High TDS with normal CYA

[spanman]

I understand that TDS is no longer considered meaningful because tests for CYA and other variables provide more useful information, and I understand that pool store automated test results should be received with skepticism. That said, a neighbor in Phoenix with an 18,000 gallon plaster pool got pool store test results that seem just plain odd. He sanitizes with liquid chlorine. What would cause such a high TDS readout while the other variables are normal, or close to normal?

 

[Newdude]

What would cause such a high TDS readout while the other variables are normal, or close to normal?

Salt for one. Many liquid chlorine pools are north of 2000 and you guys get no rain dilution so its likely much higher.

But twice in the last couple of days I saw TDS of 0 with all the other #s in range. (Plenty of TDS). The pool store tests just cant be trusted.

The 0 phosphates in your friends test is highly unlikely, and again calls the whole thing into question.

 

[JamesW]

Test for salt with a K-1766 salt test kit.

 

[Newdude]

I understand that TDS is no longer considered a meaningful because tests for CYA and other variables provide more useful information,

I want to revisit this too. TDS was never meaningful for normal chemistry operation.

As an example, if i tell you I have 57 things in my fridge, do I need to go grocery shopping for the week ? You have no idea, do you ? It could be 15 kinds of meats, a dozen veggies and various other stuff to make side dishes, OR, my door shelves are chocked with condiments and my shelves are bare. '57 things' / TDS tells us nothing.

And i just checked. I have 42 things on my fridge door alone. 57 things probably wouldn't make it through the week.

 

[JamesW]

You cannot use a meter set to "TDS" because it assumes a TDS composition of 40% sodium sulfate, 40% sodium bicarbonate and 20% sodium chloride, which is not accurate for pool water.

The meter just measures conductivity and converts it into a reading based on the assumed composition of the TDS.

Below shows a graph of conductivity vs. TDS and conductivity vs. Sodium Chloride.

5,000 ppm TDS = 3,200 ppm Salt.

 

[JamesW]

Each gallon adds 6.94 ppm FC and 11.43 ppm salt.

280 gallons of 12.5% sodium hypochlorite = 3,200 ppm salt.

 

[spanman]

Thanks everybody! I'll assume that salt is the culprit. Will see if we can run a test for salt to rule that in or out.f

 

[mknauss]

The CH result on that sheet is also very suspect. That is just above the fill water CH. Not likely unless pool filled in the last couple months.

 

[jmastron]

Thanks everybody! I'll assume that salt is the culprit. Will see if we can run a test for salt to rule that in or out.f

What is there to "rule out"? Is there a problem/visual/other issue, or just the fact that it's showing up on the paper? Agree that I'm skeptical of the test -- FC measured to the hundredth, but Total Dissolved Solids is *exactly* 5000? And the "ideal" ranges are way off -- at CYA 46 (another "precise" value), 1ppm of FC is very very far from ideal, and 7+ ppm is not a problem at all

The easiest way to avoid seeing a high TDS value is...not to test for it , ideally by doing one's own tests. I'd argue that a salt test would be far more valuable than TDS just to know it, just not as a way to rule a non-problem out.