Kind of a science question: I've been hacking the electronics of a cheap espresso machine a bit to see if I can get better results, and was wondering – espresso is recommended to be brewed at around 130 psi of pressure. That's a lot. My machine is a single-boiler steam pressure machine, and from what I can tell it barely manages 5 psi when pushing water through the puck.
When I use compressed air to step up the pressure to 60psi, the water shoots through in about 1 second – a tiny bit less than the recommended 25 sec :). So how is this supposed to work in a "real" high-end machine? Seems to me the tamped coffee is never going to be able to offer up that much resistance, to where 130 psi of back pressure could be sustained.
Best Answer
I can't speak for the new high end machines that provide pressure profiling, or the cheap domestic machines, which work who knows how, but the standard system for distributing pressure on the coffee is the e61 group head, and most machine group heads are based off the original design.
The e61 group is pretty much just a system of 3 pressure valves that allows the pressure to slowly ramp up 9 bars and then maintains that 9 bars of pressure while the pump continues to work. Some versions include other steps such as pre-infusion, or on newer high-end machines the ability to create pressure profiles which adjust the pressure at any point during the espresso shot.
Also note that all the resistance HAS to come from the coffee puck, so you may be grinding too coarse or not tamping hard enough. If you need to measure the pressure from the grouphead, espresso machine techs use a device like this.
You could probably make your own at home if you are so inclined.