I test French press brewers much like I test standard drip coffee makers. I begin by hand-washing and hand-drying each product. Then I grind enough coffee beans to meet a specific brewing ratio. For French press brewing, that's 4 ounces of ground coffee to 32 ounces of water.

I then add hot water (203 degrees F, 95 degrees C), or nearly boiling water, to the brewing chamber, stir the grounds and let them sit for 4 minutes. After that I drop the coffee plunger for each press and pour a sample cup. Next I draw a sample of the brewed coffee and measure its percentage of total dissolved solids. I use a pocket reflectometer for this test. From there I can calculate the extraction percentage for each batch of coffee I brew.

Ideally, the extraction percentage of brewed coffee should be in the range of 19% to 22%. Though this number alone doesn't guarantee delicious joe, it's a strong indicator of it. Ultimately the truth lies in a proper taste test.