Museum Grade Airtightness

Air change is defined as the ratio of air supply into a display space in relation to the volume of display space.The rate of air exchange between museum display cases and external space in the museum rooms is a vital concern on the conservation function of museum display cases.

Air exchange rate (AER) or leakage rate of display cases dictates the flow of air (a mixture of gases including water vapour, pollutants and particulates) in and out of internal display space.The air exchange rate will effectively influence the display space of display cases to buffer external environmental conditions and limit the ingress of dust and externally generated pollutants.In addition,the finally acquired micro-climate values and the control precision of climate system are also very related to the airtightness of display space.

The NPS 1999 column in the following table shows a reference about different air exchange rate. Most of museums require the AERs of museum display cases to be between 1 and 0.1 days and generally, a display case with an AER of 0.1/day or less is considered to be extreme airtight although AERs between 1 and 0.1 days are intermediate performances for which it is easier to customize museum display cases at an affordable cost and within the affordable time .

Well airtightness with AER 0.1 or less is achieved by the consistency of the display case structure, the high fabrication precision for each parts, the dedicated sealing extrusions,the glass bonding technology,the door sealing strips and the performance of opening system and so on.Well airtightness of museum display cases ensures minimal air exchange between internal display space and external atmosphere to be not more than 0.1 times per 24 hours.

Air exchange rate (AER) can be tested by standard tracer gas measurement.For this purpose, a small amount of the tracer gas is released in display space and its concentration is then recorded as a function of time. Subsequently, using evaluation algorithms the AER can be calculated from the data obtained.The first section is introducing basic physical principles of air change processes between internal display space and external atmosphere,followed by a description of tracer gases which are frequently used in daily routine, and methods determining the AER. Moreover, the applicability of carbon dioxide as a tracer gas has been compared with results obtained by hexafluorobenzene or sulphur-hexafluoride.