Dry ice bomb
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A dry ice bomb is a simple bomb typically made from a plastic bottle, water and dry ice. The simplicity and ease of construction, high bursting pressure, and sound make the dry ice bomb popular in recreational activity.
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[edit] Construction
Generally either a 20 fl oz or a 2 liter soda bottle is used. A few dry ice cubes are added, then a variable amount of water is poured in to speed up the sublimation process. The cap is screwed on tightly, and the area is cleared. As the dry ice sublimates to gas, the pressure inside the bottle increases to a high enough pressure to rupture the bottle, ranging between a few seconds to a few minutes, varying on several factors including the structure of the bottle, outside temperature, water temperature, and amount of dry ice. Often, the bottle will grow noticeably larger due to the extreme internal pressure before bursting.
[edit] Energy calculations
The burst pressure of PET bottles vary. Large bottles tend to have slightly lower burst pressures. A 2 liter bottle may burst anywhere from 140 to 180 psi (0.95 to 1.25 MPa). In theory, the amount of dry ice it takes to reach 180 psi (1.24 MPa) in a two liter bottle is about 50 g, or about 32 cubic centimeters of dry ice. Generally more would be used for safety, as it's dangerous for the bomb not to go off. The amount of energy released varies with altitude, size, burst pressure, volume of water used, and elasticity of the bottle. A two litre bottle will release between 9.2 and 15.2 kJ of energy at sea level, and slightly more at higher altitudes because the gas will expand to a greater volume when it bursts. This energy can be harnessed in simple homemade dry ice cannons.
[edit] Common uses
Dry ice bombs are most commonly used to simply make an explosion. They are also, however, sometimes buried in soft soil or sand for visual effect, sunk with a weight to create a water explosion or very large bubble, or used in dry ice cannons. They are almost never used as weapons as the timing of the explosion can not be controlled. The effects are usually minor but can be severe.
[edit] Dangers
Dry ice bombs are safer than conventional explosives, but do have some serious risks:
- Premature explosion. If too much water is poured in, or the water is too warm, burst pressure can occur within seconds, injuring the handler.
- The shock wave can be extremely loud. Permanent hearing damage can occur even at substantial distances.
- Shrapnel poses a danger to anyone within 20 yards (18 m). The cap is particularly dangerous.
- In some areas dry ice bombs are illegal[1] but even elsewhere the noise generated may violate local laws.
- Leaving an unexploded dry ice bomb can be construed as public endangerment.
Bombs which do not explode are a major safety problem. They can't be left, yet can't be safely approached. The normal advice is to puncture them from a safe distance with a pellet or BB gun.
[edit] Safety guidelines
Although dry ice bombs can be dangerous, there are a number of preliminaries and precautions that one can make to be substantially safer while producing and using.
- Only plastic bottles should be used. Using glass or metal containers is extremely dangerous.
- The location should be away from people and outside of a neighborhood where the noise may alarm the residents. A large empty parking lot or field at least a quarter mile away from residential areas is ideal.
- Ear protection should be worn at all times by all people within a close proximity 46 m /50 yards of the bomb unless the bomb is under at least three feet (1 m) of water.
- It greatly helps to have a pellet or BB gun handy when making bombs, in the case that it doesn't go off on its own. A pellet or BB gun is one of the safest methods of neutralizing it.
- One should always be aware of the direction the cap is pointing when the bomb is set as occasionally the cap may shoot straight out from the bottle at considerable speed, and can harm anyone directly within its path.
- One should always wear gloves when preparing dry ice. This will greatly reduce the risk of burns from dry ice handling.
[edit] Why does this happen?
The primary cause of the explosion is the solid carbon dioxide (dry ice) sublimating into gas by heat transfer from the water. As a gas, the CO2 molecules collide with each other and the bottle at a speed of about 500 m/s (1120 miles per hour). Since more air molecules are hitting the inside of the bottle than the outside of the bottle, it explodes.
[edit] Popular culture references
- A dry ice bomb featured on Mythbusters - episode 57 Mentos and Soda, which was first aired on August 9, 2006.
- The book One Day in the Life of a Fool by Jeremy M Gates includes a story about a dry ice bomb which failed to explode as planned, and later exploded after someone took it indoors.
