Degree of Case Seal: Air Exchange Rates

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The level of air seal of an exhibit case relates directly to its intended function and performance. Microclimatic cases require Level III or well-sealed construction.

What is meant by degree of case seal?

The degree of case seal is defined as the rate of air exchange between the exhibit case interior and the ambient environment of the exhibit space. There are four levels, or degrees, of case seal: unsealed, moderately-sealed, well-sealed, and hermetically-sealed.


DEGREE OF CASE SEAL AIR EXCHANGE RATE (Approximations)
Level I
Unsealed
one per 1 hour or less
Level II
Moderately-sealed
one per 24 - 36 hours
Level III
Well-sealed
one per 72 hours or more
Level IV
Hermetically-sealed
no air-exchange



How are the different levels of seal achieved?

Most exhibit cases are unsealed or moderately-sealed. A well-sealed case requires special attention in both the design and construction phases and must be tested to determine performance. Due to the expense and technical difficulty of achieving the fourth category, hermetically-sealed cases are only built for rare circumstances when the objects are of extraordinary significance (such as display of national treasures) and controlled, inert gas atmospheres are to be introduced.



Degree of Seal Characteristics
Level I
Unsealed
  • incorporates no gaskets or caulking to control air leakage at case seams or doors;
  • permeability of construction material is not considered;
  • no requirement for air-tight fasteners at doors or movable panels;
  • enclosure cannot be used to maintain an internal case environment;
  • applies to commercially-available, pre-assembled cases.
Level II
Moderately-sealed
  • all seams in glazing, panels and doors incorporate a gasket or caulk to reduce leakage;
  • permeability of construction material is not considered;
  • no requirement for air-tight fasteners at doors and movable panels;
  • used when the ambient environment is acceptable for object preservation.
Level III
Well-sealed
  • case must be designed and built to tight specifications;
  • all seams are gasketed or caulked to eliminate air-leakage;
  • incorporates special fasteners and gaskets for door s and movable panels;
  • uses construction materials that are impermeable to air and water vapor or employs barrier films and coating over permeable substrates;
  • required for microclimate control.
Level IV
Hermetically sealed
  • case is engineered to prevent any air leakage;
  • design includes gas impermeable materials, precision joints and seals;
  • includes a mechanism/diaphragm to counteract pressure changes;
  • required to maintain an inert gas atmosphere.



The degree of air seal can be evaluated both qualitatively and quantitatively. Leak detection can easily be performed by using ultrasonic detection equipment. Currently, several museums and exhibit production houses employ tests to establish air-exchange rates—either by pressurizing an enclosure to determine leakage rates or establishing the half-life time of an enclosure by introducing a gas and measuring its rate of loss.