What is TDS?
TDS is Total Dissolved Solids.
Do We Care about TDS Levels in Pool Water?
TDS is not very important for pool care. It used to be a way to tell when water was "old," but that was before we could measure the components that make it up more accurately. Now it's just a relic of a bygone era.[1]
We care about measuring and managing the individual chemicals, but the Total Dissolved Solids are meaningless.[2]
What Chemicals Make Up TDS?
TDS in pool water includes Salt, Calcium Hardness, Free Chlorine, TA, and CYA, and any other soluble substances that have been dissolved in the water.
TDS Meters
A TDS meter reads the concentration of ALL conductive ion species in water (sodium, calcium, chloride, nitrates, sulfates, etc etc etc). You will not get an accurate reading if it is not accurately calibrated to known chloride standards with a calibration factor determined for your pool water. That means you must know the exact concentration of your standards to ensure the meter's linearity AND the concentration of chloride ions in your pool water to establish a calibration factor.[3]
Pool Store Measurements of TDS
Pool stores often measure TDS using the Myron L Company PoolMeter.
The PoolMeter has a dual range to measure Salt and TDS. Its measurement of salt is accurate. However, its measurement of TDS and the meaning of what it is measuring with TDS are questionable.
The TDS measurement is based on a 442 Standard Solution consisting of the following salt ratios: 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride.[4]
Unless your pool water makeup matches this, which it doesn't, the TDS reading is meaningless.
What usually happens is that the pool store sets the meter to salinity and takes a reading, then to TDS and takes a reading and reports both numbers.
But think about it: how can the conductivity meter isolate just the salt in the first reading?
It can't. It measures conductivity in both tests, which must be the same reading. The only difference is the multiplier used in each setting. When testing a salt pool, only the salt multiplier is valid.
For a more in-depth discussion of the flaws of TDS testing, read TDS, IRON, COPPER and TDS.
- ↑ https://www.troublefreepool.com/threads/what-is-the-significance-of-tds-as-it-related-to-pool-balance.179719/post-1588667
- ↑ https://www.troublefreepool.com/threads/what-is-the-significance-of-tds-as-it-related-to-pool-balance.179719/post-1588669
- ↑ https://www.troublefreepool.com/threads/salt-levels.205078/post-1805174
- ↑ https://www.troublefreepool.com/threads/tds-iron-copper.195565/post-1724167
