Talk:Degree of Case Seal: Air Exchange Rates

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new intro:
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 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

Justification (jean): This proposition keeps the 1 AE per 72 hrs (0.33/day = ≤ 0.3/day) as the recommended level in the CD-rom
An AER above 1/day will required too much silica gel. anyway when people has interest on AER they want an AER, at least, below 1/d
An AER of 0 is not useful but below 0.1/day make sense. people can still request as low as 0.01/d if wanted.
Moderate degree (≤1.0/day) is what is typically achievable by retrofitting old cases.
some showcase makers could target 0.1/d in factory but to be safer after the shipping and re-install may want to claim a <0.3/d degree of seal

Degree of Seal Characteristic
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.

May: proposed after consulting few members of MGW specification group.
June, 5, 2020: People form NARA suggested to show old and new stds. Approved by Theresa Voellinger, NPS and MWG by July 1, 2020
Jean Tétreault

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