Why are the TDS and salt levels combined in the Orenda App?

How do I go about the TDS/salt factor in the LSI calculator?

When we add salt to a swimming pool with a SWG (salt water generator) to achieve the minimum salt level for a proper operation, we are directly contributing to our TDS levels. We have combined these two levels in the Orenda app, given that besides being an LSI equilibrium tool, the Orenda app is also a dosing calculator. The Orenda app will tell you from left to right, how much salt will be needed to achieve the desired input. 

Let's not forget that besides our salt, we also have other factors that contribute to our TDS level and hence, the equilibrium of the LSI. Total Dissolved Solids (TDS) is a metric used in water treatment that measures all dissolved minerals, salts, chlorides, metals, organics and many other contaminants in water. TDS also impacts water conductivity and is related to corrosion, chemical efficiency, water clarity.

Pool chemicals and their byproducts

Just about every pool chemical you can think of leaves behind something that contributes to TDS. Let's do a brief rundown of the common pool chemicals.

  • All types of chlorine, when reduced (used up), become chloride (Cl-), which is basically a salt. 
  • Liquid chlorine leaves behind sodium and chloride, which is common salt (NaCl). 
  • Cal Hypo leaves behind 4 ppm per pound per evert 10,000 gallons of calcium, as well as additional chloride.
  • Dichlor and Trichlor both leave behind cyanuric acid and chloride.
  • pH adjusters like sodium bicarbonate and sodium carbonate leave behind sodium (obviously) and alkalinity ions.
  • Algaecides, depending on their type, leave behind byproducts like nitrogen compounds, copper, etc.
  • Calcium chloride (CaCl2) directly adds to TDS with both calcium and chloride.

TDS and salt levels in my pool

As a quick example, let's say we have a salt water pool with the following readings:

- 80 ppm of total alkalinity 

- 350 ppm of calcium hardness

- 35 ppm of CYA

- 3600 ppm of salt

That means that our TDS levels will be at a minimum of 4065 ppm (80 + 350 + 35 + 3600), plus all the other solids that have been dissolved in the water since the pool filled up, which tends to be a smaller fraction, but it does add up over time. The most precise way to calculate TDS is with a test kit.