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Acoustic Emission monitoring: on the path to rational strategies for the
collection care
Michał Łukomski1, Janusz Czop2, Marcin Strojecki1, Łukasz Bratasz1,2
1Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences
ul. Niezapominajek 8, 30-239 Kraków, Poland
2The National Museum in Krakow, al. 3-go Maja 1, 30-062 Kraków, Poland
ABSTRACT
Cost analysis of various climatic control strategies in the galleries of the National Museum in Krakow,
Poland based on direct monitoring of the energy consumption and computer modelling using the
WUFIplus software, is reported. At the same time, the effectiveness of the existing climate control is
evaluated by long-term acoustic-emission monitoring of an eighteenth century wardrobe exhibited in
the Gallery of Decorative Art of the Museum. The technique allowed tracing directly damage
progress and classifying risk from various RH variations to the object. The outcome of both
investigations has supported reviewing of Museum’s policy and practice regarding climate control.
The modified approach to the climate control was first applied during comprehensive rebuilding and
renovation of the historic seat of the Princes Czartoryski Museum in Krakow, started in 2010.
INTRODUCTION
Designing and implementing optimal climate control strategies in museums is a challenging task due
to conflicting demands between the needs to maintain high standards of collection care and to
reduce energy costs and CO2 emissions. There has been a growing agreement that collections can
sustain greater variations in relative humidity (RH) and temperature than previously recommended
[1] which allows relaxing controls of the heating, ventilation and air-conditioning (HVAC) systems
and, consequently, reducing considerably the energy consumption [2]. But broadening the allowable
variations of climatic parameters has to be informed by understanding how changes in climate
conditions affect real artefacts.
There are two fundamental approaches to establishing the allowable ranges of climatic variations for
objects sensitive to climate-induced damage – an analysis of object’s mechanical response to climate
variations [3-5], or an analysis of the historic climate [6] to which the object has acclimatized.
Both approaches to predicting risk of damage of a concrete historic object in its specific environment
can be supported by non-invasive scientific methods of direct tracing damage capable of operating in
the real-world conditions in museums, historic buildings or during the transportation of works of art.
The acoustic emission method, which is based on monitoring the energy released as sound waves
during fracture processes in materials, has been particularly successful in direct tracing the fracturing
intensity in wooden objects exposed to variations in temperature and RH [7].
This study provides a comprehensive cost analysis of various climatic control strategies in the
galleries of the National Museum in Krakow (MNK) based on direct monitoring of the energy
consumption and computer modelling using the WUFIplus software. At the same time, a long-term
acoustic-emission monitoring of an eighteenth century wardrobe exhibited in the Gallery of
Decorative Art of the Museum is reported to evaluate the effectiveness of the existing climate
control, especially insufficient humidification of the air during repeating falls in the indoor RH in
winter. Already considerably cracked, the wardrobe was selected by the curators as representative of
massive furniture displayed in the gallery, but was also considered as particularly vulnerable to
climate-induced damage.
The outcome of both investigations has supported planning an effective and economic HVAC system
in a historic building of the Princes Czartoryski Museum in Krakow, currently undergoing systematic
rebuilding and renovation.
MONITORING ENERGY CONSUMPTION
The Main Building of MNK was chosen as a case for a detailed analysis of energy consumption by
systems of climate control operating in the Museum. The total surface area of air-conditioned zone
of the building is 19 500 m2. The building houses permanent and temporary exhibitions but also the
library, offices of museum’s administration as well as some of its conservation studios and
workshops. The climate control algorithm in the exhibition space followed the institutional indoor
climate guidelines which required a stable RH in the range of 45 – 60% and a stable temperature in
the range 19 – 21 oC dictated by comfort of visitors and staff. The monitoring has shown, however,
that the climatic conditions in the galleries deviate from these guidelines. Especially, RH drops below
30% during cold periods in winter due to insufficient air humidification, a typical situation for many
museums in central and northern Europe.
