# Lessons from SEALAB: Underwater Habitat Exploration

## Table of contents

## Abstract

The article selected presents a discussion about the underwater habitats known as SEALAB I and SEALAB II, the article was adapted from 'Living under the Sea' by Dr. Joseph MacInnes which originally appeared in the March 1966 issue of 'Scientific American' magazine. The selected article discusses several topics that included the special structural details and location of SEALAB I and SEALAB II, how the divers would deal with decompression sickness when working in underwater habitats, and the saturation diving experiments that were performed on the divers in SEALAB I and SEALAB II. Combining this week’s unit lessons regarding liquids and gases are essential to the science needed and required for living and working in an underwater environment.

## Article Review with Unit Lessons Learned from Physics

The article selected was greatly appreciated, and presents a discussion about the underwater habitats known as SEALAB I and SEALAB II, the article was adapted from 'Living under the Sea' by Dr. Joseph MacInnes which originally appeared in the March 1966 issue of 'Scientific American' magazine. The selected article discusses several topics that included the special structural details and location of SEALAB I and SEALAB II, how the divers would deal with decompression sickness when working in underwater habitats, and the saturation diving experiments that were performed on the divers in SEALAB I and SEALAB II. Using this week’s unit lessons regarding liquids and gases are essential to the science and the analysis needed for a complete article review of living and working in an underwater environment. Unlike solids, liquids and gases do not have a definite shape unless they are kept in a container. Fluid is matter that can flow without a specific configuration. Both liquids and gases are instances of fluids. Water and air move from place to place if they are not confined in a vessel. Water and air show a comparable pattern of motion, but water and air density are very different because it depends on the property of matter. Gases usually have smaller densities than liquids have because the average distance between molecules is greater in gases than in liquids. The density of gases is greatly dependent on pressure and temperature.

## Pressure in Fluids

A resounding issue found in the article was regarding pressure in fluids. This week’s unit lessons explained that when a force (F) acts on an area (A) in a fluid, the pressure (P) can be expressed as P=F/A. Here, the only consideration is the magnitude of F, so P is a scalar and its unit is the Pascal (N/m2 =Pa). Atmospheric pressure at sea level is about 101,300 Pa =1 atm. So, when the divers go deeper in the ocean to work then they will experience a greater pressure as presented by the unit lessons example of when you go deeper in a swimming pool or in an ocean. When Edwin A. Link set out to build an underwater vehicle in 1956 he needed to understand the relation between pressure and depth. Edwin A. Link needed to consider that one column of water with height h below figure in a large swimming pool. The area (A) of the top face is the same as that of the bottom face. The pressure (Pt) on the top face creates a downward force, or PtA. The pressure (Pb) creates an upward force, or PbA. Also, the weight (mg) due to gravity points downward. The water is at rest, and thus its acceleration is zero. Here the column is in equilibrium. Edwin A. Link was able to apply Newton’s second law, and the summation of the vertical forces is zero: PbA-PtA-mg=0. Use m=ρV=ρAh. Then, Pb=Pt + ρgh. Edwin A. Link clearly could see that the pressure at a deeper level is greater than the pressure at a shallow level if the density is not changing or if it is incompressible. In the case of gas, the density varies according to the vertical distances or if it is compressible, the formula only works when h is very small. For instance, the density of our atmosphere varies significantly from the earth’s surface to higher altitudes. The important thing is that the pressure difference between lower and higher levels comes from the height, or the vertical distance, not the horizontal distance within the fluid.

