Controlled Atmospheres

The gases primarily used for controlled-atmosphere fumigations are carbon dioxide, nitrogen, or a mixture of these gases. These gases, in effect, are applied to reduce the oxygen content in a storage.

Carbon Dioxide

The addition of this gas not only reduces the oxygen content but also acts directly on stored pests by acting as a dessicant on the insect body fluids. Carbon dioxide is usually applied from trucks or tanks that contain liquid carbon dioxide. Appropriate vaporizers and regulators are then used to supply gaseous carbon dioxide to the structure to be fumigated. The gas is usually applied at the top of a silo and because it is denser than air the gas will move down through the grain mass. The silo or warehouse to be fumigated must be tightly sealed, and carbon dioxide must be added daily to maintain a 60-70% concentration to be efficient. A system must also be used in the structure that will not allow the pressure inside to build up during application. Usually a vent is placed in the roof or a hatch or portion of polyethylene used to seal roof vents is opened during application.

Nitrogen

Nitrogen can also be used as an inert gas to control oxygen concentrations. A nitrogen generator is required, and normally the oxygen concentration must be reduced to approximately 1%. Fumigation time can vary but usually requires approximately 10 days for good control.

Inert Atmosphere Generators

There are several inert atmosphere generators that result in a mixture of nitrogen and carbon dioxide being applied to the structure to be fumigated.

To use a controlled atmosphere for fumigation, the following factors must be considered.

1. Temperature of the commodity and within the storage area should be 21°C (70°F) or higher to have an effective fumigation. Lower temperature will effect the mortality of the insect present in the structure or commodity.

2. A vent or pressure relief system must be present in the warehouse or silo to prevent structural damage due to pressure build up.

3. Application time can take up to 12 h with daily recharging, which will normally take 3-4 h.

4. Total fumigation time can be from 4 to 10 days depending on temperature and gas tightness of the structure.

5. Even though these gases are considered to be nontoxic, an air pack must be used when entering a facility that has low oxygen content during fumigation.

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