All right, I re-did the test. This time I targeted a fairly high 9 dGH (~64 ppm of Ca). And measured the GH using the API GH Test kit - counting the drops until the 5 ml solution in the test tube turns from yellow to green.
After applying the salt I would stir it, let it sit for 5 minutes and stir again... I did this 3 times so each test sample got about 15 minutes before measuring GH and TDS.
This time using 3 Liters of RODI water.
Test 1:
Null test: 2 ppm.
0.83 grams of CaSO4: 169 ppm (338 uS/cm). API test turned green on the 9th drop consistent with the target.
Test 2:
Null test: 2 ppm.
0.7 grams of CaCl2: 237 ppm (474 uS/cm). API test turned green on the 9th drop - but turned out somewhat
darker green compared to the CaSO4 test, which somewhat suggests that the dGH might have been closer to 8 rather than 9 and I should have added slightly more CaCl2. Anyway, inconsequential for the overall result.
One curious thing about this is that the numbers aren't scaling linearly at all, as I would have expected.... relative to my first test I would have expected the CaSo4 solution at 64 ppm of Ca to yield closer to 200 ppm (64 ppm Ca / 20 ppm Ca x 62 ppm TDS) = 200 ppm . and the CaCl2 to be closer to 400 ppm. I can't explain this discrepancy - other than perhaps the TDS meter. Of course not many among us will target 64 ppm of Ca with RO water, so I guess it's more of academic interest, but still.
Either way, at least with the compounds I have in my possession, my TDS meter (Hanna Instruments), and how I did the test, it still appears that CaSO4 yields a significantly lower EC/TDS than CaCl2 for equal amounts of measured Calcium, but I am totally not comfortable about drawing a hard conclusion on this until we find an explanation for the discrepancy between mine and
@Hanuman's measurements. Regardless of EC/TDS, the Calcium content (GH) is ultimately what we aim for here, but we also want to use the compound with the least collateral
damage in terms of EC/TDS and this ought to be fairly predictable.
Cheers,
Michael