Total dissolved solids

lawjohn

Well-known member
May 2, 2013
62
I've followed this forum and used the forum's recommendations as well as the pool math app since about 2013 and have noticed that there is not a lot of comments about total desolved solids.....Frankly I've ignored them too, but I wonder why Lowery Consulting includes them in their LSI calculation and others mention them too.

I recall that LSI calculation is an older one, but many companies in the pool industry still list the LSI calculation in their warranties including mine. In addition Taylor's calculation wheel in my K-2006 kit uses the LSI Calculation without the TDS included....with all that said I wonder why some include TDS and others don't in the LSI Calculation. Comparing the LSI and TFP CSI are virtually the same close balanced readings in comparison I've made. Just a curiosity question for someone who is knowledgeable on the subject...many thanks Lawjohn
 

JamesW

TFP Expert
Mar 2, 2011
21,145
I think that the book that comes with the test kit gives you an adjustment for the TDS.

Note that the Taylor wheel requires you to calculate the "Adjusted Alkalinity" and not the total alkalinity.
 

lawjohn

Well-known member
May 2, 2013
62
In the manual it says TDS in most pools is less than 2000 having little effect on water balance and is not included on the LSI Wheel. The LSI calculator wheel is set for the following readings. Total Alkalinity, calcium hardness, ph, and water temperature to then give a LSI reading. If you do the manual calculation the formula indicates Alk = measured Alkalinity minus cyanuate alkalinity. In addition, if you Google LSI calculator, Pentair has a LSI on line calculation which it includes what the wheel has plus you also include the CYA value to arrive at the Saturation index....however TDS is not included, but the manual gives a reasonable reason as to why it's generally not included......so I guess that's the answer all along....all the best to you James...
Lawjohn
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,379
Tucson, AZ
The thing to understand about LSI versus CSI is that LSI is a phenomenological model that was built by simply looking at how calcite formation (scale) was affected by various water parameters (pH, alkalinity, dissolved solids, temperature, etc). It's akin to a regression analysis or statistical model. CSI, on the other hand, is a model of calcite formation that is based entirely on "first principles" meaning that it is a quantity that is derived from the actual underlying chemical and physical properties of calcite formation. Both indices look at the same outcome ("does scale form or dissolve?"), they just take different approaches to get there.

TDS is related to amount of "stuff" dissolved in water. If you take a sample of filtered water (to remove suspended solids) and then gentle evaporate away all the water, what is left are the "dissolved solids". These dissolved chemical species affect or modify how water dissolves or forms calcium scale through their modification of the ionic strength of the solution. However, because field technicians and people in the industry are not trained chemists and don't really care about the atomic details of what's going on, or the physics of it for that matter, they desire to just have simple parameters to measure and then use a phenomenological approach to arrive at an answer that's "good enough" to direct their activities. The details don't really matter to them. Back in the old days, there were no easy field methods for measuring every individual chemical component found in water and, because water analysis often involve samples drawn from lakes & rivers, trying to figure out every particular dissolved chemical in the water would be a very difficult task. So TDS, which is often measured by proxy using electrical conductivity, was used to gauge how "pure" or "impure" the water sample was relative to the need (boiler fill water, cooling tower water, etc). If the TDS was too high for the intended application, then that would signal to the operator that the water needed to be treated first.

In the old pool management days, operators rarely ever measured things like cyanuric acid or calcium hardness. They only wanted to know how "contaminated" the water was so they could either dump it and start over or keep going and add chemicals to it. TDS was used as a proxy for that analysis. If the water in the pool had been there for several seasons with lots of sanitizer and other chemicals going in but not a lot of fresh water to dilute it, then the TDS would be high. That would signal the operator or owner to probably consider dumping the pool. Lots of lore and old-wive's tales built up around that in terms of TDS telling you that your sanitizer wouldn't work or that your pump would die, but none of that was ever grounded in reality. TDS is simply not that informative of a parameter now that people can measure all sorts of chemicals species independently.

Operators and industry types like Lowry that hype TDS simply don't understand the underlying physics and chemistry of what's going on. They do it because their target audiences are typically service techs and pool "boys" that need very simple concepts to do their weekly rounds. It's unfortunate that it spills over into the residential world of pool ownership and makes people think there is something special about it...As a variant of Clarke's 3rd Law states -

Any technology, no matter how primitive, is magic to those who don't understand it.
 

JamesW

TFP Expert
Mar 2, 2011
21,145
TDS matters to the CSI. Many pools are salt where the TDS is above 3,200 ppm.

Anyone who dismisses the TDS in CSI is not doing it correctly.

In any case, it really doesn't make sense to use a slide rule now that everyone has a computer. Time to get out of the 1960s and step into the 21st Century.