THE VENTILATION OF ENCLOSURES TO REDUCE INTERNALLY GENERATED POLLUTANTS AND SIMPLE TECHNIQUES TO MEASURE AIR EXCHANGE RATES WITHIN ENCLOSURES: REPORT ON PROGRESS TO DATE

Andrew Calver

Museum of London
United Kingdom

ABSTRACT

The presentation will outline the work undertaken to date for the research programme outlined below. The key work so far has been to develop equipment for simplified tracer gas measurement of the air exchange rate of display cases and other enclosures following on from the work briefly shown at IAQ2000. This information can be used to measure ventilation rates for passive ventilation systems or conversely to determine the air tightness of the case. The equipment used in the experiments is readily available, compact, battery powered and can be used within existing enclosures. The start up cost for the equipment and numerous tests is approximately the same for a single commercial test making the measurement of air exchange rates a viable additional tool for investigations into air quality and enclosures. Results to date from the study of the internal levels of key carbonyl and sulphur compounds measured on the same ventilated and non ventilated enclosures will also be presented. It is hoped that this will stimulate discussion of the merits the pros and cons of ventilating enclosures for different types of materials, different display or storage conditions and climates.

Summary of current research programme:
Since the Museum of London opened in 1976 it has had a policy of ventilated display cases wherever practicable - that is to deliberately enhance the air exchange rate using filtered ventilation ports. This approach is contrary to the current trend for very low air exchange rate cases. This approach originally appears to have been to circulation the air to prevent the build up of dust and stagnant or damp air but more recently the aim has been to reduce the concentration of internally generated pollutants. In this case the primary aim was to reduce the concentration of potentially damaging organic compounds emitted by display case construction materials, however, more recently concern has focused on the potential for the objects themselves to out gas undesirable compounds. The Museum of London is a social history museum with a wide range of materials and object types from archaeological and historical contexts. Pollution studies have shown several archaeological objects from waterlogged sites produce gaseous sulphur compounds, which have proven to promote the tarnishing of silver objects housed in the same case. The primary method of providing ventilation has involved variously sized vents located at the top and bottom of the case. The vents are fitted with particle filters to reduce dust ingress and the airflow through the case is assumed to be driven by stack pressure. Some previous pollution monitoring results have suggested that the cases without ventilation have higher pollution concentrations however no visible ill effects have been observed on objects in these cases.
With the move by other museums towards very well sealed display cases with air exchange rates of less than 0.1 air changes per day and the adoption of this specification by many case manufacturers it was felt that the museums strategy should be reviewed. The addition of vents can add considerably to the cost of a case and benefits of reduced air exchange such as humidity buffering and the reduction of externally generated pollutants may also be lost. However, experience has shown that the ventilation adds a measure of protection against the build up of high relative humidity levels caused by condensation within cases against cold external walls - a common problem in the Museum of London as the building is humidified to 50% in the winter and insulation levels are poor. Proposals for major gallery redevelopment's meant that in order to justify the need or otherwise for ventilated display cases further research would be required. In theory air movement through the vents in the case will dilute the concentration of internally generated pollutants leading to lower equilibrium concentrations. There is increasing empirical and experimental evidence to suggest the damage caused by carbonyl compounds such as acetic acid on susceptible may only occur above certain concentrations. If this is the case then control by dilution could be a simple and effective mitigation technique that can be retrofitted to many types of traditional case and storage enclosure.
To determine the effect in real situations pollution monitoring studies will be carried out in existing display cases and storage enclosures especially where previous monitoring data exists. Levels of acetic acid, formic acid, acetaldehyde, formaldehyde, hydrogen sulphide and carbonyl sulphide will be measured internally and externally using diffusion tubes in the enclosures normal ventilated mode and then again after the vents have been sealed to reduce as far as possible the air exchange rate. One of the key factors governing the internal concentration is likely to be the ventilation or air exchange rate of the enclosure as this governs the dilution factor and influences the emission rate. Although it assumed that the ventilated cases do in fact ventilate the enclosure more than an similar unventilated case no measurements have ever been undertaken at the Museum of London to check this. Therefore not clear what effect, if any, the various types and designs of vents have had. Indeed in almost all reported cases of pollution levels measured in cases and enclosures the air exchange rate has not been reported. While a variety of methods have been used in the past to measure the air exchange rate of cases the most readily available method is tracer gas decay. This is an effective technique, which is commercially available in the UK and relatively simple if you already possess a suitable portable gas analyser. Unfortunately if you do not possess the equipment the current method is prohibitively expensive for routine measurements and requires external equipment and thus sampling ports in the case making it difficult to use on existing cases or enclosures without such a facility.
The initial challenge was to find a cost effective method to measure air exchange rate in cases that could ideally be used in both existing cases containing objects in the galleries. Previously reported simple attempts to measure air exchange rate have used changes in relative humidity (using water as a tracer gas) or pressurisation measurements but both of these techniques have limitations. As the tracer gas technique is effective and relatively simple the first approach has been to investigate the availability of smaller and cheaper gas sensors. An inexpensive carbon dioxide sensor connected to a radio telemetry monitoring system was on trial at the museum for measuring carbon dioxide levels in galleries (as a measure of air quality). This led to an examination of using CO2 as a tracer gas rather than the Nitrous oxide (N2O) used commercially. Initial results showed promise. However, as CO2 is an atmospheric gas and bioeffluent the background concentration varies depending on occupancy. Following some initial studies a relatively inexpensive data logging N2O sensor was obtained and tested alongside the CO2 sensor. Initial results are promising with good repeatability and the methods will be tested alongside the commercially available method in an attempt to test their accuracy. Simple methods of tracer gas delivery have also been explored using domestic supplies of CO2 (for inflating cycle tires) and N2O ('whippets' for aerosol cream) to avoid the need for bulky and expensive cylinders and regulators.

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Andrew Calver
Museum of London
150 London Wall
London, EC2Y 5HN
United Kingdom
E-mail: acalver@museumoflondon.org.uk