NON-SPHERICAL HOLES AND WAVY TRACER GAS DECAY CURVES. A COMPARISON OF THEORY AND REAL LIFE WITH RESPECT TO LEAKAGE OF DISPLAY CASES

Frank Ligterink, Hubertus Ankersmit and Maarten van Bommel

Instituut Collectie Nederland / Netherlands Institute for Cultural Heritage (ICN)

ABSTRACT

Conservators and curators from time to time will have to decide on the design of new display cases or the implementation of measures to improve existing display cases. Sometimes cases have to be well sealed in order to prevent penetration of harmful components from the external environment. In other situations a certain level of ventilation is preferred in order to avoid the accumulation of harmful components emitted either by objects or construction materials the interior of the case.

To guide the design of cases, at least a crude quantitative estimate of the effects of various measures that influence the leakage/ventilation rate of a case is necessary. In principle the equations and graphs in Stefan Michalski's article 'Leakage prediction for buildings, cases, bags and bottles' [1] allow for such an estimations.

In his article Stefan Michalski distinguishes three basic transport mechanisms which determine the leakage of display cases: 1) air flow through holes and cracks, 2) diffusion through (stagnant air in) holes and cracks, and 3) permeation through case wall materials. The airflow through holes and cracks is driven by pressure differences across the holes. Equations are given for pressure differences caused by air density differences between inside and outside due to temperature and relative humidity differences (stack pressure), and pressure differences caused by thermal and barometric pumping. The equations developed in the article are based on the assumption of homogeneous conditions within the case. Stack pressure is considered constant. Flow and diffusion equations through the holes and cracks are developed for simplified geometry's, that is tube shaped holes and rectangular cracks.

In real life, display cases have seldom simplified geometries and temperature and relative humidity differences are not constant. It is tempting to create more precise models that can account for this level of detail . From a practical (non-physicist) point of view, however, the current set of equations is already complicated to work with. In this study the practical use and the validity of the leakage predictions made by Michalski are tested experimentally. Exchange rates of a display case with holes and cracks of different sizes, shapes and positions are measured using carbondioxide as a tracer gas, while internal and external temperatures, relative humidities and air speed are measured simultaneously. The decay in the carbondioxide concentration allows for calculation of the actual exchange rate, which is compared with values calculated with the theoretical ones. Aim of this work is to produce a ready-to-use set of equations that can be implemented in a spreadsheet program to predict exchange rates of display cases.

Reference:
[1] Michalski S., "Leakage prediction for buildings, cases, bags and bottles", Studies in Conservation, 39 (1994) 169-186.

Frank Ligterink*, Hubertus Ankersmit and Maarten van Bommel
Instituut Collectie Nederland
Gabriel Metsustraat 8
1071 EA Amsterdam
The Netherlands
E-mail: frank.ligterink@icn.nl

(*): Author to whom correspondence may be addressed