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RH Control: Use of Silica Gel
Hybrid silica gels, developed during the last decade, are generally more effective for museum applications, particularly in maintaining a range between 40 -60% RH. This means that a smaller quantity of gel will condition a larger space. Many factors must be considered when calculating the appropriate quantity of gel for an application.
It may be useful to consider the table below as a guide, or "rule of thumb" for well-sealed and moderately-sealed cases. The table also includes information regarding the volume of silica gel and the space required to contain it. The table uses volumes for basic tray containers, however, gel containers can be made or purchased in many efficient formats, including as:
- tiles, cassettes and panels;
- rigid plastic or fabric tubes;
- and bags.
Containers must restrict the depth of gel to less than 2" and materials used to make containers must be non-hazardous and permeable to moisture vapor.
RH Control: Mechanical Systems
Miniature HVAC equipment is available for generating specific climate conditions within exhibit cases. These mechanical systems feed conditioned air into the case and out into the room or re-circulate it back thorough the system. A wide range of humidity levels can be produced, however, the units have several important drawbacks. Miniture HVAC systems:
- raise the risk of electrical fire;
- introduce the possibility of mechanical failure which can harm objects;
- introduce heat and thermal instability;
- introduce vibration and noise;
- require the proximity of water near the case.
Exhibit case design must consider these deficiencies. Mechanical systems can be located within a case if insulation and vibration dampening devices are used; alternatively they can be located in another easily accessed, adjacent space. Staff maintenance can be considerable, therefore, the direct plumbing of the equipment is recommended.
Locating Absorbers in Cases
Micro-environmentally controlled cases incorporate moisture absorbers, such as silica gel and chemical pollutant absorbers, such as activated charcoal. Their location within cases varies according to: case style and size, the type of environmental system and quantity of absorber and budgetary considerations.
Passive designs generally utilize an interior environmental maintenance chamber that adjoins the display chamber. The simplest design locates the equipment behind display objects, within exhibit pedestals and furniture, or within interior panels. The critical feature is that the equipment is within the same air space as the display objects. Designs should ensure:
- no object contact with the environmental products;
- sufficient space for the quantity of absorber;
- adequate air circulation over the absorber;
- practical access for installing and servicing.
The following diagram illustrates several practical locations.
Accessing Absorbers in Cases
Micro-environmentally controlled cases must incorporate a practical method for accessing environmental absorbent materials (e.g. silica gel or activated charcoal). Access should not interrupt the case seal, and accommodate the initial installation. Servicing of equipment and should not require moving or contacting the display objects. Design should consider the frequency of absorber maintenance; i.e. control that requires a specific RH for the duration of an exhibit usually needs more frequent access than is needed for RH buffering designs (which can run an entire year without having to access the gel). Absorbers can be loaded before objects are installed; and a small door or port can be used to access the absorber. Five methods of access for micro-environmentally controlled cases are illustrated below:
- Access through the Display Chamber
- Removable Access Panels
- Hinged Access Doors
- Sliding Access Drawers
- Prefabricated Access Doors
The successful performance of microenvironmentally controlled cases relies on uniform conditions within the enclosure. Air mixing and the even distribution of humidity are critical features that are commonly overlooked. Restricted air circulation and impermeable barriers within the display chamber keep objects from the benefits of the climate control system. Ensure that air passes freely over environmental absorbers and that conditions are quickly equalized within all areas of the display chamber. To ensure adequate air circulation two methods are commonly used:
- a sufficient perimeter gap on all sides of the display platform or deck; leave as large a gap as possible for air movement into the maintenance chamber (from 5/8 to 1" diameter for small and average size vitrines); or
- a perforated display deck or plinth (the surface should be at least 40% open). Avoid drilling wood products, it is best to fabricate from metal or conservation appropriate plastic materials.
The use of ventilated cases is appropriate only when the conditions of temperature and relative humidity throughout the exhibit space meet the established conservation requirements. Mechanical, forced air systems are used in special circumstances or in large cases, when it is determined that natural convection venting is not sufficient. Fans should be carefully selected; select low CFM rated equipment to avoid excessive air flow; select low noise and vibration equipment; select fans that accept filter media. Filters are required to ensure against the entry of dust and insects. Fans can run indefinitely and filters can be replaced after several years, depending on the room's airborne particulate and contamination levels. A practical design combines a fan with an equally- sized filtered vent, located far apart. If only one fan is employed, it should direct air out of the case. If two fans are used one can push air out and the other one pull air in. Placement of fans is less important than the vent location in passive systems.
Passive ventilated cases employ air vents or ports and rely on natural convection for air circulation. This design is appropriate only when the environmental conditions throughout the exhibit space meet the established conservation requirements. If passive venting is not sufficient, forced air (fan driven) designs should be employed.
Ventilation vents can be unobtrusively placed with- in decorative features of a case; i.e. in reveals, kick space and behind graphic panels. Two vents are sufficient for small or moderate sized cases. The location of the vents is important:
- lateral vents (installed at the same height) will restrict excessive pumping of air (chimney effect)
- vertically installed vents allows warmer air to rise and exit the upper vent pulling in cooler from below (destabilizing the interior RH)
In all designs, filters are required to ensure against the entry of dirt and insects. High quality filters can be serviced after six months or one year, depending on the room's airborne particulate level.