Editing Submersible (section)

==Technologies==
{{unreferenced section|date=October 2024}}
Technologies used in the design and construction of submersibles:
 
*[[Buoyancy]] control
*[[Marine thruster]]s
*[[Pressure vessel]] with external pressure load
*[[Life support system]]s
*[[Through-water communications]]
*[[Manipulator arm]]
*[[Submarine navigation]]

'''Absolute pressure:''' At sea level the atmosphere exerts a pressure of approximately 1 bar, or 103,000 N/m<sup>2</sup>. Underwater, the pressure increases by approximately 0.1 bar for every metre of depth. The total pressure at any given depth is the sum of the pressure of the water at that depth ([[hydrostatic pressure]])and atmospheric pressure. This combined pressure is known as absolute pressure, and the relationship is:

Absolute pressure (bar abs) = gauge pressure(bar) + atmospheric pressure (about 1 bar)

To calculate absolute pressure, add the atmospheric pressure to the gauge pressure using the same unit. Working with depth rather than pressure may be convenient in diving calculations. In this context, atmospheric pressure is considered equivalent to a depth of 10 meters.
Absolute depth (m) = gauge depth (m) + 10 m.

'''Depth measurement: Pressure monitoring devices'''

The pressure the is more important for structural and physiological reasons than linear depth. Pressure at a given depth may vary due to variations in water density.

To express the linear depth in water accurately, the measurement should be in meters (m). The unit "meters of sea water" (msw) is a by definition a unit for measurement of pressure.

'''Note:''' A change in depth of 10 meters for a change in pressure of 1 bar equates to a water density of 1012.72&nbsp;kg/m<sup>3</sup>{{cn|date=February 2024}}

Single-atmosphere submersibles have a pressure hull with internal pressure maintained at surface atmospheric pressure. This requires the hull to be capable of withstanding the ambient hydrostatic pressure from the water outside, which can be many times greater than the internal pressure.

Ambient pressure submersibles maintain the same pressure both inside and outside the vessel. The interior is air-filled, at a pressure to balance the external pressure, so the hull does not have to withstand a pressure difference.

A third technology is the "wet sub", which refers to a vehicle that may or may not be enclosed, but in either case, water floods the interior, so underwater breathing equipment is used by the crew. This may be scuba carried by the divers, or a breathing gas supply carried by the vessel.

===Buoyancy===
{{Main|Buoyancy}}
When an object is immersed in a liquid, it displaces  the liquid, pushing it out of the way. 

Once the object is partially immersed, pressure forces exerted on the immersed parts are equal to the weight of water displaced, Consequently, objects submerged in liquids appear to weigh less due to this buoyant force. The relationship between the amount of liquid displaced and the resulting up-thrust is known as [[Archimedes' principle]], which states:

"when an object is wholly or partially immersed in a liquid, the up-thrust it receives is equal to the weight of the liquid displaced."{{cn|date=February 2024}}

Buoyancy and weight determine whether an object floats or sinks in a liquid. The relative magnitudes of weight and buoyancy determine the outcome, leading to three possible scenarios.

'''Negative Buoyancy:''' when the weight of an object is greater than the up-thrust it experiences due to the weight of the liquid displaced, the object sinks.

'''Neutral Buoyancy:''' if the weight of an object equals the up-thrust, the object remains stable in its current position, neither sinking or floating.

'''Positive Buoyancy:''' when the weight of an object is less than the up-thrust, the object rises and floats. As it reaches the liquid's surface, It partly emerges from the liquid, reducing the weight of the displaced liquid and, consequently, the up-thrust. Eventually, the reduced up-thrust balances the weight of the object, allowing it to float in a state of equilibrium.

=== Buoyancy control ===
During underwater operation a submersible will generally be [[Neutral buoyancy|neutrally buoyant]], but may use positive or negative buoyancy to facilitate vertical motion. Negative buoyancy may also be useful at times to settle the vessel on the bottom, and positive buoyancy is necessary to float the vessel at the surface. Fine buoyancy adjustments may be made using one or more [[variable buoyancy pressure vessel]]s as  [[trim tank]]s, and gross changes of buoyancy at or near the surface may use ambient pressure [[ballast tank]]s, which are fully flooded during underwater operations. Some submersibles use high density external ballast which may be released at depth in an emergency to make the vessel sufficiently buoyant to float back to the surface even if all power is lost, or to travel faster vertically.