DiFluid R2 Extract coffee refractometer for TDS and extraction

What does a refractometer measure and what does the reading tell you about your coffee?

A refractometer measures how strongly a drop of coffee bends light. The more substances from the ground coffee are dissolved in the water, the stronger the refraction. The device converts it into TDS, the share of total dissolved solids in percent. The value describes the strength of the cup, not its flavour. Combined with the coffee dose and the beverage weight, it gives you the extraction yield, the share of the ground coffee that actually ended up in the drink. That figure explains why a coffee tastes sour and thin or bitter and dry, and shows whether to grind finer or coarser. The DiFluid R2 Extract measures from 0 to 30 percent TDS with a resolution of 0.01 percent, an accuracy of plus or minus 0.03 percent and a precision of plus or minus 0.02 percent. It uses a dual 2D CMOS image sensor with two temperature sensors and sends its readings over Bluetooth to the DiFluid Café app.

From reading to extraction yield

A TDS reading alone does not tell you whether a coffee is well extracted. For that you need two more numbers: the coffee dose and the weight of the finished drink. A scale that reads to 0.1 g, such as the Acaia Pearl Pitch Black, gives you both. The Brew Lab does the maths. Without a refractometer it estimates TDS from grind size, ratio, temperature and contact time. Once you enter your measured TDS and beverage weight, the whole result is calculated from your numbers instead of the model.

The benefit shows most clearly when you change only one variable. Brew the same coffee twice with the Graycano Dripper, once ground finer and once coarser, and measure both cups. The difference in extraction yield tells you which direction your recipe needs.

Where measuring fits in the setup and which gear comes before it is covered in the pour over setup in three steps.