equipment
Pressure gauges
Vertical position