## Pascal’s and Archimedes’ Principles

Water towers use Pascal’s law to provide water to homes. Pascal’s principle states that any externally applied pressure is transmitted undiminished to everywhere in a completely enclosed fluid at rest. This is the same analysis of the above equation: Pb=Pt + ρgh. The bottom pressure is equal to the sum of top pressure, which is the externally applied pressure, and the static fluid pressure due to the weight of the fluid. For instance, this is the case with the mechanism of a hydraulic car lift when the static fluid pressure is zero. The first experiments that Edwin A. Link completed regarding the underwater vehicle were moving the vehicle and determining the balance and weights needed to maintain the underwater vehicle’s position. The unit lesson indicated that a beach ball is very hard to push under the surface of the water. The water, in fact, pushes back, and this upward force is called the buoyant force; its cause is the pressure of fluids, and its magnitude depends on depth. The net upward force is called the buoyant force F= PbA-PtA= ρghA=mg=weight. Notice that the buoyant force does not depend on the shape of the object. Archimedes discovered this property more than 2,000 years ago. Archimedes’ principle states that a fluid exerts a buoyant force to an immersed object. The magnitude of the buoyant force equals the weight of the displaced fluid.

Bernoulli’s Equation and Its Application. Knowing that the underwater vehicle would be in motion Edwin A. Link needed to consider the motion. When fluids are in motion, they move in a variety of ways. By observation water flows calmly in a shallow stream and violently in a steep valley. The air blows sometimes very gently and sometimes vigorously with great speed. In order to characterize the type of fluids, compressible/incompressible and viscous/non-viscous categories are used. When the density of a fluid is almost constant, the fluid is said to be incompressible. The unit lesson explained that most liquids are incompressible, but all gases are not. When a fluid does not flow easily, like molasses, it is a viscous fluid. Water on the other hand flows very easily because its viscosity is very small. Incompressible and non-viscous fluids are called ideal fluids; this is great to describe the motion of fluids with mathematical equations. For steady flow, Bernoulli studied the behavior of ideal fluids. Its result is in his equation: P+1/2 ρv2 + ρgy=constant. In this equation, y is the elevation at any point and v is the fluid speed. If the flow speed is not changing, the above equation goes back to the pressure equation when water is at rest. If the flow is horizontal, that is, there is no elevation between two points, the pressure is related with the speed. The higher fluid pressure makes the slow-moving flow and vice versa. In addition, when the volume flow rate Av=constant, if the cross-sectional area of a tube is large, the fluid speed is small and vice versa. With these fluid equations, we can describe the motion of liquids in a plumbing system, the speed changes of oil in a pipe, and even the dynamics of an airplane wing, very essential for Link’s underwater vehicle.

Ideal Gas Law. Edwin A. Link had to also look at the ideal gas law when considering his underwater vehicle. When a gas has a very low density (the average distance between molecules of the gas is very large), it is called an ideal gas. The ideal gas law is the relationship between pressure, volume, and temperature. More exactly, the absolute pressure (P) of an ideal gas is proportional to the temperature and is inversely proportional to the volume (V): P =constant *T/V. If the temperature is not changing, that is, T=constant, the pressure P is inversely proportional to the volume V of the gas: PV=constant. This is called Boyle’s law. Similarly, if the pressure P is constant, the volume V is proportional to the temperature T: V/T=constant. This is called Charles’ law.

## Closing and Conclusion

In conclusion, the selected article discusses several topics that included the special structural details and location of SEALAB I and SEALAB II, how the divers would deal with decompression sickness when working in underwater habitats, and the saturation diving experiments that were performed on the divers in SEALAB I and SEALAB II. This week’s unit lesson regarding liquids and gases are found to be essential to the science needed for living and working in an underwater environment. Edwin A. Link needed to understand and apply the science learned from this week’s lessons to begin and complete the underwater vehicle started in 1956.

## References

- Cutnell, J., & Johnson, K. (2004). Physics (6th ed.). Hoboken, NJ: Wiley
- Hardy, K., Koblick, I., & MacInnis, J. B. (2016). Ed Link’s submerged portable inflatable dwelling (SPID). Journal of Diving History, 24(86), 42-26. Retrieved from https://libraryresources.columbiasouthern.edu/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=a9h&AN=114708312&site=ehost-live&scope=site
- Hewitt, P. G. (2015). Conceptual physics (12th ed.). Boston, MA: Pearson.

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