How to determine CH from TDS reading

[alexaki]

Hello All,

I'm new here so please go easy on me!.

I have a 1 yo Quartzon pool with a SWG. I recently purchased a TDS meter to measure total dissolved solids. Is there a calculation I can apply to my TDS value to estimate my Calcium Hardness value?

I would appreciate all input.

Many Thanks in advance.

 

[PoolGuyNJ]

None that I know of. You're going to need a proper test kit anyway, most of which will have the CH test anyway. The TDS meter's usefulness is limited to saturation index calculations.

Scott

 

[duraleigh]

Welcome to the forum :lol: You will have no application at all for the TDS meter. TDS is irrelevant to pool water chemistry. As poojguynj mentioned above, you really need a drops-based test kit to accurately test your water.

Pool School has several articles including "ABC's of Pool Water Chemistry" which will get you headed down the right path.

A side conversation on the effect of TDS on CSI has been moved here.

 

[JasonLion]

Welcome to TFP!

TDS is the sum of everything dissolved in the water. If you happened to know every single level except CH it would in theory be possible to determine CH by subtracting everything else from TDS. However, there are two practical problems with trying to do that. First, there isn't any practical way to know all of you other levels, there are just too many things that could be in the water. And second, none of the tests are completely precise. By the time you subtracted out everything else there would be hardly any precision left so the resulting number wouldn't be much better than just guessing.

There are situations where you can use TDS to get a good guess at the salt level, since salt usually is a high percentage of the TDS level. But that approach will not work for CH. Still, even if you were trying to guess at the salt level, it would be better to simply get a salt tester.

 

[alexaki]

Thank you very much for your tips and feedback.

I purchased the Taylors test kit for CH (10,11,12)
25ML Pool water
20 drops -10
5 drops - 11
count drops 12 until water changes colour from red to blue.....

Unless I'm color blind the water colour is a pinky color to start with after the 5 drops (of #11) and really doesn't change color no matter how many drops of #12 I put in..

 

[JamesW]

There might be metal ion interference. Try this modified test.

Make a sample of 50 % pool water and 50 % distilled water. Mix well and follow these steps to test:

1) Rinse and fill large comparator tube to 25 mL mark with water to be tested.

2) Add 5 drops R-0012. Swirl to mix.

3) Add 20 drops R-0010. Swirl to mix.

4) Add 5 drops R-0011L. Swirl to mix. If calcium hardness is present, sample will turn red.

5) Add R-0012 dropwise. After each drop, count and swirl to mix until color changes from red to blue.

6) Add the original 5 drops of R-0012 to the number of drops needed in step 5, and multiply the total number of drops by 20. Record as parts per million (ppm) calcium hardness as calcium carbonate.

What are you getting for TDS from the meter?

 

[alexaki]

TDS came in at 4350..I have a 12k gallon pool running a SWG..I think my CH is at around 250...How does this TDS reading compare?

 

[JamesW]

TDS is mostly salt, especially when large amounts of salt are added, such as with an SWG. Calcium has to be measured with a test that is specific for calcium.

TDS or salt meters measure the conductivity of the water and then apply a conversion factor depending on the supposed chemical makeup of the water and what units of measure you want to report in.

TDS meters might report in units of NaCl (sodium chloride), KCl (potassium chloride), or TDS - 442 (developed by the Myron L Company to represent the makeup of natural fresh water). TDS-442 is 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride

Meters should be calibrated with a standard solution to ensure accuracy.

 

[alexaki]

Thanks for the response James....Given TDS is mainly salt then a TDS of 4350 is what I would say is about right for my pool.

 

[JamesW]

You also have to know what units your TDS meter is reporting in. The instructions for the meter should specify the assumed chemical makeup of the water and what the reported units are. Unless the unit is reporting in units of sodium chloride, the TDS value won't represent the amount of salt (NaCl) in the water.

For example: If your TDS meter is reporting in units of TDS-442, then your salt would be about 2,800 ppm to 3,000 ppm. To get an accurate measure of the salt (sodium chloride), you should use the K-1766 Taylor Salt Test

What units does the TDS meter report in?

 

[alexaki]

The instruction sheet doesn't really say...it just says PPM(Parts Per Million)

 

[JamesW]

What make and model is the meter?

 

[JamesW]

Overall, TDS is not super important. It can be somewhat useful in detecting unusual problems. For example, if the meter showed an unusually high TDS reading that did not correspond to the known ionic components of the water, it would suggest that further investigation might be necessary to identify the unknown ions.

Unusually high TDS reading could come from such ions as phosphates, nitrates, sulfates, potassium, magnesium etc.

Note: 1,000 ppm TDS-442 is the same number of ions as 687 ppm NaCl. Therefore, if the Electrical conductivity were the same for the same number of ions, then 4350 ppm TDS-442 would convert to 2988 ppm NaCl.

*[edit]I was assuming that the ratios of sodium sulfate, sodium bicarbonate and sodium chloride were on a molar basis. However, now I think that they might be on a mass basis. If the ratios are on a mass basis, then 1,000 ppm TDS-442 will have the same number of ions as 725 ppm of sodium chloride (NaCl). Either way, the number is about the same, and the correct conversion is determined by the measured values of conductivity vs. the concentration. Also, the ions are not just sodium and chloride, so there will be some inaccuracy based on that.[end edit]

However, different ions conduct electricity differently, and you can't use a single conversion factor. The chart below shows that 4350 ppm TDS-442 converts to about 2850 ppm NaCl, which is about 655 ppm NaCl per 1,000 ppm TDS-442.

It would be helpful if you got a standard salt solution to use as a reference, such as 3,200 ppm GLX-CAL-SOL. You could use that to determine a TDS reading that would correspond to a proper salt concentration. It would also help determine what units your meter is reporting in.

Here is a conversion chart for TDS-442 to NaCl.

http://www.myronl.com/products/solutions.htm