How To Measure TDS
Unfortunately for your wallet, a device called a refractometer is required to measure the amount of dissolved solids in your brew. Refractometers for coffee can be somewhat expensive ($200-$1,000), but they are considerably cheaper and much smaller than a gas chromatograph coupled to a mass spectrometer, the other VERY fancy and scientific way of measuring dissolved solid. This magic tool works by measuring the deflection of light as is passes through the coffee. (Barista Hustle has a great article explaining this light bending property further if you’re interested in nerding out a bit more.) Along with your refractometer, you’ll need these items:- Distilled water
- Alcohol wipes
- Dropper
- Paper towel
- Small bowls or cups
- Filtered syringe*
*Optional: If you are brewing using a metal filter like a Prismo or espresso shot, there will be suspended solids that are not water soluble. You will also want to use a filtered syringe to eliminate the suspended solids to get an accurate TDS reading. If you are using a paper filter, then your brew will be entirely dissolved solids.
Step 1: Set refractometer to zero using distilled water
- Turn on your refractometer.
- Add 3-4 drops of room temperature distilled water.
- Take a zero set reading.
- Wipe down refractometer lens with an alcohol swab.
Step 2: Put a small sample (around 2 oz) of well-integrated brewed coffee into a small bowl or cup
- Make sure to stir the coffee to unionize the liquid. Stratified coffee will give you an inaccurate measurement.
- Be sure to also allow the brewed coffee to cool to approximately 100°F or below. If you don’t have a thermometer, aim for room temperature or cooler.
- If you’re using a non-paper filter, this is where you use the filtered syringe
Step 3: Take a small sample of brewed coffee with a dropper or syringe
- Place 3-4 drops of brewed coffee on the eye of the refractometer.
- Close the lid and wait 10 seconds for the sample to reach a temperature close to the refractometer.
- Press go and take a reading. Do this 2-3 times to ensure a proper reading.
Step 4: Use a clean paper towel to remove the sample
- Wipe clean with an alcohol swab to reset for the next coffee sample.
Optimal TDS Readings For Different Brew Methods
For a pour-over dripper like Stagg [X] or Stagg [XF], the typical TDS measurement will be somewhere between 1.2-1.7 TDS. That’s right, your pour-over coffee is actually about +/- 98.5% water! For espresso-style shots with Prismo, we love the flavor most with TDS readings of 3.5-5.
Pour-Over: 1.2-1.5
AeroPress® Coffee Maker: 1.4-1.7
French Press: 1.4-1.7
Prismo: 3.5-5
Espresso: 8-12
Ex: 320 grams of brewed coffee x 1.43 TDS = 457.6
457.6 / 20 grams ground coffee weight = 22.88
22.88 = your extraction percentage