·Table of Contents
·Conservation and Restoration in Art and Architecture
Importance of Measuring Approach in Measurements Performed During the Restoration and Conservation WorksD. Krstic, Croatian Conservation Institute, Croatia
V. Mudronja, Faculty of Engineering and Naval Architecture, Croatia
F. Meder, Croatian Conservation Institute, Croatia
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.
|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|
|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
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%
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.
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.
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