Regulating Relative Humidity Rates in Dynamic Fumigation Systems

Abstract

The use of controlled atmosphere fumigation as an alternative to chemical biocides has proven to be effective in eradicating insect pests in museum collections. However, some operational problems arose during the implementation of controlled atmosphere fumigation. One of the problems was the fall in the relative humidity (RH) value inside the insecticidal bag while purging the dry inert gas. Moisture-containing and moisture-sensitive materials slowly lose water and undergo dimensional and chemical changes if moved from an atmosphere with ambient levels of water vapor to a much drier one. Previous studies have been conducted in order to determine solutions to this problem. In the 1990s, the Getty Conservation Institute and the J. Paul Getty Museum developed a simple humidification system to regulate RH by humidifying the inert gas stream before being passed into the treatment bag. However, the exact control of RH using this system was difficult. Even after being modified, this system is still inapplicable when using carbon dioxide as a fumigant, due to the possibility of the formation of carbonic acid. Therefore, RH control during the treatment is still regarded as a difficult challenge. This research introduces a simple humidification system, innovated by the present authors. The system controls RH by flushing water vapor gradually into the fumigation bag in order to compensate the fall in RH that results from replacing air with ambient RH by dry inert gas. A water vapor stream that is generated by a vaporizer (humidifier) is introduced into the bag through a polyvinyl chloride (PVC) piping unit installed in the upper part of the bag. The vapor is distributed through holes on both sides of the PVC pipe, compensating the fall in RH. The volume of water vapor is proportional to the volume of the dry gas flushed into the bag. Achieving complete control of RH is dependent on the operator’s expertise and instantaneous monitoring of RH via data loggers installed in the bag. The complications and challenges in the operation and implementation of the system and subsequent solutions are discussed in the research. The results proved that the system managed to maintain RH within the desired range (50 ± 5%) for the desired duration. The system overcomes the limitations of previous trials and can be used with all fumigants (argon, nitrogen, and carbon dioxide). It enables the operator to control RH during treatment, and allows instantaneous and accurate follow-up to the RH values inside the fumigation bag throughout the treatment.