TDS: SUm of Salt, CH, CYA, and TA?

[blueman2]

I purchased a TDS meter and for testing my RO system, but then decided to test my pool with it. Just because I can . Am I correct that TDS is the sum of Salt, CH, CYA and TA in ppm? It seems pretty close. Here are my readings:

TDS: 2100 ppm (using my TDS meter)
salt: 1500 ppm (using 0.5 conversion of the 3000 ppm showing on my Aquarite SWG)
CH: 320 ppm (Taylor test)
CYA: 60 (Taylor test)
TA: 80 (Taylor test)

Total of components are 1960 vs TDS reading of 2100. Given that there are probably some metals and other stuff in the water, this seems like a reasonable result. So is this the way the math works for TDS? Or do other components need to be adjusted the way Salt is adjusted?

This is purely for sake of curiosity.

 

[Richard320]

There might be other salts that you don't test for (like sulfates) that go into TDS, but you are basically correct. Everything that is in the water that isn't water is dissolved solids.

 

[JoyfulNoise]

A TDS meter really doesn’t measure TDS, ie, Total Dissolved Solids. TDS can only truly be measured by evaporating off all the water in a known volume and measuring the weight of the remaining solids (thermogravimetric analysis). It also has to be done carefully to take into account the hydration of certain salts and the presence of suspended, but undisdolved, solids.

What your meter is measuring is electrical conductivity (EC) of the water sample and then it uses a correction factor, k, to approximate TDS. The values for k can vary widely depending on the details of the water sample being used.

Typically for RO water you are measuring very low TDS and so the meter would not be as accurate at the higher salinities found in a pool. You could recalibrate the meter for the pool BUT you would need to do a more rigorous analysis of your pool water using thermogravimetric techniques to get a true reading of TDS versus TSS (Total Suspended Solids) as TSS doesn’t contribute, for the most part, to EC.

 

[Griswald]

Quote "salt: 1500 ppm (using 0.5 conversion of the 3000 ppm showing on my Aquarite SWG"

Huh??? Where did this come from?

 

[blueman2]

@JoyfulNoise and @Griswald, you both nailed it. @Richard320, I think misled you with my numbers. As noted, I have an EC/Temperature meter, which converts to TDS using roughly 0.5 conversion. My erroneous conversion of 3000ppm from my aquarite was, well, erroneous. I misunderstood the need for conversion. So the real math was:


3000 ppm salt (from Aquarite)
320 ppm CH (Taylor Test)
60 ppm CYA (Taylor Test)
80 ppm TA (Taylor Test)
==========
2100 ppm (4200 us/cm EC auto converted by the meter into PPM)

So obviously, my EC meter is waaaay off. It needs to read at least 3460, probably even higher since there are other dissolved solids that I did not test for. I am surprised the TDS meter is so far off, however. It was rated to read correctly up to 9999 TDS and auto corrects for temperature.