I test French press brewers like I test standard drip. Coffee makers. I start by hand washing and hand drying each product. Then I grind enough beans to meet a certain ripeness ratio. For French press brewing, it's 4 ounces of ground coffee to 32 ounces of water.

I add again. Hot water (203 degrees F, 95 degrees C), or nearly boiling water, in the brewing chamber, stir the grounds and let them sit for 4 minutes. I then leave enough plunger for each press and insert a sample cup. I then 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, extraction percentage Brewed coffee Should be in the range of 19% to 22%. While this number alone does not guarantee delicious barley, it is a strong indicator of it. Ultimately the truth lies in a proper taste test.