Degree of Case Seal: Air Exchange Rates

Back to complete list of Exhibit Technical Notes

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 airtightness degree of the display case is defined as the air exchange rate (AER) between the interior of the display case and the ambient environment of the exhibition space. The NPS 1999 column in the table below remains a reference. The AIC Wiki degree of case seal considered the following points: In general, the lower the air exchange rate, the lower the amount of moisture absorbers required. It is also necessary to assess the risk of pollutants emitted by the materials of the display case and the objects in it. Generally, when there is interest in the AER, the target is usually 1 day or less. A display case with an AER of 0.1/day or less is considered to be very airtight, but not all display case models can achieve this target and not all display case manufacturers have the expertise to achieve and maintain this leakage rate in the long term at a reasonable cost. AERs between 1 and 0.1 days are intermediate performances for which it is easier to design showcases at a reasonable cost and within a reasonable time frame. Most of the AERs requested by museums are also between 1 and 0.1 days. The degree III of case seal of AIC Wiki 2020 matches the recommended degree III of NPS 1990 for some continuities.

	NPS 1999 (original)	AIC Wiki 2020 (replacing NPS 1999)
Degree of Case Seal	Air Exchange Rate (approximations) (per day as reference)	Air Exchange Rate (based on CO2 or N2O)
I Unsealed	1 AE per 1 hr or less (≥ 24/day)	> 1.0/day
II Moderately sealed	1 AE per 24 - 36 hrs (1 - 0.67/day)	≤ 1.0/day
III Well-sealed	1 AE per 72 hrs or more (≤0.33/day)	≤ 0.3/day
IV Very well-sealed		≤ 0.1/day
V Hermetically-sealed	no AE

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.
IV Very well-sealed	case must be designed and built to tight specifications; design includes gas impermeable materials, precision joints and seals; required to maintain a high-performance case.
V 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.