Evaluating the performance of a coffee maker is trickier than it might sound. You need to know what good drip coffee is and, according to the Specialty Coffee Association, there are essential criteria for brewing well. Brewing time and water temperature top the list. Hot water should come into contact with grounds for no less than four minutes and no more than eight. The ideal water temperature range is between 197 degrees Fahrenheit (92 degrees Celsius) and 205 degrees Fahrenheit (96 degrees Celsius).

To see how well each coffee maker meets that challenge, we log the length of their brew cycles. We also employ thermocouple heat sensors connected to industrial-grade data loggers. That enables us to record the temperature within the coffee grounds while brewing is underway.

After brewing coffee, we take sample readings of the produced coffee liquid with an optical refractometer. Given that we factor in the amount of water and freshly ground coffee used, that data lets us calculate the Total Dissolved Solids percentage of each brew. From there we arrive at the extraction percentage. The ideal range is commonly thought to be between 18% and 20%.

We also back up measured data with a good old-fashioned taste test. Over-extracted coffee tastes bitter and sharp, while under-extracted coffee is usually weak or sour. To be certain, we brew identical test runs a minimum of three times to get a sense of the average results.