Actual power consumption in real time by every subsystem of the HVAC operating in the building was
monitored. The total energy consumption in 2010 was 4 150 000 kWh at 250 000 Euro with 64%
spent on heating for comfort and remaining 36% on the climate control for conservation.
Importantly, 32% of the total expenditure was spent on drying the air in summer whereas only 4%,
that is eight times less, on humidifying the air in winter. The average ventilation rate in the building
was 0.7 air exchanges per hour.
In the next step of the project, computer modelling was used to simulate energy consumption under
various climate control scenarios. The accuracy of modeling was verified by comparing the calculated
values with those measured during the monitoring described above. The comparison allowed
assessing the predictive power of the modelling with precision, as the monitoring had provided costs
of technological processes into which climate control by the HVAC systems operating in the building
can be split.
MODELLING ENERGY CONSUMPTION
Calculations of the energy needed to implement various climatic control scenarios were performed
using WUFI PLUS - software calculating coupled heat and moisture transfer in multi-layer building
components exposed to outdoor climate. The method of calculation is presented in Ref. [8].
Construction of the building and materials used were taken into account as well as heat and moisture
gains from people working in the building and visiting it. The latter number was assessed by taking
into account the number of tickets sold during 2010. The model is capable of calculating energy
demand for keeping predefined climatic conditions inside the building. The real energy demand was
calculated by taking into account the efficiency of HVAC subsystems used for heating, humidifying
and drying.
Several climate control scenarios were considered. Three different bands of allowable RH variations
were analysed, namely: 45-60% RH, 35-60% RH, and 35-65% RH for various ventilation rates in the
building: 0.4, 1.2 and 2.5 air exchanges per hour. The exchange rate of 0.7 per hour measured in the
building was also considered in the simulations. The results are presented in Fig. 1.
0,4 0,7 1,2 2,5 0,4 0,7 1,2 2,5 0,4 0,7 1,2 2,5
0
2000
4000
6000
8000
10000
12000
14000
35 - 65% RH35 - 60% RH
Energy demand (MWh)
drying
humidifying
heating
45 - 60% RH
Fig. 1. Yearly energy demand for different climate control scenarios in the Main Building of the National
Museum in Krakow. Results for three different allowable bands of RH variations at four air exchange rates in
the building are presented.
The total difference between measured and calculated energy does not exceeds 6% whereas the
biggest discrepancy (up to 20%) is obtained for air drying – the most complex technological process.
As expected, the cost of humidifying air increases when the lower RH stabilisation level is raised from
35 to 45% RH, whereas the cost of drying air increases when the upper RH stabilisation level goes
down from 65 to 60% RH. Further, a strong correlation between the energy consumption and the air
exchange rate is observed for all analysed RH ranges. The cost of heating for comfort does not
depend on the allowable RH band selected but increases significantly with increasing ventilation rate.
The economic benefits from various modifications of the current climate control strategy in the
building – that is keeping RH in the range of 45 – 60% at the ventilation rate of 0.7 - are summarised
in Fig. 2. The allowable temperature band is kept unchanged in the climate control modifications
considered.
The first possible modification of the current guidelines is an extension of the allowable RH range
from 45 – 60% to 35-65% RH. The modification results in a reduction of the total yearly energy
demand by 13%, mostly due to the diminished air drying in summer. The second possible
modification is a reduction of the ventilation rate from 0.7 to 0.4 exchanges per hour by limiting
mechanical ventilation of the building. This modification would result in a further reduction of the
energy demand by 34%. Hence, the total energy savings obtained by the above modifications of the
allowable RH band and the ventilation rate is 42%. Energy savings in other possible scenarios can be
analysed from the data shown in Fig. 2.
0,4 0,7 1,2 0,4 0,7 1,2 0,4 0,7 1,2
0
1000
2000
3000
4000
5000
6000
7000
8000
35 - 65% RH
35 - 60% RH45 - 60% RH
Energy demand (MWh)
drying
humidifying
heating
13 %
34 %
42 %
Fig. 2. Economic benefits from various modifications of the current climate control strategy in the Main
Building of the National Museum in Krakow.
However, to make the decision by museum’s managers for conservation and collection care possible,
risk of damage to vulnerable objects exhibited should be assessed as precisely as possible for various
climate control scenarios. Long-term acoustic-emission monitoring was used to trace directly
physical damage in an eighteenth century wardrobe exhibited in the Museum’s Gallery of Decorative
Art and classify in this way risk to the object from various RH variations. The measuring technique
and the results of the monitoring, which ran for more than two years, are presented below.
ON-SITE ACOUSTIC EMISSION (AE) MONITORING
Acoustic emission is defined as the energy released due to micro-displacements in a structure
undergoing a deformation. The energy passes through the material as ultrasound and sound waves,
and is detected on the surface using a piezoelectric transducer which converts the surface vibration
to an electrical signal. The AE experimental setup and details about the data processing and analysis
are published in Ref. [9]. Raw data recorded during the monitoring were post-processed with the
help of a computer program searching for individual AE events, extracting them and calculating the
most important AE features, that is, amplitude, energy, duration and frequency distribution.
A wardrobe dated to 1785 was chosen for the AE monitoring as both representative of massive
furniture displayed in the Gallery of Decorative Art of the National Museum in Krakow, and
vulnerable to climate-induce damage. The wardrobe richly decorated with ornamental carving is a
work of supreme craftsmanship. The oak structure is entirely veneered with walnut and decorated
with inlays of ivory, tin, graphite and various types of wood in which the maker created rich
ornaments including allegorical figures (Fig. 3). Fluctuations in ambient RH are considered to be one
of the main factors that contributed to the past deterioration of the wardrobe. They have caused
cracks on the front and side walls in the areas where cross-grained wooden elements were
assembled in the structure: the four-part frame has acted as a restraint for the central plank as
illustrated in Fig. 4.
Fig. 3. Wardrobe, 1785, the National Museum in Krakow (MNK 4-Sp-732)
Fig. 4. Damage area on a side wall of the wardrobe (a); cracks visible on the veneered decoration are related to
the construction of the wall evident inside (b) - anatomical directions of the construction elements are
presented on the schematic drawing of the structure (c).
(a)
(c)
(b)
From the technical point of view, a long-term, continuous monitoring of the micro-damage
development in an object displayed on the gallery is a challenging task as a very low level of signal in
comparison to environmental noise is expected in such environment. Monitoring required highly
effective filtering of any possible noise resulting from processes different than fracturing in the wood
structure. To prevent recording electrical and unwanted acoustic signals, the anti-correlation
measuring scheme was utilized. Two identical AE sensors were connected to the opposite sides of
the wardrobe, close to existing crack tips, at such a distance that events recorded by one sensor were
out of the range of the other. By discarding events recorded by two sensors simultaneously, a serious
reduction of the noise was achieved. A further reduction of the noise was possible by application of
60 kHz high pass frequency filtering after the initial 45 days of the monitoring. It has been
demonstrated [7] that signals with a high-frequency content are associated with fracturing of the
wood structure whereas signals of low-frequency characteristics are typical of ambient noise.
Parallel to the AE monitoring of the wardrobe, temperature and RH were measured every hour in the
gallery with help of the permanent museum system of the microclimate monitoring. Comparison of
the AE measurements with the microclimatic parameters in the gallery allowed performing risk
analysis for the monitored object presented in the following sections.
AE INDUCED IN THE WARDROBE BY THE CLIMATIC VARIATIONS
The results of almost two years of monitoring the AE and climatic conditions in the gallery are
presented in Fig. 5. As one can see, managing indoor climate in the gallery was subordinated to
comfort of visitors and staff. Therefore, the temperature was maintained at approximately 20°C
throughout the year with periods of slight increases or decreases in summer and winter respectively.
