bdswan wrote:Can anyone weigh-in on how they calculated the line in Figure 6: Corrosion risk zones that corresponds to a value of ~0.8 for the S1 Corrosion Index?
The DIN 12502 spec that they reference defines S1 in terms of the concentrations of chloride, sulfate, nitrate & bicarbonate ions. It's not clear to me how they extrapolated the total hardness limits from that equation that were used to calculate the corrosion boundary, which defines one side of their recommended "Core zone" (Figure 8, p. 12 &13).
Close inspection of these figures shows that the Larrson-Skold Index at 0.8 line in Fig. 6 would actually bisect the "Core zone" as it is displayed in Fig. 8, effectively cutting the zone in half. Careful measurement shows that the slope & intercept of this boundary in slightly different in each of these plots (Fig. 6, Fig. 8 on p.12 & Fig. 8 on p.13).
I noticed some of that also, and perhaps Marco Wellinger (welone
) will give an authoritative reply, but FWIW, I'll give my interpretation of it:
The difference between figs 6 and 8 on pg 12 look like a simple slip in the graphic for fig 8. Note that the ticks and labels on the x axis (alkalinity) are not aligned, and that the "SCAA standard" line appears to have shifted down below the 40ppm alkalinity where it belongs. To my eyes, fig 6 looks fine - hard to tell because it's scaled differently (shows alkalinity out to 300 ppm.)
The dashed line on fig 6 labeled "Larson-Skold Index at 0.8" goes from the origin through the (150,250) point, so is clearly hardness = 5/3 * alkalinity
. I think that line would describe a Larson-Skold Index of 0.67 under the simplifying assumption that all the non-carbonate hardness is due to chloride and sulfate, and that there is no chloride or sulfate other than that associated with Mg and Ca. They may have some other assumption for calling it an index of 0.8.
I think the core zone on figure 8 pg 13 is the one to use - partly because it's scaled big enough to see clearly. If you draw a 'hardness = 5/3 * alkalinity" line on that one, you will see this:
And as you say, that line does bisect the zone. But the upper limit of the zone does follow the slope of that line so I just assumed that they chose their upper limit based on about 10ppm hardness above that line.