Current at early times

It is interesting to note the early conductivity response of the sample to the application of the 60 volts required by the standardized RCT. Figure 7 is a plot of effective specimen conductivity calculated from the RCT current for the two specimens from Sample F during the first 4 minutes of the RCT. The solid lines shown are only meant to guide the reader's eyes. The dashed lines delineate the sample conductivities using IS. The figure shows the effects of two phenomena: a decrease in current due to the accumulation of polarization charge, and an increase in current due to ohmic heating.

Figure 7: Early-time conductivity, as measured by the RCT current, taken from Sample F.

Using these data from Specimen F, it is difficult to distinguish the proper time at which to measure the RCT current to ensure an accurate estimate of σ_IS. One could argue that the proper value should be determined from an extrapolation to zero time using the values measured at one minute and later. Although this method would be relatively accurate for the 50 mm specimen, it would not be justifiable for the 100 mm specimen. The estimate using the value at t=0 seems to be a reasonable compromise. Since the values in the figure only vary by approximately 4 percent during the first 4 minutes of the test, it would seem as though sufficiently accurate estimates of sample conductivity could be made using the DC current measurements at any time during the first few minutes.