Though average RH was about 40%, a distinct low-high seasonal cycle in RH cycle was caused by
heating in winter. According to the recent European standard (EN 15757, 2010) the seasonal cycle
was obtained by calculating, for each reading, the 30-day central moving average which is the
arithmetic mean of all the RH readings taken in a 30 day period composed of 15 days before and 15
days after the time at which the average is computed. The seasonal RH cycle ranged from 47% in
May 2011 to 32% in February 2012. The most pronounced falls in RH recorded in December 2010,
February 2011 and February 2012 were due to spells of particularly cold dry weather, when the
outdoor air drawn into the museum was heated to the set temperature but was insufficiently
humidified by the air-conditioning system.
091 182 273 364 455 546 637 728
10
20
30
40
50
60
AE energy (arb. units)
Temperature (C) / RH (%)
12 March 2010 12 March 2012
Time (day)
no filtering 60 kHz high-pass filtered
50k
100k
150k
200k
250k
Fig. 5. Indoor climate in the Gallery of Decorative Art in the National Museum in Krakow as plots of
temperature and RH recorded every hour (grey line). The RH seasonal cycle is also shown by calculating, for
each reading, the 30-day central moving average (black line). AE energy recorded for the wardrobe is shown as
vertical bars. Periods when no signal filtering or 60kHz high pass signal filtering were used are indicated.
A multi-step analysis of the data performed has focused on the risk of RH variations to the monitored
object as thermal expansion or contraction have a minor effect on the overall dimensional changes of
the wood as compared to its response to moisture. As the drops in RH during winter and the wood
shrinkage they produce are the primary condition of concern, correlation was sought between the
measured AE energy and the minimum values of RH recorded in the gallery during winter. It should
be born in mind that the cracks in the wardrobe would propagate when two conditions are
simultaneously met. On the one hand, a fall in RH must go beyond a certain critical level, on the
other, the variation must last longer than the response time of the wooden panels to bring about
their dimensional change. The panels are approximately 10 mm thick. If one assumes that both panel
faces have the same permeability to the diffusion of water vapour, the 95% response time in such
case falls within the range of 5 – 7 days depending on the air velocity around the panel, that is, if the
adjacent air layer is calm or turbulent (ASHRAE, 2007, Rachwał et al., 2012a). However, the response
time of a panel can dramatically increase when wood is coated with any layers hindering the
moisture flow: the 95% response times for a 10 mm panel coated with light, medium or heavy
varnish are approximately 10, 20 and 40 days, respectively (ASHRAE, 2007). An increase in the
response time can be brought about also by the asymmetric diffusion through the uncoated back-
face of the panel when the top face is coated with any kind of a finish layer blocking the moisture
flow.
The detailed procedure of the analysis is illustrated in Fig. 6 for a time interval of one week
corresponding to a response time of a 10 mm thick panel with two faces diffusively opened as
discussed above. In the first step of the analysis, the weekly simple moving average for each RH
reading is calculated which is the arithmetic mean of all the RH readings taken in one week before
the time at which the average is calculated. In this way, the short-term fluctuations are smoothed
and the longer-term cycles to which the elements might have responded are emphasized. To each AE
signal, represented by vertical bars in the figure, a corresponding value of the one-week RH average
is attributed. The procedure is illustrated in Fig.6 for three selected AE events.
672 679 686 693 700 707 714
10
20
30
40
50
60
AE energy (arb. units)
RH (%)
Time (day)
11 January – 1 March
20k
40k
60k
80k
100k
Figure 6. Indoor RH – original values recorded every hour (grey line), as well as simple moving averages
calculated for a one-week time interval (black solid line). RH values related to three exemplary AE signals are
marked.
In the next step of the analysis, the number of RH drops during the monitored period is established
as illustrated in Fig. 7 for the time period shown in Fig.6. The smoothed one-week average RH plot is
recalculated into a series of discrete decreases and increases of 1% RH. The number of RH drops to
each RH level can be then derived.
