|Ask The Expert: TDS and salt levels
Can I measure the total dissolved solids (TDS) to get an approximation of my salt level?
Initially, when the pool is first filled with fresh water, the majority of the TDS in the water is due to the concentration of salt. Therefore, the TDS can be a good indicator of what the salt level is. However, keep in mind that any time you add a chemical to the water you are increasing the dissolved solids level. As the TDS level increases, particularly over several seasons if water is not completely drained and refilled, the measurement of TDS no longer represents an approximation of the salt level.
Salt is only one component of the total dissolved solids. However, salt is highly soluble and is generally factored out of the TDS measurement. In order to get a true TDS reading you need to subtract the salt level from the TDS reading. Therefore, both tests are performed. You may want to know the true TDS in order to make a Langelier calculation or determine if it is time to drain and refill a pool and you need to measure both levels and make a calculation. For example, a TDS measurement of 5,000 ppm and a salt measurement of 3,000 ppm would mean that you have 2,000 ppm true TDS (5,000 TDS 3,000 salt = 2,000 ppm true TDS).
Answer provided by Joe Sweazy, Technical Sales and Service Manager for HACH Company/ETS Business Unit
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