Editing Depth gauge (section)

== Types ==
{{further|Pressure sensor|Level sensor}}

=== 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 [[Graduation (instrument)|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 ===
{{main|Bourdon gauge}}
[[File:Manometer anim 02.gif|right|thumb|Bourdon tube]]
The Bourdon tube depth gauge consists of a curved tube made of elastic metal, known as a [[Pressure measurement#Bourdon gauge|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 [[Indicator (distance amplifying instrument)|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 ===
{{main|Pressure measurement#Membrane}}

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 [[Diaphragm (mechanical device)|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 gauge]]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]] This signal can be processed to provide a signal proportional to pressure, which may be digitised for further processing and display.

===Piezoresistive pressure sensors ===
[[File:Tauchcomputer Suunto Vyper Air.JPG|right|thumb|Dive computer showing depth display]]
[[Piezoresistive pressure sensor]]s 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.<ref name="omega" >{{cite web|url=https://www.omega.com/en-us/resources/types-pressure-sensor |title=Pressure sensor |date=17 April 2019 |website=www.omega.com |access-date=9 December 2019 }}</ref> These pressure sensors are commonly used in [[dive computer]]s.<ref name="amsys" >{{cite web|url=https://www.amsys.info/sheets/amsys.en.wp02.pdf |title=How to measure absolute pressure using piezoresistive sensing elements |website=www.amsys.info |access-date=9 December 2019 }}</ref>
{{expand section|date=December 2019}}

=== Pneumofathometer ===
<!-- target for redirect [[Pneumofathometer]] -->
[[File:Gas panel 1.png|thumb|left|Surface supplied diving gas panel for one diver:
{{unbulleted list|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 }}]][[File:Pressure gauge on Siebe Gorman manual diver's pump P3220126.jpg|thumb|Pressure gauge on Siebe Gorman manual diver's pump, indicating delivered pressure in pounds per square inch (black) and feet sea water (red)]]
[[File:Surface supply air panel for 4 divers P3053737.jpg|thumb|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 valve]]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. The type of high precision gauge used is also known as a [[caisson gauge]]. Precision is typically 1% to 0.25% of full scale.