For your reading pleasure, here are a couple more tests to measure the effect of varying pump pressure on the rate of espresso extraction. Last time
the results suggested that increasing pump pressure above ~8 bar actually decreased
the extraction rate, and I wanted to verify this result.
In the first experiment, I kept dose and grind constant and simply measured how much espresso I got in a 25 second extraction at various pressures. Once the pump came up to full pressure, the pressure was maintained as flat as possible. In particular, I was looking at the lower pressure realm, because some data in Illy's book suggested the the flow rate might go up around 5 bar. I saw no evidence of this:
In the second experiment, I kept the dose and grind constant and set the flowmeter to terminate the shot after 48 ml of water had been pumped into the boiler. This resulted in shots that were fairly uniform in volume (19.0-21.4 grams). To get the average flow rate, I divided the volume in grams by the seconds of flow (total seconds minus 8.5 seconds dwell time). This data suggests that the flow rate peaks somewhere around 8 bar:
1. I've been saying for years that it's the pressure/volume relationship in the first few seconds
of the extraction that is most critical in determining flow rate. I still believe this, so I kept the parameters in the initial 4 seconds of each extraction constant.
2. I tasted all the shots, especially in the second test, where similar volumes permitted realistic comparisons. It would be very easy to draw hare-brained conclusions from these, but I believe I'd have to run a lot
more shots with a blind taste panel to get any definitive results.
3. I also have a hare-brained theory for why espresso seems to come out best around 8-9 bar, but I'll save it for when I can actually prove it (if ever).
4. I bought a new 1.5% pressure gauge to calibrate my setup. I believe the pressures in the graphs aren't perfectly accurate, but they're pretty close to what the cake is actually seeing.