by another_jim on Mon Feb 20, 2006 5:32 pm
Considering safety devices may be the last stage in the design, after the major elements have been thought out.
The basics:
-- Espresso is a 25mL single or 50mL double of coffee brewed from 6 to 9 grams or 12 to 18 grams of ground coffee
-- Brewing time is 25 to 30 seconds
-- Brewing pressure is 8 to 10 bar
-- Brewing temperature is 90C to 95C
-- The machine must also be able to generate 120C to 135C steam for frothing milk
The classic commercial espresso machine uses hand lever or spring powered pistons or a centrifugal motor for pressure, a large boiler and heat exchanger for the steam and water heating, and the coffee is handled manually with a portafilter and basket. The time, temperature, and pressure can be held within about 1/3 intervals of the acceptable range
The classic home machine has a much smaller boiler or thermoblock with two thermostats, one for brewing, one for steaming, and a small vibratory pump. In most home machines, the espresso is "out of range" for both pressure and temperature. The home machines we talk about on HB are the very expensive kind (since we're hobbyists) which do brew within commercial tolerances or better.
One trend is towards convenience -- Superautomatic grind and brew without requiring the operator to handle the coffee at all. Pod machines use preground capsules of coffee so the operator doesn't need to tamp or grind. The challenge for these machines is to equal the quality of the classic commercial machines. Any better mousetrap in the convenience area will make you very, very rich, since this is where the real money is going.
The other trend is towards precision -- as the convenience machines become as good as classic espresso machines, the "hand made" espresso at cafes or enthusiast home had better improve, or the cafes will be out of business, and the hobbyists out of a hobby. The action here is electronic pressure and temperature controls, brewing and steaming systems that are independent and can be controlled within 0.5C, pump designs that have controllable and flutter free pressure ramps. Improvements in this area will get you lots of admiration from us, and almost no money at all.