Finally, the energies of AE events corresponding to intervals with the same value of RH± 0.5% RH
were averaged, that is added and divided by the number of RH drops to that RH value. As the
fracturing and the AE signals are caused just by RH drops, all AE events recorded were related in this
way to a drop of at least 1% in the one-week RH average. As 58,000 a.u. was established in the
calibration procedure as the energy released during the fracturing of 1 mm2 of the wood area, or 0.1
mm of crack propagation for the 10 mm thick panel, the AE energy can be recalculated into the
equivalent crack propagation. Fig. 8 presents plots of the crack propagation averaged per RH drop to
a given RH level. For comparison, the figure also shows plots obtained by the same data analysis but
using time-intervals of two weeks and one month, corresponding to possible longer response times
of the panels. The plots immediately reveal which response time is correct. As the process leading to
damage is related to RH drops, the damage should be more and more severe with decreasing RH
value. Such a tendency is observed most clearly for the one-week time interval, which indicates that
the damage progress is due to RH variations of an approximate duration of one week or less.
672 679 686 693 700 707 714
30
32
34
36
38
40
42
44
RH (%)
Time (day)
Fig. 7. Indoor RH as simple moving averages for one-week time interval recalculated into discrete decreases
and increases.
48 46 44 42 40 38 36 34 32 30
0,00
0,01
0,02
0,03
0,04
0,05
0,06
1 week
2 weeks
1 month
Crack propagation per RH drop (mm)
RH (%)
Fig. 8. Plots of average AE energy per RH drop to a given RHMIN versus RHMIN.
The total AE energy registered during the two-year monitoring period was 703,000 a.u. Thus, the
total area fractured was 12.1 mm2 corresponding to 1.2 mm of crack propagation for the 10 mm
thick panel, or 0.6 mm per year. The recorded crack propagation in the entire object is relatively
small for any practical assessment of the damage. So the principal conclusion from the two-year
monitoring carried out is that the climatic conditions in the gallery are relatively low risk for the
collection.
However, amazing sensitivity and reproducibility of the AE sensors in detecting extremely small
sources of acoustic signals has allowed further refining the analysis. Fig. 9 shows a plot of cumulative
crack propagation which has occurred as a results of the RH drops in the entire two-year monitoring
period. Calculations for the time intervals of one week, corresponding to realistic response times of
the panels are provided.
48 46 44 42 40 38 36 34 32 30
0,0
0,2
0,4
0,6
0,8
1,0
1,2
Crack propagation (mm)
RH (%)
Fig. 9. Plots of cumulative crack propagation resulting from the RH drops in the two-year monitoring period to a
given level of the one-week RH average.
The plots allow to derive acceptable RH falls if a conservation professional or a curator select a
‘tolerable’ magnitude of the fracture that is the magnitude below which object’s damage is
considered insignificant. For example, an improvement in the present climate in the gallery by
maintaining one-week average RH over 38% at all times will halve the current yearly crack
propagation in the wardrobe to 0.3 mm/year. If the allowable one-week RH level is further increased
to 40%, the yearly crack propagation will be reduced to mere 0.13 mm/year.
CONCLUSIONS - THE RE-EVALUATED INDOR CLIMATE GUIDELINES AT MNK
Basing on the outcome of the investigations, the National Museum in Krakow has embarked on
reviewing its policy and practice regarding climate control so that the display conditions are better
tailored to clearly identified needs of the collections on one hand and the potential of a particular
building for the climate-control on the other. The modified approach to the climate-control was first
applied on the occasion of comprehensive rebuilding, renovation and renewal of the historic seat of
the Princes Czartoryski Museum in Krakow, started in 2010.
