Blind Test Extraction Measurements
Tests were done with a Mojo Refractometer. Coffee dose was measured using a 0.1gm scale before tamping. Espresso was pulled into a cup sitting on the scale and the cup was pulled when the shot approached the target beverage weight. Measure results show that extraction is in the ideal 18% range and is very consistent across both devices and from shot to shot. Both the "steam" and "soak" preheat performed identically in this test.Brew Temperature Measurement / Preheating
The brew temperature was measured with a quick responding 30 gauge fine wire thermocouple, inserted over a plastic block machined to fill up almost the whole filter basket. This ensures all water flows down the hole in the middle past the thermocouple. A fine hole was drilled in the bottom of the block to simulate the correct water flow rate of espresso extraction. This resulted in the emptying of the water chamber ( filled to the top, around 80ml ) in 25 seconds - see sudden drop in temperature graphs . A calibrated thermometer was used and connected to data acquisition software on a PC via RS232.
To ensure correct brewing temperature of 90-94 degrees, it is necessary to preheat the water chamber. Three preheating methods were tested:
Steaming ( top of Mypresso handle and water chamber put in kettle above water level, 2mins above boiling water)
Soaking ( filling water chamber with boiling water and soaking for 20 seconds, then repeating once, then fill and pull )
Steam wand preheat ( filling hot water in water chamber and heating it with another espresso machine steam wand ).
All three resulted in consistent and acceptable brew temperatures. Brew temperature hardly dropped during the pull - very good temperature stability. Extraction measurements and taste confirm correct brew temperature compared to a commercial machine.
My theory about the consistency of brew temperature is that we are starting from a know reference temperature, which is 100 degree rolling boiling water. If you preheat the device for the same amount of time every time, each pull will be very close in temperature. As for stability, I believe that the water chamber temperature will reach equilibrium with the filled boiling water quickly, after which the temperature of the whole device will only drop slowly. As it is a gas which is pushing the water down, there will be minimal drop in temperature ( compare with room temperature pumped into the boiler of entry level single boiler machines! ). You can experiment with lower temperatures by varying the wait time before pulling - open you find the sweet spot for a specific coffee just repeat the same wait time and you will get the same results.