I did my little experiment just for the 'fun' of it, it took longer than I thought, I made a few experimental design faults and it didn't really give me any definitive answer. I received my answers from from the supplier and manufacturer. The supplier wrote;
In terms of determining different ionic species a TDS meter is incapable of doing this.
If you take your sample to a lab for analysis they can give you a TDS factor that you can input into the meter that will more closely correlate what TDS actually is.
However, if you wish to determine specific NaCl content an Ion meter that measures this would be better suited.
The B-721 is an Ion meter designed to determine the NaCl level and this can be viewed here;
Compact Salt Meter LAQUA twin - B-721
An ion meter is beyond my budget and I'm not sure a measurement as % salt by weight is suitable. The manufacturer wrote;
A TDS meter is an electrical conductivity meter that converts the electrical conductivity reading to an approximate TDS value, based on a user determined multiplication factor of the conductivity value. When this factor = 1.0, the conductivity = TDS value.
All ionic species in water will contribute to the conductivity reading (and therefore the TDS reading). When there are multiple species present, it is impossible to know the concentration/contribution of just one species.
When CaCl is added to a pool, the conductivity/TDS will increase. It is impossible to determine a NaCl & CaCl ratio based on the conductivity/TDS reading alone.
You will be able to determine things like—is it higher than it was yesterday? It is going up? Is it going down? How much is it going up? Which pool has a higher level of dissolved salts? Also, if you take a reading before you add only CaCl, you can assume that the increase was only due to CaCl addition.
I hope this confirms your understanding of TDS. If you have additional questions please let me know.
Which from both responses it is pretty much what my understanding of a TDS meter was and similar to other descriptions within the forum so no big surprise. I can happily use my TDS meter to monitor how my salt level changes over time. Both responses seam to indicate that a correction or adjustment of the multiplication factor can be used to correct for known ionic species. I can assume any increase between TDS reading before and after a CaCl addition is due to the CaCl but I can't differentiate between calcium and chloride ions.
After a bit more searching through the forum I found this;
Nearly all of the TDS in a saltwater pool is from the sodium chloride salt. A fresh non-salt pool that has saturated calcium carbonate from adding calcium chloride for calcium and sodium bicarbonate for bicarbonate will have a TDS of around 525 ppm and have around 350 ppm salt (chloride as ppm sodium chloride). That is, the TDS of calcium and bicarbonate is not very high...
So I guess it's around 200ppm for the sodium, calcium and bicarbonate ions.
Anyway, for what it's worth here's my results for the TDS of a balanced swimming pool in an esky. I used a clean esky and an Ehiem 1048 hobby pump (10L/min) for circulation. My TDS meter is a salt meter with the result given as mg/L, a range of 0 - 9999mg/L, resolution of 1mg/L, an accuracy of salt ± 5mg/L +1%FS and calibrated in a 6440ppm solution where 1mg/L = 1.001142303ppm.
Results:
6000ppm standard solution. - 6040mg/L (6047ppm) (+0.666666%)
30L Tap water - 135mg/L (135ppm)
+ 2.1g cyanuric acid - 135mg/L (135ppm) +0ppm
+ 8.6g boric acid - 135mg/L (135ppm) +0ppm
+ 11.6g CaCl - 565mg/L (566ppm) +431ppm
+ 3.7g NaCO3 - 630mg/L (631ppm) +65ppm
+ 180g Salt - 6705mg/L (6712ppm) +6081ppm
+ 6 hours - 6820mg/L (6829ppm) +117ppm
+ 18 hours - 7040mg/L (7048ppm) +220ppm
+ 8 hours - 7055mg/L (7063ppm) +15ppm
Each reading was taken half an hour after the addition, just before the next addition. Sodium carbonate accounted for 65ppm which could be deducted from a TDS reading when evaluating for salt. Calcium chloride increased the TDS reading by 431ppm but a good proportion of that is due to the chloride ion that is accounted for in a salt reading. The salt raised the TDS by 6081ppm which is very close to what was predicted from PoolMath for that amount of salt. Both CaCl and NaCO3 additions where also close to the PoolMath predictions for both.
The TDS continued to rise over the 32 hours by 352ppm. Clearly I should have waited at least 24 hours between additions but I was worried that evaporation would affect the outcome.
I was surprised that I found no TDS reading for both cyanuric acid or boric acid, I was sure both were ionic species in water. Each one cold take longer to completely dissolve or disassociate and could be the reason for the climb in TDS. To test this I have set up to solutions, 8g of boric acid and 2.2g of cyanuric acid, each in 2L of tap water. Those concentration represent about 15 times the recommended levels for a pool. After 24 hours both showed no increase in TDS.
So at the end of it I think I could comfortably reduce my TDS reading by ~100ppm or mg/L to more accurately read for the salt. I found no direct comparison between a TDS meter and a Taylor K-1766 within the forums and have bought one to compare them. The postage from the US cost me more than the kit.