The museum itself – the oldest in the country - has existed for more than 200 years. It has been
housed in the current building in the historic centre of the city since 1901. The research and
conservation team of MNK which advised on the preservation aspects of the renovation plan had the
following challenge: ensure that this renowned museum meets the standards of the present day and
preserve the historic architecture and exhibition design that has defined the museum. The devised
climate-control guidelines require that during the summer temperature is kept between 22 – 25 oC
and is lowered to between 18 – 22 oC in winter. As an emergency measure, the temperature can be
further lowered on very cold days, when the indoor RH falls below 35%, but never below 16 oC to
ensure minimum comfort of visitors and staff. The RH is kept between 40 and 60%, the efficient
central humidifying and dehumidifying systems being responsible for the provision of the required
conditions. The air entering the air-conditioning system is a mixture of outdoor air and re-cycled
indoor air. Indoor carbon dioxide concentration is used to evaluate indoor air quality and building
ventilation following the recommendation of the ASTM D 6245 - 07 standard [10]. The CO2 sensors
are installed in selected rooms of the museum. When the CO2 level is above 1150 ppm, that is the
level at which, according to the standard, approximately 20% of visitors find the level of body odour
unacceptable, the outdoor air is supplied into the building, otherwise the indoor air is recycled.
With these guidelines, size of the ducts through which the conditioned air will be supplied to the
galleries could be considerable reduced. Consequently, lowering of the ceilings behind which the
ducts are hidden was limited and the impact of installing the duct system on the original high ceilings
in the rooms minimized.
ACKNOWLEDGMENT
The research was supported by grant from the Polish Ministry of Science and Higher Education
supporting activities of COST Action IE0601 „Wood science for conservation of cultural heritage”.
References
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to changes in ambient relative humidity’, In: Painted Wood: History and Conservation., eds.
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two-dimensional calculation using simple parameters’,IRB Verlag,Fraunhofer-
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AUTHOR BIOGRAPHIES
Michał Łukomski graduated in physics from the Jagiellonian University in Krakow, Poland in 1999,
and received a PhD in 2003 from the same university. For the next two years he was a research
fellow at the Windsor University in Canada. In 2006, he joined the staff of the Institute of Catalysis
and Surface Chemistry, Polish Academy of Sciences, Krakow. His research focuses on the response of
historic materials to changes in environmental parameters and the investigation of painted surfaces
by advanced optical techniques. Email: nclukoms@cyf-kr.edu.pl
Janusz Czop has been an active restorer since 1985. His major conservation works include panel and
easel paintings, wall paintings, sculpture and altar works. Janusz Czop is the Chief Conservator of the
National Museum in Krakow. Following the Museum’s preventive conservation strategy Janusz Czop
created in 2004 a scientific laboratory – LANBOZ with the aim to carry out nondestructive analysis of
objects and monitoring the storing and display conditions in the Museum. Email:
jczop@muzeum.krakow.pl
Łukasz Bratasz graduated in physics from the Jagiellonian University in Krakow, Poland in 1996, and
received a PhD in 2002 from the same university. In the same year he joined the staff of the Institute
of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, where he is a research
fellow. Since 2007, he has worked as the senior scientist at the Laboratory of Analysis and Non-
Destructive Testing of Artefacts in the National Museum in Krakow. His research focuses on
microclimatic monitoring, computer modelling of environmentally induced mechanical damage, and
acoustic emission. Email: ncbratas@cyf-kr.edu.pl
Marcin Strojecki graduated in physics from the Jagiellonian University in Krakow, Poland in 2003, and
received a PhD in 2009 from the same university. In the same year he started to work in the
Laboratory of Analysis and Non-Destructive Testing of Artefacts in the National Museum in Krakow as
a research specialist. From 2010 he joined the staff of the Institute of Catalysis and Surface
Chemistry, Polish Academy of Sciences, Krakow, where he is a research fellow. His research focuses
on acoustic emission as a non-destructive method of diagnosing damage in historic materials. Email:
ncstroje@cyf-kr.edu.pl
