TDS - Further Reading

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 were able to more accurately measure the components that make it up. 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 is 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.

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. Its measurement of TDS and meaning of what it is measuring with TDS is 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.[3]

Unless your pool water makeup matches this, which it doesn't, the TDS reading is meaningless.

What usually happens is the pool store will set the meter to salinity and take a reading and then to TDS and take a reading and report both numbers.

But, think about it, how can the conductivity meter isolate just the salt in the first reading?

It can't. It just measures conductivity in both tests, which has to be exactly 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 you can read TDS, IRON, COPPER and TDS.