If you have ever made yourself a cup of coffee and accidentally added too much sugar, or what sweet tea drinkers call “Just the right amount”, you have probably noticed that a good quantity of the sucrose sits undissolved at the bottom of the cup. Why is that? Sugar dissolves easily in water, doesn’t it?
Water can only hold so many solids in solution before it cannot hold anymore. It can become over-saturated, simply meaning that there is just no more room. When this occurs, things that normally dissolve, like our sugar, can not. Total Dissolved Solids (TDS) is what the name suggests: the measure of the amount of dissolved solids, the combined total of all inorganic and organic substances in a body of water: Anything that is in the water that is not Dihydrogen Monoxide (aka: H2O).
Anything you add to a swimming pool is going to increase something and everything you add will increase TDS. For example, a gallon of sodium hypochlorite (containing 2.34 pounds of solids) will add 12 ppm of FAC (Free Available Chlorine) to 10,000 gallons of water and will increase the TDS by 28 ppm. 15 oz. of trichlor (stabilized chlorine tablets/granular) will add 10 ppm of FAC to 10,000g, but it will also increase the CYA (Cyanuric Acid) by 6 ppm and the TDS by 10 ppm. Calhypo (Calcium Hypochlorite) added at a rate of 20 oz. per 10,000g will add 10 ppm of FAC, but will also increase your Water Hardness by 8 ppm and your Total Dissolved Solids by 15 ppm. Even a cup of muriatic acid (containing 1.87 pounds of solids per gallon) will raise the TDS by 1.4 ppm.
Total Dissolved Solids is often referred to as “The Age of the Water”. Take into consideration that the water straight from the tap, even before you add anything, has a level of Total Dissolved Solids. It is important in swimming pool maintenance that we know what that starting level is. The TDS level is not something that will decline over time – it only increases. The only means we have to lower the level of dissolved solids is to replace water. It is recommended in pool and spa care that we drain the vessel and replace with fresh fill whenever the Total Dissolved Solids level increases by 1500 ppm over that starting measure.
A common industry expression: “Dilution is the Solution to Pollution”
Evaporation is also going to be a factor in an ever-increasing Total Dissolved Solids level. When water evaporates, the only thing that leaves the swimming pool is water. The solids that were present in the evaporative loss are left behind, this increasing the total in the water that remains. Then, the replacement water only adds additional solids to the solution.
Total Dissolved Solids are measured using a TDS meter. These electronic testers vary in price, from less than $100 to well over $1,000. The meter serves as an indirect means of determining the level, what this actually measures is the electrical conductivity (EC) of a solution; this occurs by sending a charge between the two electrodes of the device. Ionized particles such as salts and minerals increase the EC, because of this our results can be misleading. Not all solids dissolved in water are ionized, however they due still contribute to the TDS level. Take the sugar in our cup of coffee for example, though dissolved and a contributor to Total Dissolved Solids; it is undetected by our TDS/EC meter.
The function of the TDS meter also helps to explain Galvanic Corrosion (which would be a killer name for a heavy metal band). GC (galvanic corrosion) can be recognized when metal components (light rings, handrails, ladders) submersed in water turn black. This is usually an indication of a high TDS, but the Total Dissolved Solids itself is only a contributing factor and not the actual cause. As NACE International explains it: “Galvanic corrosion (also called “dissimilar metal corrosion” or wrongly “electrolysis”) refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive-electrolyte. It occurs when two (or more) dissimilar metals are brought into electrical contact under water.” As we discussed above in the operation of a TDS meter, a high level of ionized particles (which is what we measure when we measure Total Dissolved Solids) will increase the EC (electrical conductivity) of water.
So, do you need to run right out and purchase a TDS meter? The Total Dissolved Solids level in a swimming pool can take years before it increases by 1500 ppm over your starting value, so you could bring a water sample to the pool store and have it tested quarterly. What about the Saturation Index you learned about in CPO® Certification Class? Because your factor for TDS is either 12.1 or 12.2 (NSPF Guidelines), you could honestly guess for this calculation; if you were to shoot for a LSI of 0, in a worst case scenario you would only be off by ).01 in either direction. With a balanced range of -0.03 to +0.03, you would still hit the mark. A spa, however, is a completely different story and I would say that those who maintain one probably should own a TDS meter. In a small hot water environment, the Total Dissolved Solids level can increase by that amount in the matter of weeks, or even in days (depending upon bather load/usage).
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