The Water Course

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Make Predictions — Protect Your Equipment

TWC 3.04 – Recap and Glossary

3.04 Recap


  • The Langelier Saturation Index (LSI) gives baristas and cafe owners an inexpensive and easy way to predict how their water will affect their coffee flavour and whether it will damage equipment.
  • To use an LSI calculator, you need to all the measurements we taught you about in Chapter 1: GH, KH, pH, and TDS.
  • The LSI scale ranges from -2 to 2. A negative reading suggests the water is aggressive and prone to causing corrosion, and a positive reading implies that the water has potential to form limescale.
  • A slightly positive LSI reading means you can create a very small amount of scale, which can have the effect of preventing corrosion by leaving a slight layer of scale on the inside of your boiler — it’s a bit like coating a pan in Teflon.
  • An LSI reading above 0.7 indicates a strong likelihood that you will develop severe limescale.
  • A reading below zero means you are a risk of corroding your equipment.
  • It is useful to get LSI readings of your water direct from the water mains. It is also useful to get LSI readings of the water after it has passed through your water softening system, if you have one.
  • The water control chart can help you understand when the hardness and buffer (alkalinity) are well balanced. With this information, you can brew your coffee to have optimal flavour.


New Words

Balanced (water)  Water that has an LSI reading of zero. This means it will neither be scale forming or corrosive.

Bypass (water treatment)  A system of blending used in water softening where some source water is allowed to avoid passing through an RO membrane or an Ion exchange column and is instead mixed into water that has travelled through the softener. This system allows baristas to increase the hardness of their product water.

Supersaturated (water)  Water that has an excess of hardness and alkalinity and will give an LSI reading above zero.