![]() The coefficient of variation is an indicator of the relative scatter of the values, and it was initially suggested that a large value would indicate localized corrosion. It clearly only makes sense for the current, as the mean potential will vary according to the reference electrode used. The coefficient of variation is the standard deviation divided by the mean. Cottis, in Techniques for Corrosion Monitoring (Second Edition), 2021 5.4.3.6 Coefficient of variation For this reason the coefficient of variation of current is not now in common use. This problem is a manifestation of a more fundamental difficulty of the coefficient of variation, as it is only meaningful for a one-sided distribution (one where all values are either all positive or all negative). However, it is now recognised that it has a major problem since the mean current can be zero (indeed the expected value is zero), the value of the coefficient of variation may go to infinity (in principle it can also go to minus infinity, as the mean may be positive or negative, but the absolute value is normally taken). ![]() The coefficient of variation is an indicator of the relative scatter of the values, and it was initially suggested that a large value would indicate localised corrosion. Cottis, in Techniques for Corrosion Monitoring, 2008 Coefficient of variation However, using the standard deviation as the basis for accuracy, then the ASTM C1701 method produces less variable results. From this it appears that the variability of measurement by the NCAT method was slightly superior compared with the ASTM C1701 method. When the mean permeability was higher than 0.5 cm/s, the CV values for both methods were comparable. Also, as a general trend, the CV decreased as the mean permeability increased. The average CV for the ASTM C1701 method was around 30%, while the average CV value for the NCAT permeameter was around 10%. The CV is higher for the ASTM C1701 method because the mean permeabilities are lower than the means for the NCAT permeameter. As shown, the CV is generally higher for the ASTM C1701 method compared with the NCAT permeameter method, although the standard deviation is lower. The relationship between the CV and the mean permeability values for all locations on all test sections is shown in Figure 5.4. Variability can also be looked at in terms of the CV. While in general the variability remained within a close range, for test section C3 the variability was much higher for the NCAT method. Generally, the mean was higher for the NCAT method compared with ASTM C1701. Measurement repeatability based on coefficient of variation for permeability values measured using the ASTM and NCAT methods.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |