What is the role of TDS?

Ahh total dissolved solids or TDS – the magical number people seem to get unnecessarily obscessed with.

I will be frank: stop giving a shit about TDS.

Here's why you should stop focusing on it so much:

• The TDS meters you use are actually measuring electrical conductivity and just assuming they can translate this to TDS using a conversion
• The measurement from these devices varies with temperature quite a bit and between the different devices + calibration
• Different ions in water affect conductivity differently, so a solution of 1 molar sodium chloride and 1 molar magnesium sulfate give very different values
• Sticking a pen in water and reading "118" doesn't tell you anything about what is in the water and how coffee taste will be impacted

Why is TDS so popular?

Firstly, because it's easy to measure with a pen – so naturally people have gravitated to this as a simple way to measure the water they have at home.

But you're able to take pure water and add specific minerals to it, so TDS measurements are no longer important to you, the budding wizard.

Secondly, TDS has long been used in coffee for more sensible measures of coffee extraction, i.e. how much solids are present in the coffee.

This makes sense, because a general measure of mass of solids in the coffee does tell us something about the overall strength of the coffee. Though usually this is measured with a refractometer (which uses light), which is an actual measurement of dissolved solids, unlike these electode pens you stick into water.

Thirdly, the SCA released guidance information on suitable "water for coffee" including some TDS recommendations.

For many people, the mineral content and chemistry of water is not well-understood, so when some authority recommends "TDS should be ~150" and to use between 75-250, people will follow this quite blindly.

Lastly, people may have experience with using different water and observing it's effects on coffee taste. They measured TDS, and now they draw conclusions that TDS above X is bad, TDS below Y is bad, and so on. All while not actually knowing what is in the water.

Let's move beyond that and focus on sensible scientific units, creating our own well-understood mixes, and further understanding the sensory contributions of each component.