Pressure – the force acting over an area, where the force is perpendicular to the area.
- p = F / A
where p is the pressure, F is the normal force, and A is the area. Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point.
Pressing your finger against a wall will not making any lasting impression. The same finger pushing a thumbtack can damage the wall. Although the force applied to the surface is the same, the thumbtack applies more pressure because the point concentrates that force into a smaller area.
Some examples of pressure measurements
- car tires – 30 – 40 psi
- “normal” blood pressure – 120mm Hg / 80mm Hg (gauge press)
Changes in pressure
- car tire pressure measured with a standard pressure gauge – 35psi at sea level, 40psi at 12,500 feet above sea level. The air at 12,500 is less dense so the air pressure in the tires seems to be greater, as there is less pressure on the outside of the tire – atmospheric pressure, gauge pressure
- scuba divers – at 15 feet feel a 1/2 atmosphere pressure increase from the water pressing down – hydrostatic pressure
- boiling water at 5,000 feet elevation with reduced pressure and lower boiling point will take much longer (about 25% increase in cooking time)
- Alvin’s latest upgrade began in 2005 – the three-inch thick titanium sphere, intended to carry Alvin’s crew, will allow scientists to reach a depth of 6,500 meters (21,000 feet) where the surrounding pressure reaches 10,000 psi. … a new syntactic foam created to withstand the crushing forces of “deep marine exploration” without becoming compressed by exterior pressure, will provide additional buoyancy. …
In the photo NASA astronaut/NEEMO 16 commander Dottie Metcalf-Lindenburger, Andrew Abercromby (NASA Deputy Project Manager for the Multi-Mission Space Exploration Vehicle) and ESA astronaut/NEEMO 16 aquanaut Timothy Peake pose with a DeepWorker submersible during the NEEMO 16 undersea exploration mission in June 2012.
Since 2001, NASA has used Aquarius for its NEEMO (NASA Extreme Environment Mission Operations) program, to study various aspects of human spaceflight in a similar environment. Like the environment of space, the undersea world is a hostile, alien place for humans to live.
Exploring the other 71%
- Ask – NOAA’s Aquarius underwater habitat currently sits in about 60 feet of water, 4.5 kilometers offshore of Key Largo, Florida, on a sand patch adjacent to deep coral reefs in the Florida Keys National Marine Sanctuary. At this depth, the pressure on the divers and the habitat is 2.5 times surface pressure. What are some of the limitations to underwater exploration? How much difference does staying a the bottom make? How many people need to be accommodated? How much stuff do they have to bring with them?
- Imagine – Through saturation diving techniques, Aquarius allows scientists to live and work underwater 24 hours per day for missions lasting approximately 10 days. Scientists can conduct research and observe thing that would be difficult to observe if diving from the surface. : What are some other examples of living away from home? How are living in high pressure and very low pressure similar? Can some of the technologies used for the International Space Station be used on Aquarius?
- Design, Build – The Entry Lock, contains bench space for computers and experiments, power equipment, life support controls, small viewports, and bathroom facilities. The Main Lock, which includes berths for a six-person crew, computer work stations, two large viewports, and kitchen facilities.
In addition to not having the usual living accommodations, the Aquarius habitat also has a double-lock pressure vessel, life-support controls, 4 adjustable legs each with 25 tons of lead ballast. The pressure of the water at the 60 foot depth presents many engineering problems. The design of Aquarius provides structure and extensive monitoring and sensing devices to ensure the safety of the crew, equipments, and the Aquarius habitat. What are some of the solutions that are unique to undersea living?
Aquarius was completed in 1986. Teams of scientists move in and get to work.
- Improve – Aquarius was damaged during Hurricane Hugo in 1989 and was repaired and modified.
- Aquarius “moon pool” – the air pressure in this chamber is slightly greater than the water pressure outside Aquarius at this depth (usually 50-60 feet below sea level). This keeps the water out, so the divers and scientists can move between the water and the interior of the habitat
- differential pressure can be strong enough to provide strength to a structure. A full soda can containing soda and carbonation under pressure is strong enough to withstand crushing. When the can is empty, the pressure differential is zero, and it is easy to crush.
- Under pressure, gas dissolved in liquid. Underwater, the water provides the higher pressure, and people, air tanks and underwater structures have to work against that pressure. Divers need to allow time for gas dissolved in their blood underwater to return to normal levels as they move toward the surface to prevent the “bends”.
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Here are some challenges for you to work on…
- Which is stronger – you or air? – simple activity that may surprise you.
- gather some household items. Demonstrate a variety of results applying pressure. Document and explain your results.
- on an airplane flight, tightly cap a soda bottle while you are at cruise altitude. Check the bottle when you land.
- research systems for living at 60 feet below the surface. Provide details about how ordinary living conditions have to be changed.