IMPACT tool - showcases
Background
An online app to calculate indoor pollutant concentrations in showcases. IMPACT means Innovative Modelling of Museum Pollution and Conservation Thresholds. This is the name of the project where the IMPACT tool originated. The current version (2) was coded by Josep Grau-Bove, in order to continue offering access to this useful model. The original research was carried out by Nigel Blades, Declan Kruppa, May Cassar (UCL Institute for Sustainable Heritage), and Terje Gr?ntoft (Norwegian Institute for Air research).
Instructions
The model predicts indoor concentrations of pollutants in showcases. The main otuput is the Showcase/Room ratio (%), which is the percentage of outdoor pollutant concentrations that we can expect in showcases. To find this value, the model estimates the deposition flux of pollutants to surfaces. The deposition flux depends on the type of Surface and its area. It also depends on a value known as deposition velocity, which has been experimentally determined for a diversity of common surfaces in showcases. To run a simulation, you need to input the volume of a showcase, the area of every surface where pollutants can deposit, T, RH and the level of ventilation. Ventilation is expressed in Air Exchange Rate (AER, 1/h), which indicates the number of times in an hour that the air in the showcase is refreshed. The model shows concentration in a showcase at equilibrium. This means that, in a showcase with no
deposition, the internal concentration will eventually be equal to the external, even if it takes a very long time. The time to equilibrium can be explored with the plot Concentration Through Time.
Acetic acid emission
The tool helps plan or interpret measurements of acetic acid emission. It was developed within EU project MEMORI by Paul Lankester and David Thickett as a part of
Internally generated acetic acid is emitted more when the temperature or relative humidity are higher. Therefore, timing of measurements can impact upon the result if the climate parameters vary over the year. For example, the emission is often higher in summer months compared to winter month. The tool plots the acetic acid emission over a period of 90 days as a function of the measurement start date. The emission is expressed as a % of the maximum possible emission which would be recorded by measurements carried out over 90 days of the worst-case combination of high temperature and relative humidity.
References
A description and the main equation of the model can be found in:
- Kruppa, D.; Blades, N.; Cassar, M.; (2002) A web-based software tool for predicting the levels of air pollutants inside museum buildings developed by the EC impact project. 5th EC Conference, Cracow, Poland.
- Grøntoft, T. and Michele R. Compilation of tables of surface deposition velocities for O3, NO2 and SO2 to a range of indoor surfaces. Atmospheric Environment (2004).
Required dehumidifier capacity
The tool was developed within EU project MEMORI by Paul Lankester and David Thickett as a part of
The tool calculates the dehumidifier capacity that is to say how much water vapour needs to be removed from a showcase each hour (g/hour) to keep objects displayed at a specified relative humidity. The parameter is calculated from the room relative humidity, the air exchange rate of the showcase (per day or per hour), and its volume. The room and showcase temperature of 22 oC is assumed.
Silica gel lifetime
The tool was developed within EU project MEMORI by Paul Lankester and David Thickett as a part of
The tool calculates the lifetime of dry normal (type I) silica gel in a showcase before it needs reconditioning. The calculations assume the showcase is at a similar temperature to the room. If there is significant case heating (greater than room by more than 2 oC) then the calculations will be inaccurate. Internal and external lighting; direct sunlight (even through double blinds in some instances); equipment in or underneath the case, and positioning near heaters can all cause case heating.
The RH of the dry silica gel is used as the initial RH. With sufficient drying time, well-spaced gel and an oven that is not overloaded, 5% is routinely achievable.
If the showcase contains significant amounts of buffering material such as wood products (MDF, plywood etc.), fabrics, or wooden objects, then the calculations will be less accurate. If the case is monitored, this will show up as a significant delay in the RH dropping after adding dry silica gel (several days) and the RH not dropping to a low value.
There must be good exchange between the silica gel compartment and the display volume. Many modern designs have been found wanting in this. For a metal baseboard, at least 25% of the surface needs to be covered in 8 mm diameter holes or 15 mm slots. If the silica gel is placed in case furniture in the display volume, they need 15 mm gaps to allow the dry air to circulate. Silica gel can also be used as a humidity buffer which evens out RH variations inside the showcase around a long-term RH average. Silica gel obviously does not require any reconditioning in such operation. You can set both the initial silica gel RH and RH specified in the showcase to the same long-term average indoors to see the range of the RH variations achieved inside the showcase, that is, to assess the RH stabilization capacity of silica gel.