Total Dissolved Solids -TDS

What is Total Dissolved Solids - TDS

Find and bookmark our home page at

I often see forum posts asking the following question

"I am using a (whole house water softener or, in-line filter on the end of my espresso machines feed pipe etc..) and when I check the Total Dissolved Solids (TDS), I get a reading of 250 for the unsoftened water and 265 for the softened water. It looks as if my softener is not working properly."

It's a common mistake and caused by not testing correctly, the TDS of water softened using an ION exchange resin will be pretty much the same as it was before. The TDS meters work my measuring electrical conductivity, a good indicator of the dissolved solids (more dissolved solids, higher conductivity).

Hardness in water is caused by dissolved Calcium and Magnesium Carbonate (CaCo3 and MgCO3). There are two types of soft water, for want of better terms let's call them "natural" and "artificial".

  • "Naturally" soft water , has lower Calcium and Magnesium Carbonate (CaCo3 and MgCO3). Naturally soft water will show a lower TDS
  • "Artificial" soft water produced by ION exchange (resin softeners) work by exchanging equal amounts of calcium (and magnesium) Ions for Sodium ones…because sodium carbonate does not form limescale (it stays in solution) you now have "soft" water. But of course it's still as electrically conductive as before, possibly slightly more so, because sodium carbonate has slightly higher electrical conductivity. So the TDS either doesn't change or rises slightly. It is also why it's OK to drink naturally soft water, but not advisable (especially for people on a low sodium diet) to drink "artificially" softened water, and of course there's the possibility of more bacteria, because of resin bed contamination.

To check the hardness of the water after it passes through your little in-line filter, or household water softener, you need a special test kit (available for a few £), usually a tablet that's dissolved in the water and the colour indicates how hard/soft it is. The TDS meters are really an ideal way of measuring the effectiveness of Reverse Osmosis systems

I personally don't like those little in-line filters that sit in the cold water tank of your espresso machine and think your better off not using them. Try and find another way to provide boiler safe water, they also don't do very much for very long. The same can be said for Jug filters, which quickly become relatively ineffective.