Determining Exhibit Case Air Exchange Rates
Back to complete list of Exhibit Technical Notes
The rate at which air exchanges between a case and the exhibit room is a major conservation concern. There are several measuring techniques available to assist museum specialists in obtaining this information, however, they are not simple routine procedures, and will not be necessary for most cases.
Why do we need to be concerned about air exchange?
Air moves in and out of exhibit enclosures; the rate at which the case's internal air is replaced can greatly affect its conservation performance in two ways:
- In cases with low-grade construction materials (and vulnerable display objects) ventilation is used to ensure that volatile organic compounds do not build- up to damaging concentrations within the case. Measuring the rate of air exchange is useful in confirming effective ventilation.
- In cases that provide protective microclimates the enclosures must be well-sealed to ensure successful performance. Measuring the rate of air exchange is used both to confirm an effective seal, and for environmental control to quantify the amount of moisture or pollutant sorbents required.
Exhibit case fabricators are beginning to market cases that have been tested for air exchange; these firms can provide the purchaser with considerable information regarding the enclosure's projected performance.
What approaches are used to measure air exchange?
Two approaches are used in museums to measure exhibit case air changes rates:
- Room initiated test:" an inexpensive tracer gas is introduced into the exhibit room through the air handling system. Before this step, a gas sensing device is introduced into the exhibit enclosure. The internal air is periodically sampled and the results are graphed to determine the air half-life and mean air life.
- Case initiated test: alternatively, this procedure introduces a detectable gas or airborne particulates into the case until a high level is achieved. Before the gas is released, a gas sensitive meter is introduced into the case to periodically sample the decline of the gas or particulate level. Once the level is stabilized the decline in gas is graphed to determine the air half-life and mean air life of the case.
|Air changes per hour:||measured by assessing how many times per hour or per day the air in the sample area is totally replaced with new air|
|Air half-life:||time it takes, given the calculated air exchange rate, for one half of the sampled air to be replaced with new air|
|Mean air-life:||time it takes, given the calculated air exchange rate, for all of the air in the sample area to be totally replaced with new air.|
What techniques are currently being used to quantify case air exchange?
Several techniques are available to measure air exchange, however most are rather involved.
- Smoke Test
- Technique: Smoke or airborne particulate is introduced into an enclosure and is visually monitored to establish the decline (decay) rate and air exchange.
- Equipment: Airborne "smoke powder" and bellows; or actual smoke generating material that is ignited in the case.
- Comments: A quick and practical method; most useful for assessing ventilation in vented case designs and making general assessments.
- Gas Chromatograph
- Technique: A portable gas chromatograph is used to sample the gas filled case at its highest gas level and then periodically to detect the rate of the gas concentration decline. The gas commonly used is sulfur hexafloride.
- Equipment: Gas chromatograph, a gas canister filled with tracer gas, syringe injector to introduce the gas.
- Comments: Gives very accurate results; equipment is very expensive; temperature sensitivity makes equipment less useful in field applications.
- Air Quality Monitor
- Technique: A battery run, air quality monitor is set within an enclosure to sample the decline rate of carbon dioxide or other common gas. Before this step, the tracer gas is released into the case and the highest level is recorded; periodic samples are taken and graphed (the declining levels of gas are shown on a LED display).
- Equipment: Electronic gas monitor, a gas canister filled with the tracer gas.
- Comments: Gives accurate results; equipment is getting less expensive; requires periodic calibration but self calibration is possible.
- Air Sampling Pump
- Technique: Through a small port in the case wall, an air-sampling pump is used to periodically sample or "sniff" gas levels within a case. Before this step, a tracer gas is released into the case and its decline is recorded.
- Equipment: Air sampling pump; comparison test strips or use of a commercial testing laboratory.
- Comments: A practical and relatively low-cost technique; requires drilling a hole into case wall, floor or ceiling.
- Note: Because barometric pressure can change during the testing period and affect the test results, it is recommended that cases be fitted with a small bladder (e.g. a ⁄ inch tube leading to a non-permeable, inflatable foil bag, such as a bag of Marvelseal 360) to neutralize any pressure change.
3. Pressurization Testing
- Air Pressure testing
- Technique: Two small ports are drilled into the wall of a case; an airtight fitting is mounted into one to receive a one-way compressed air fitting; an air pressure meter is fitted to the other hole. The case is put under pressure; the length of time a case remains under pressure can indicate its air exchange rate when calculated mathematically.
- Equipment: Compressed air, an air pressure meter, an airtight fittings.
- Comments: A mechanical method that requires drilling holes into the case. A drawback is that the pressure can deform case materials and give false readings.
Products, Manufacturers, and Suppliers
Mention of a product, manufacturer, or supplier by name here for information only and does not constitute an endorsement of that product or supplier. Listed materials have been used successfully in past applications. It is suggested that readers also seek alternate product and vendor information to assess the full range of available supplies and equipment.
Instruments and Meters
- Gas Chromatograph available from vendors of scientific instruments (eg Hewlett Packard)
Air Quality Monitor
- aq-501 Unit, Metrosonics Inc, Rochester, NY, 14692
- Draeger accuro Bellows Pump and Detector Tubes, Pittsburgh, PA, 15275
- Value Plastics Inc, Fort Collins OH, 80525
- Marvelseal 360 (aluminum foil), Marvelseal 1177 (transparent), Ludlow Corp, Laminating and Coating Division, Homer, LA, 71040
- Foil-O-RAP FR 2176 (aluminum foil), Foil-oRAP FR 7750 (transparent), Film-O-Rap FR 3300 (fire resistant and transparent, Bell Fiber Products, Columbia, GA, 31993
Exhibit Fabricators (Firms providing exhibit cases with established air exchange rates and expandable case bladders.)
- Small Corp, The Small Corporation, Greenfield, MA, 01302
- Artifact Preservation Services, New York, NY, 10128
- Total Safety Hazco, Dayton, OH, 45459
- sulfur hexafloride, argon and carbon dioxide available from chemical supply houses and compressed air suppliers
Suggested Further Reading
Calver, A., A. Holbrook, D. Thickett and S. Weintraub. " Simple Methods to Measure Air Exchange Rates and Detect Leaks in Display and Storage Enclosures." In ICOM-CC 14th Triennial Meeting, The Hague, 12–16 September 2005: Preprints , ed. I. Sourbes-Verger, London, James & James, 2005, pp. 597-609.
Michalski, S., 1994. “Leakage Prediction for Buildings, Case, Bags and Bottles” Studies in Conservation, 39:169 -186.
Watts, S., D. Crombie, S. Jones and S. A. Yates. [https://www.padfield.org/tim/cfys/mm/watts/watts.pdf "Museum Showcases: Specification and Reality, Costs and Benefits"] Museum Microclimates, Copenhagen, November 2007, Edited by T. Padfield and K. Borchersen