Temperature vs TDS of Water

In my Water Tasting Classes, we use low-end TDS (total dissolved solids) meters to check the TDS of still waters before tasting them.

Not only are the TDS meters not the top of the line, another reason they don’t always match the reported TDS (bottled waters put this info on their websites – Fiji is 220 for example) is because TDS measurement with these meters is temperature dependent.

I wanted to see *how* dependent, so I collected some hot tap water out of the sink (or did I heat it on the stove? don’t remember) then kept measuring it as it cooled. Eventually when it hit room temp I put the glass in the freezer to keep cooling it.

As you can see, the water starts at around 57C (134F) and ends at 6C (43F), and the associated TDS is 150 at the high temperature and 70 at the low temp.

Note: the standard accepted temperature to measure TDS is 25C (77F), which is warm room temperature. Interestingly, I did this experiment at home in the Mission where the TDS is a bit over 100ppm, while at my office in the Financial District it’s much lower – 70ppm or so if I recall correctly. (This is the ppm that the SFPUC says it should be.) Here on the chart if we look at where the water is around 25 degrees, it’s around 100ppm at my house. Suddenly I am feeling like maybe I should get the water at home tested…

Perhaps I’ll repeat this experiment at the office.