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Importance of Measuring Approach in Measurements Performed During the Restoration and Conservation Works D. Krstic, Croatian Conservation Institute, Croatia V. Mudronja, Faculty of Engineering and Naval Architecture, Croatia F. Meder, Croatian Conservation Institute, Croatia Contact

Abstract

The conservation and restoration works are, by definition, related to a number of diverse measurements. Moreover, by taking advantages of modern technologies, there are constantly new measuring methods in such works using highly sophisticated measuring equipment. It often happens in practice that measurements are carried out for the sake of measurements only. This habit stays behind us and fully defined requirements have been set on measurements. In preparing measurements, the following questions are asked:
1. What is the objective of measurements? Can the measuring procedure fulfil the set requirements?
2. Has the equipment used in measurements been calibrated?

1. INTRODUCTION

The result of every analysis in conservation and restoration activities depends on a number of measurement data. The question is how reliable these measuring data are, or in other words, what is the quality of these data. In this context the so-called metrology approach in carrying out the necessary measurements will be further considered. Theoretical explanation of the metrology approach will be given using the example of measuring dampness which is otherwise today a very serious conservation (and metrology) problem. It certainly is a serious metrology problem but only as a function of seriously set requirements on measuring dampness. There has not been up to now a procedure for measuring dampness whose level of uncertainty of measurement could meet the increasingly strict requirements regarding precise determination of the amount of dampness.

For the monuments of historical and art value mainly non-destructive measuring methods are applied. They can also serve for detection of areas contaminated by greater amounts of water, as well as for the control of the level of efficiency in the drying process. Moreover, they allow a series of measurements at the same spot depending on the time with no contact to surface, free of material structure interference.

Since dampness is the main initiator of destructive processes in cultural monuments, certain methods and measuring instruments have to be used to determine its content, distribution and origin, in order to make proper decisions regarding the restoration. Monitoring dampness in architectural monuments includes also measuring and registering of crypto-climatic parameters which is especially important in distinguishing condensation from rising damp. For determining the share of hygroscopic moisture, chemical analyses of soluble salts are of crucial importance. Very often there are combined influences of two or more forms of moisture, and they have to be determined with great care. The most difficult problem are the measurements in-situ and their quantification, the lack of which would render the term "damp" indefinite and ambiguous. Heterogenity of the monumental material further complicates the problem. Measurement results will differ for every analysed material, e.g. if several materials are used in the construction of a wall (wood, brick, plaster) at air relative humidity of 50% after achieving the balance, the water content in wood will be about 11%, in bricks 1.5-2.5% and in plaster about 1% (conditions of a normally dry wall).

These considerations make it clear that the required level of accuracy in measuring dampness depends on a whole series of factors, but it should certainly be related to the objective planned in the given case. Therefore, it needs to be determined whether the measuring system is capable of meeting the set requirements in measuring the dampness.

2. CAPABILITY OF MEASURING SYSTEM

For measuring dampness various measuring methods (measuring instruments) can be used, such as e.g. dielectric hygrometer, microwave methods, thermographic methods, atomic methods, thermogravimetric methods. Which measuring method will be used for a certain case, depends on the requirements set for the measurement itself. Therefore, let us consider the so-called hygienic classification of walls presented in Table 1. [1]

Type of wall	Perfectly dry (native humidity)	Hygienically dry	Hygienically tolerable in some cases	Damp	Very damp
Common brick	1%	up to 3%	up to 4%	from 3% to 9%	above 9%
Light, absorbent stone (sp.gr. 1.9)	up to 4%	up to 6%	up to 7%	from 6% to 15%	above 15%
Other materials, natural or artificial	native humidity must be measured after drying in open air	up to 2% above native humidity	up to 3% above native humidity
Table 1: Hygienic classification of walls according to moisture content by weight


If we look at the data presented in Table 1, it is obvious that hygienic classification of the wall cannot be reliably carried out on the basis of the results of measurements if the uncertainty of measurements is not indicated in the parts of humidity percentage. Generally, the measuring system is expected to be able to meet the set requirements, Figure 1.

Fig 1: Capability of the measuring system

Fig 2: Rule of consistency with the specification

In Figure 1 T represents the scope of requirements, LSL the low standards limit and USL the upper standards limit. Cg is the capability of the measuring system.

