A number is easy to remember.
60 ppm.
80 ppm.
100 ppm.
That's one reason TDS has become such a popular way to talk about coffee water.
But there is a problem.
TDS tells you how much dissolved material is present.
It doesn't tell you what that material is.
And in coffee water, composition matters.
The 60 ppm problem
Imagine two waters.
Both measure:
60 ppm TDS.
Water A contains a magnesium-forward mineral profile.
Water B contains a calcium- and sodium-rich profile.
The meter sees two similar conductivity-derived estimates.
The coffee sees two different chemical environments.
That is the problem with using TDS alone.
What does TDS actually measure?
Most handheld TDS meters estimate dissolved solids from electrical conductivity.
Different ions conduct electricity differently.
Meters also use conversion factors.
So TDS should be understood as an estimate.
It is useful.
But it isn't a complete chemical analysis.
Composition gives the number meaning
Consider a water containing:
Ca²⁺
Mg²⁺
HCO₃⁻
Cl⁻
SO₄²⁻
The relative amounts of each ion create the composition of that water.
Change the ratios and you change the water.
Even if the total measured TDS remains similar.
This is why water formulation is more interesting than simply hitting a ppm number.
Hardness adds another layer
Calcium and magnesium contribute strongly to hardness.
So two waters with similar TDS can have different hardness.
Or similar hardness but different calcium-to-magnesium ratios.
The number alone doesn't reveal that.
Alkalinity adds another layer
Bicarbonate contributes strongly to alkalinity.
Two waters can therefore have similar TDS and hardness but different buffering capacity.
That can strongly influence how coffee acidity is perceived.
Again:
same number
different water.
Why TDS is still useful
This doesn't make TDS useless.
Quite the opposite.
TDS is excellent for:
batch consistency
monitoring dilution
checking mineralisation
spotting unexpected changes
quality control
It gives you a fast number.
The mistake is treating that number as the entire recipe.
Composition first
At The Water Artist, we prefer this order:
1. Define the ionic composition. 2. Consider hardness. 3. Consider alkalinity. 4. Set a practical TDS target. 5. Brew. 6. Taste.
TDS becomes a checkpoint.
Not the destination.
A useful thought experiment
Suppose someone tells you:
"My competition water is 100 ppm."
The useful follow-up question is:
100 ppm of what?
What is the calcium?
What is the magnesium?
What is the alkalinity?
What counter-ions are present?
How was the TDS measured?
Now the number starts to become useful.
The Water Artist
A TDS number without composition is incomplete information.
The number tells you how much.
The composition tells you what.
And the coffee tells you whether it worked.
Water is the canvas.
You're the artist.