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?[edit | edit source]
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.
|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 Unseal||1 AE per 1 hr or less (≥ 24/day)||> 1.0/day|
|II Moderately||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?[edit | edit source]
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|
|IV Very well-sealed||
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.