Usually, the demands on the capability of the measuring system range from Cg = (0.1 to 0.3) T.
The question is how to determine the capability of the measuring system. Most often the capability of the measuring system is related to the uncertainty of measurement U of the applied measuring procedure. Moreover, especially over the last ten years the uncertainty of measuring results has had to be indicated. If, namely, only one figure is noted for the measuring result then the quality of this figure remains questionable for the user of such information. In other words, the uncertainty of measurement always needs to be indicated along with the measuring result, since it is the uncertainty of measurement which defines the quality of the indicated result, that is:

X = 8 ± U, k

where:
X - is the measured characteristic,
8 - is the measurement result (usually arithmetic mean n of repeated measurements),
U - is the extended uncertainty of measurement,
k - is the factor of extension (usually k = 2 is used which corresponds to approximately 95% probability),

Taking into consideration the above mentioned, and checking whether the set requirement has been fulfilled or not, one should act in accordance with the presentation in Figure 2.

Based on the above considerations the capability of the measuring system may be expressed as:
Cg = 2U, that is Cg = 2U/T ^. 100%

3. ANALYSIS OF REPEATABILITY AND REPRODUCIBILITY

It often happens in practice that the concept of measurement uncertainty is not clear and that it is reluctantly applied. Moreover, it is almost a rule if we consider the measurements in conservation practice. In this practice there is often the need to monitor certain parameters (dampness, temperature, etc.). Repeating the question what the measured values are for, there is a need to know the reliability of these measured values. Frequently, monitoring is done by different people (observers), using different instruments. Regardless of this diversity, the reliability of the obtained results has to be known, i.e. the capability of the measuring system as a whole. In that case, the so-called study of repeatability and reproducibility (R&R) of measurement results is performed in practice.

3.1. Repeatability
Repeatability is the dispersion of measuring results done by the observer in multiple measurements of identical characteristics on the same elements (specimens) using the same instrument, Figure 3.

Fig 3: Repeatability of measuring results

Fig 4: Reproducibility of measuring results

Repeatability is the value that determines to the greatest extent the influence of the measurand in the measuring system variation.

3.2. Reproducibility
Reproducibility is the dispersion of measuring results obtained by a greater number of measurands in multiple measurements of identical characteristic and using the same or different measuring instruments (Figure 4). In case only one measurand participates in the measuring system, the reproducibility is defined as dispersion of measuring results obtained with multiple measurements of identical characteristics using the same or different measuring instrument over a longer period of time.

Reproducibility is the value which determines to the greatest extent the influence of the measurand in the measuring system variation.

3.3. Repeatability and reproducibility
The total dispersion of measuring results due to common effect of repeatability and reproducibility determines the capability of the measuring system, i.e. Cg = R&R or Cg = R&R/T ^. 100%.

Based on the performed studies it may be said that in the absence of information on uncertainty of measurements, the capability of the measuring system may be assessed by the analysis of the repeatability and reproducibility, where uncertainty of measurement U is equivalent to R&R/2.

4. CONCLUSION

This paper considers only some of the characteristics of the metrology approach in measurements. We are aware of the fact that many measurements for the needs of conservation and restoration works are carried out for the sake of measurements themselves. However, the question is not whether measurements are done or not, but what is obtained by measurements. Bad practice needs to be changed every day, and the measurement should be regarded as a very expensive, important and responsible discipline. Therefore, all the modern quality management assumptions have to be taken into consideration, and when talking about measurements then particularly the requirements of the ISO 10012 standards. Obviously, regular calibration of the applied measuring equipment and the provisions of traceability of related standards should no longer be doubted even in conservation and restoration practice. In Croatia, in performing the measurements, we are starting to behave as described in this paper. The first step is systemic insisting on regular calibration of the used measuring equipment.

References

1. Massari, G. and Massari, I.: Damp Buildings, old and new, ICCROM, Rome 1993.
2. Arendt, C.: Technische Untersuchungen in der Altbausanierung, R. Müller GmbH,Köln 1994
3. ISO 5725: 1994 (E) Accuracy (trueness and precision) of Measurement Methods and Results-Part 1: General principles and definitions calibration laboratories
4. ISO 10012 - 1: 1992 Quality assurance requirements for measuring equipment

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