The Pressure in an Air Cannon

Welcome to All Sensors “Put the Pressure on Us” blog. This blog brings out pressure sensor aspects in a variety of applications inspired by headlines, consumer and industry requirements, market research, government activities, and you.

The Pressure in an Air Cannon

A compressed air cannon may sound a little weird, but it’s surprising what a sudden pulse of air can do. For simply having fun, the AirZooka can blow a harmless ball of air up to 20 feet. Also for play is the Zero Blaster that launches 2-to-6-inch diameter non-toxic fog rings, which travel up to 14 feet. The fog allows the user and observers to see the toroid shape of the pressure pulse.

AirZooka Air Gun

Image Source: AirZooka on Amazon

The sudden release of a diaphragm allows pressure to build inside a chamber. With a restriction on the opposite end the pressure is increased according to Boyle’s law (P1 x V1 = P2 x V2).

One do-it yourself website for an air-powered cannon recommends attaching a bicycle pump or air compressor to build up pressure to 50-80 psi on a design that uses water pipe in a sealed chamber with a release valve. Depending on the design, air cannons are also used to launch potatoes, tennis balls and other stuff.

The world’s largest air cannon, built in the Czech Republic in collaboration with the TV show “Wonders of Nature,” can shoot air more than 300 feet. Its effectiveness was demonstrated by knocking down a wall of cardboard boxes. Smoke added to the pulse shows the toroid shape of the moving air.

Popular Mechanics Air Cannon DemoImage source: Popular Mechanics

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Using Pressure to Salvage Underwater Treasures and History

Welcome to All Sensors “Put the Pressure on Us” blog. This blog brings out pressure sensor aspects in a variety of applications inspired by headlines, consumer and industry requirements, market research, government activities and you. In this blog we’ll be discussing the role pressure plays in salvaging underwater treasures and history.

Using Pressure to Salvage Underwater Treasures and History

Finding a long lost ship that carried gold, silver or other cargo that could survive an underwater hiding place for dozens, hundreds or even thousands of years is just the beginning of an underwater salvage operation. If a simple chain or rope cannot be used to retrieve the underwater treasure, the next step is buoyancy – using the pressure difference between a fluid (gas or liquid) to exert an upward force to overcome the weight of the submerged object.

Lift bags, parachute lifting bags and other terms are used for the lower density objects that can be either open or closed structures to create the lifting force to raise a submerged item. At certain depths, air can be used as the lifting force.

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Lifting bag courtesy of Water Weights.

For an ideal gas, Boyle’s Law states that the product of pressure and volume is a constant when the temperature is held constant.

p * V = constant

In addition, every 10 meters or 33 feet of depth, the water pressure increases by an additional bar (14.5 psi) above atmospheric pressure.

As a result, the amount of air required to lift a payload depends on the weight of the load and the depth of the bag. For example, lifting 5.6 tons with air bags at a depth of 20 meters requires 5m3 of air at 3 bar (45 psia), which is a volume of 15m3 at surface pressure (1 bar or 14.5 psia).

The compressibility of air creates problems in deeper salvage operations.  As one salvage company notes, “At depth, the air in the bags will be crushed by the water pressure until it is almost as dense as the water around it.”

When the lifting structure and submersed object rise, the pressure decreases and the air inside the bag expands. This can cause a dangerous situation by accelerating the rate of ascension. Observing how far a submerged ball can be launched into the air is a rather easy demonstration of this problem.

To counter and control this effect, the pressure relief valve in one company’s enclosed floatation and salvage pontoons releases pressure at 2.5 psi above ambient pressure.

For deeper salvage operations, fluids other than air are used. In any case, the pressure difference provides the ability to raise submerged objects and understanding the forces involved enables a successful and safer process.

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Do you have a pressure sensing question? Let me know and I’ll address it in an upcoming blog.
-Han Mai, Senior Marketing Specialist, All Sensors Corporation ([email protected])