Humidity control for Record Preservation
Galleries, Museums and Libraries are public places requiring ventilation air that carries a high moisture load.
Throughout relatively short time cycles of 24 hours, the atmospheric air pressure at any location may remain virtually constant. However, temperature and moisture content vary and this is reflected in the way relative humidity (RH) alters throughout the day and night. The consequences of these naturally varying humidity ranges can be disastrous if unconditioned air is allowed to come into contact with material of a sensitive and hygroscopic nature.
Galleries, Museums, Libraries and Archives
Galleries, Museums and Libraries are public places requiring ventilation air that carries a high moisture load. For these types of buildings, a dehumidifier treating the load at its source, the fresh air, is most suitable, i.e before it enters the building. Archives have less public access and therefore less ventilation so treating the return air is more appropriate.
In properly temperature and humidity controlled stores, the preservation life of most records can be significantly increased. In the case of colour photographs, the preservation life can be increased from 25 years at 24°C, 50% RH to 1,000 years at 2°C, 40% RH. Similarly, poor paper life can be extended from 50 years at 20°C, 50% RH to 200 years at 16°C, 25% RH. The following represents typical recommendations for long-term preservation storage conditions.
• Computer print-outs
• Maps and plans
• Charts 20°C, 50% RH
Photographic Media – Black and White
• Sheet film
• Cine film
• Glass Plate Photos <18°C, 35% RH
Photographic Media – Colour
• Sheet film
• Cine film <5°C, 35% RH
• Computer tapes and disks
• Video tapes
• Audio tapes
• Magneto-optical disks
• Optical Media
• Compact and mini disks
• Laser discs 18°C, 35% RH
Air conditioners are designed to cool buildings with the equipment controlled by a thermostat. Air conditioners provide temperature stability only. In a temperature-only-controlled building, humidity will continue to fluctuate with typical yearly ranges of 20 to 80% RH. Munters equipment removes moisture from the air using a dessicant wheel, which easily attracts and holds water vapour. Desiccant dehumidifiers are uniquely suited to removing moisture from air at low temperature and humidity.
The desiccant is impregnated into a wheel. Air passes easily through the flutes, contacting the desiccant. The wheel rotates slowly between two air streams. The incoming air stream gives off its moisture to the desiccant. The process air is dry as it leaves the wheel. The now humidity-laden wheel rotates slowly into a second, smaller heated air stream. This exhaust air stream, referred to as the reactivation air, warms the desiccant, allowing the desiccant to give off the captured moisture, which is carried away by the reactivation air. The newly dried desiccant material is then rotated back into the process air, where it absorbs moisture once more, recommencing the DH cycle.
Moisture is the most important factor in environmental control for records as excess humidity causes corrison on metals and microbial activity on organic materials. In order to limit chemical deterioration of records, it is today widely accepted that 50% RH is the upper limit of acceptable humidity with the lower limit being 20% RH.
Temperature and pollutants are also important factors in preserving documents. In general, the humidity and temperature have to be as stable as possible and pollutants should be kept at a minimum.