TDS monitoring

[Drd]

I use unstablized liquid chlorine which is added using the HASA liquidator feeder dispenser. A lot of the salt in the chlorine falls out of solution in the tank and never reaches the pool so that keeps that TDS down but I use dry acid because of health concerns to adjust my PH and I just learned this adds a sulfate (TDS) to the pool over time and can combine with Calcium (which is in tap water) and become a big problem so it's important to control that. So I'm trying to find the best remedy without switching to Muriatic Acid. I guess I could put in a softener that removes the calcium in the water I use for the pool but that increases the salt level too. Is there a way to test for sulfates and what would the limit be? I should add I only us liquid chlorine to shock when needed to avoid adding calcium in large amounts and I add a descaler treatment once a month too which does keep my tiles clean along with equipment. Or would just doing a partial drain each year be the best over all solution?

 

[Gene Clean]

I wouldn't worry over TDS. As long as your CSI is around -0.3 you shouldn't be seeing much buildup at all. I know this doesn't answer your question, but why are you looking to control TDS? Not sure if you're working with vinyl or plaster.

 

[Texas Splash]

Your questions bring to light a few intersting issues, so I'm going to solicit the feedback of @JoyfulNoise to help when he gets a moement. Thanks for using the forum.

 

[JoyfulNoise]

A couple of things to note -

1. The only way to keep sulfates out of the pool is to stop using dry acid. Muriatic acid is perfectly safe if used correctly and, if you don’t like the fumes of the full strength acid, many stores sell 15% MA (10 Baume) which has almost no odor to it.

2. Water softeners do NOT add or increase the salinity of water. They use salt as a source of sodium ions (Na+) to remove calcium ions (Ca++) from the resin during regeneration but all the brine water is washed away with a standard backwash and rinse cycle that is programmed into the softeners regeneration cycle. The “sodicity”, ie, sodium concentration, of the water is increased by the softener but NOT it’s salinity (chloride ion).

3. Calcium sulfate scale is not typically a problem because you need both high calcium and high sulfate concentrations for it to happen. What is bad is “sulfate attack” on cementitious surfaces, ie, plaster and grout. Sulfates in excess of 300ppm in water can cause damage to cement as it degrades cementitious materials when they are exposed to wet/dry cycling of sulfate laden water.

4. The HASA Liquidator tends to build up salt and calcium in it BUT that doesn’t mean your chlorine is added salt free. The liquid carrying the hypochlorite is still a concentrated brine solution. You are most definitely still adding salt along with your chlorine. Also, please note, when you see salt scale in the tank, you should empty it and clean it out. Excess sodium chloride in the tank increases the salinity of the hypochlorite solution in the bottom of the tank which reduces the stability of the hypochlorite. Hypochlorite solutions breakdown faster as sodium ion concentration increases. This is why bleach manufacturers are very careful to control the salinity of the brine feed stock when making hypochlorite using the chlor-alkali process. Excess divalent metal ion (Ca, Fe, Ni, etc) concentration also reduces stability so periodic cleaning of the tank is very important.

 

[JoyfulNoise]

Also, while there is a back-titration method of determining sulfate ion concentration, it is a laboratory scale test that is not possible as a pool side test. TDS meters are not ion specific, they react to any change in solution conductivity. So in a multi-component system such as pool water, it’s extremely difficult to get any accurate measure of a specific ion concentration using a TDS meter.

If you are truly interested in your sulfate levels, you’ll have to send a water sample to a testing lab that is willing to test pool water (most are used to drinking water testing). Typical drinking water test protocols will include sulfate as a reportable criterion and a full set of tests runs around $100-$150.