TDS (Total Dissolved Solids)

We have had problems recently concerning the maximum allowable level of TDS in public swimming pools. The problem is that TDS includes table salt (NACL). High doses of salt, or liquid chlorine (which contains salt), will increase your TDS possibly above recommended levels even though salt is perfectly harmless. TDS also measures Calcium Carbonate (CACO3) and other salts in the water. If you use powdered chlorine in your pool, or use large amounts of buffer you can increase the level of calcium salts in the water.

The ocean and our ocean pools have a salt content of about 32 ppt, or a TDS of approximately 34000. Yet, even these pools are effectively treated and tested without any problems. So is there a maximum TDS? Can we test easily for TDS? What are we actually testing with TDS? Even we were surprised at what we found and we have included links to the most comprehensive sources.

TDS is a measure of conductivity in water. As conductivity is greatly affected by the amount of salt in the water it can be used as a means of measuring salt content. Conductivity is also affected by temperature.

In Australia TDS/conductivity meters are primarily used to measure the salt level for salt chlorinators. When testing a sample for salt, you must adjust it for temperature difference. Conductivity is compared to known constant values at 25 degrees. This means you must increase your result by 2% for every degree below this and decrease your result for every 2% above this. As most poolshops are testing water at about 20 degrees they should be increasing their results by 12%, although some meters do this calibration for you.

This is trivial compared with the much larger problem of TDS. Conductivity meters can only be calibrated to measure one type of salt at a time, for which the conductivity is known. This means there is no generic TDS measurement, which the swimming pool standards required. Any TDS measurement will be affected by the combination of salts in a pool. This means that for every pool the meter can only be accurately calibrated for that pool and not for any other pool and the only correct way to get a true TDS reading would be to determine the composition of salts in the water (i.e. the percentage of CACO3, NACL and other salts). Without this the TDS readings will not give any useful information. A meter calibrated for NACL for salt chlorinated pools really is sufficient for testing. TDS really is an inaccurate method of testing that has been used for years without much understanding of how it works. It is however the easiest way to test for salt content and is a good guide for those with salt chlorinators. Do not however be concerned about high or low TDS or certain levels of TDS as you can see it is affected by many factors and in salt water pools it will be very high.

See: Practical Considerations for Conductivity and TDS MeasurementReefKeeping – TDS,
PPOA – What is the fuss over TDS?

Oakton TDS Tech Tips [PDF]

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