Simple methods to measure air exchange rates …

1 Simple methods to measure air exchange rates and detect leaksin display and storage enclosuresAndrew Calver* and Andy HolbrookMuseum of London150 London WallLondon EC2Y 5 HNUnited KingdomFax: +44 (0)20 7600 1058E-mail: ThickettEnglish Heritage23 Savile RowLondon W1S 2 EFUnited KingdomSteven WeintraubArt Preservation Services315 East 89 StreetNew York CityNY 10128, USA*Author to whom correspondence should be addressedIntroductionThe air exchange or ventilation rate of display cases, picture frames and storageenclosures has a significant effect on their effectiveness as tools for preventiveconservation. Well-sealed enclosures provide a buffer against external humidityvariations and reduce the ingress of externally generated pollutants andparticulates.

2 Where internally generated pollutants are a concern, deliberateventilation of the enclosure may be an appropriate mitigation measure . When abuffer such as silica gel is to be added to enhance relative humidity (RH) stabilitythe air exchange rate (AER) of the enclosure is a key variable (Thomson 1977).In the authors experience it is difficult to determine an approximate AER of anenclosure from visual inspection, and AER in enclosures ranging from less to greater than 50 air changes per day (ac/d or acd 1) are common (Table 1).This makes the routine use of 1 ac/d for a well-sealed display case in buffercalculations (Thomson 1977) difficult to justify (Weintraub 2002) and explainswhy buffering works very well for some (Guichen and Gai 1984) but not others(Ashley-Smith and Moncrieff 1984).

3 Low-cost gas detectors and innovative gas delivery methods were used for aresearch project to investigate the impact of ventilation on internally generatedpollutants (Calver 2001). Subsequent use during several enclosure installationssuggested that it would be useful to further investigate the technique and producea standard of air exchange rateTracer gas techniques have previously been used to measure the AER of museumenclosures. Early studies involved the loss of water vapour (Padfield 1966,Thomson 1977). The disadvantage of water vapour is that any moistureabsorbent material will attempt to buffer the change in and Ramer (1983) purged cases with nitrogen, introducingcarbon dioxide as a tracer and also measuring the ingress of oxygen, monitoringthe concentrations with gas liquid chromatography.

4 Cass et al. (1989) usedsulphur hexaflouride (SF6) and electron capture gas chromatography, Daniel andMaekawa (1992) measured the ingress of atmospheric oxygen (O2) using anoxygen analyser. Cassar and Martin (1994) commissioned the UK BuildingsServices Research Information Association (BSRIA) to develop a test for theAER of display cases (Potter 1998) which uses dinitrogen oxide (nitrous oxide,N2O) as a tracer. It is now common in the UK to include a target AER inAbstractIndustry-standard measurementtechniques of air exchange using tracergas decay have been adapted for usewith low cost gas detectors andportable tracer gas delivery experimental and practicalapplication has led to the production ofa recommended protocol and dataanalysis spreadsheet freely available onthe Internet.

5 Using these tools it isrelatively Simple to calculate the airexchange rates of display cases, pictureframes and storage units. A range ofleak detection equipment has beenevaluated to support the use of low airexchange enclosures as an importantpreventive conservation exchange , tracer gas, leak detection,half time, leakageVOLIIPUBLISHED IN THE14 THTRIENNIALMEETINGTHEHAGUEPREPRINTS597sp ecifications for enclosures (Reed 1999, Bacon and Martin 2000) validated bythis commercial test. However, the service is relatively expensive (about 1500for two enclosures or 10 50 per cent of enclosure purchase price). Therefore,only a small sample of enclosures is tested during an installation (Bacon andMartin 2000).

6 Furthermore, during a busy installation schedule the results areoften not available in time to rectify defective enclosures (Erimen and Tate 1999).Demystifying air exchangeIn the past few years several users, also experimenting with low costs gasdetectors, have contacted the authors. It was felt a standard measurement protocolfor these detectors plus an accompanying data analysis spreadsheet to make airexchange calculations a Simple exercise would be is some dispute over whether air exchange per unit time has any validscientific meaning (Brimblecombe and Ramer 1983, Stanley et al. 2003). It isargued that in an enclosure a complete air change or loss of tracer gas neveroccurs, following a process of exponential decay the concentration will approachbut never reach the external value.

7 Thomson (1977) gives a good description ofexponential decay and demonstrates that for most practical purposes themathematical argument is irrelevant in real-world applications. When a tracer gasis introduced into an enclosure, it is mixed with the air and the change inconcentration is used to model the movement of air into and out of theenclosure. By adding a tracer gas with similar characteristics to air, we canmeasure the transfer of external air (a mixture of air, gaseous pollutants,particulates, water vapour) into and out of an enclosure. This movement is drivenby a combination of infiltration (convection) through cracks and holes, diffusionthrough smaller cracks and holes and permeation through enclosure the exchange is due to diffusion we should account for the molecular massof the tracer gas compared with air (Brimblecombe and Ramer 1983).

8 Thevarious driving forces for air exchange and their contribution to leakage aredescribed by Michalski (1993) in a comprehensive COMMITTEE FOR CONSERVATION, 2005 VOLIIT able 1. Selection of air exchange rates measured with a PM3010 nitrous oxide detector 2000 2004 Enclosure typeSpecification Measured ac/dMeasured ac/dNotespost-modification (if applicable)Test display case sealed and vertical joint taped withaluminium tapeCapital Concerns sealed with Marvelsealdisplay caseWorld City Gallery panel installed and top transport display caseglazing sealedCreative Quarters covered with aluminium display casetapeDemonstration of sample caseacrylic caseWorld City Dickens caseWorld City Gallery Guys display caseHigh specification storage Before London properly sealed aesthetic prototype display caseprototype onlyStandard metal storage unitcabinetOutside display Dynasty display caseDeliberately 32 3 (vents Case used to monitor impact of vented(vented)sealed)

9 Ventilation on internally generated pollutantsRoman display case 5 Deliberately (vents Case used to monitor impact of ventedsealed)ventilation on internally generated pollutantsTudor display case T19 Deliberately9730 Obvious vents sealed to test leakageventedTudor display case 22 Vented>1000 (sic)Installed above an air conditioning return air ductMeasurement of air exchange by tracer gas decayOf the various tracer gas techniques used for measuring ventilation in buildings(Liddament 1996), the most practical for museum enclosures is single zoneconcentration decay (ISO 2000, ASTM 2000). After introduction into a sealedenclosure, the concentration of the gas will decrease as air enters and leaves.

10 If thedriving forces for air exchange remain constant, the concentration of the tracergas is found to decay exponentially against time. Plotting the natural logarithm ofthe exponential decay curve against time should result in a straight line, the slopeof which is the AER in air changes per unit time (Figure 3). For this article allmeasurement are reported as number of air changes per exchange rate (N) of a particular gas is driven by the difference inconcentration of the gas inside and outside the enclosure. Thus it is important tomonitor and subtract the background measurements for atmospheric gases suchas CO2, O2and H2O. For example, the apparent air exchange using the oxygensensor in Figure 6 would be ac/d rather than ac/d if the backgroundwere general form of the equation for calculating air exchange per unit time isgiven in Equation 1.