by Sean Griffin, February 19, 2010
TDS ~ Total Dissolved Solids
Remember that science experiment when you were a kid? Keep pouring and stirring sugar (or salt) into a glass of water, and eventually it will stop dissolving - this happens when the water reaches it's saturation point. The TDS gets too high - this can happen in your swimming pool, too.
Total Dissolved Solids (TDS) is the measurement of any dissolved solid in a liquid. In pool water the lower the TDS the closer the water is to being pure (H2O). Common dissolved solids are magnesium, potassium, sulfates, carbonates, irons, other metals, minerals, carbonates/bicarbonates, anionic and cationic substances, and salts.
Suspended solids in a liquid, both organic and inorganic, can be in molecular, ionic, or micro granular form. Dissolved solids are considered to be less than 2 micrometers and able to avoid filtration, due to their small size. High TDS can over-saturate your pool water causing water quality problems.
Pool water with a high TDS reading can be aggressive or corrosive and can be the cause of cloudy water, elevated hardness, scale, and an unwanted metallic taste. Dissolved solids can affect your Ph and can reduce the efficiency of chlorine.
TDS measures anything that has ever been introduced into the pool water. These solids can enter your pool water in a many ways, from the wind, fill water, rain. Detergents, dust, sweat, body oils, etc. Even when you are adjusting your chemistry by using your pool chemicals you are contributing to the TDS reading as these chemicals dissolve. The goal is to keep TDS at a minimum but you can expect that reading to rise as the years go on.
TDS is measured using two different methods. One approach involves evaporating water and leaving behind a residue of solids. These once dissolved solids can be weighed by an analytical balance to get an approximate calculation of what is in solution. There is room for error as some solids evaporate with the actual liquid but this is the more accurate way. The second method, more practical and better known in the pool and spa industry, involves using a TDS meter which measures the electrical conductivity of water. A third method to test your TDS is by using test strips which test pool water Total Dissolved Solids.
Regardless of how you test what has dissolved in your pool or spa water it is an important factor when trying to achieve top quality water. The effect that TDS has on water varies from pool to pool. Some theorize that water with high TDS can act as a shield hindering sanitizers from breaking down contaminants and limiting effective circulation. Kind of clogging or choking up the water. It is also believed that these solids act as a bubble trapping particles and keeping them in the water.
As a service technician in the Northern Virginia area I can tell you that most pools I cared for with non problematic water, went without TDS testing. TDS readings were typically taken when water clarity and or scale became issue even though the other aspects of the water seemed to be in balance. The affect TDS has on pool water chemistry is still being studied. There are seawater pools with an extremely high amount of solids that have been dissolved but are able to be maintained and kept sanitary without problems.
If you consistently experience pool water clarity issues such as hazy, cloudy pool water or trouble with algae or if you develop calcium deposits - you may have TDS issues. Test the pool water and if at a high level* ~ the remedy is to dilute the pool water - meaning, drain out some of the existing water and replace it with water that has a lower TDS reading.
What's an ideal TDS reading for a pool?* A wide range actually - some would say from 300ppm to 3000ppm. All H2O has the same ability to dissolve, but other water chemistry parameters may allow for a higher acceptable level of Total - Dissolved - Solids....you'll know if you have a problem!