Showcase Leakage Testing
The importance of knowing the Air Exchange Rate (AER) of a museum display case cannot be overstated when considering microclimate control. Leakage is extremely common, and while a showcase may be tightly sealed when first assembled in the factory or gallery, most showcases (brand new, very expensive, or both) usually leak more than you would expect after installation.
An accurate showcase AER is needed to accurately calculate the amount of ProSorb needed to maintain humidity levels, to determine how to best mitigate off gassed pollutants, or to adjust the output of a microclimate generator to ensure that an optimal environment is assured.
Keepsafe offers two methods of testing showcase AER:
Leakage under Pressure
Air is fed into a showcase at a rate just high enough to slightly pressurize the case. The target pressure approximates the "stack pressure" which leads to air leakage under normal gallery conditions. The rate of air flow into the case is measured and the AER calculated.
A carbon dioxide monitor and logger is placed in the showcase. Carbon dioxide is injected and the showcase is normally sealed. The monitor is left in the showcase for a period of hours or days and the reduction in carbon dioxide levels due to showcase leakage is recorded. The data is collected and the AER can be calculated.
Once leakage has been noted, leaks can be located and mitigated using a variety of leak indicating devices and procedures.
What is "normal" case leakage?
Paul Marcon of the Canadian Conservation Institute has written: "Most museum display cases are in the range of one-half an air change per day (acd) to 20 acd. A one cubic metre (35 cu. ft.) glass case with a cube shape and no gaps greater than 0.3 mm (0.01 in.) is near 1 acd. A case with long gaps of 1.5 mm to 2 mm on the top and bottom will have a leakage rate of 20 acd or more. For more information see the article by Stefan Michalski in Studies in Conservation 39 (1994) p.p. 169-186."
A new case from an experienced manufacturer may have an AER of 0.1 acd, but as you can see from Marcon's notes above, it takes only a slight misalignment or imperfect seal to create a much leakier environment, and a dramatic change in requirements for microclimate control.
Please enquire about our testing services, or call us to discuss training and outfitting you to to do your own leakage testing.