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Extending shot time with pump series resistance
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GeorgeRYoung
Senior Member
GeorgeRYoung
Joined: 2 Feb 2012
Posts: 29
Location: Ottawa ON Canada
Expertise: Just starting

Espresso: delonghi Bar 42 modified,...
Grinder: delonghi DCG50 modified
Posted Mon Feb 20, 2012, 12:04pm
Subject: Extending shot time with pump series resistance
 

Picked up a used deLonghi Bar42 espresso machine after my non-pressurized Bar12 died. The Bar42 uses pressurized filters with a spring-loaded blow-off valve, and produces a fine double espresso in 27 or so seconds, lots of crema and taste. This may be blind luck or perhaps deLonghi carefully calibrated the spring to match the pump characteristics.

Because the filters are pressurized, the pump pressure stays essentially constant at the value needed to open the blow-off valve. And because of the pump pressure-flow curve, the flow rate is essentially constant. This means a single comes out in 13 seconds or so, good crema, watery taste. I assumed the lack of flavour was because the coffee and water are not in contact long enough.

Unfortunately the Bar42 single filter is the same as the double filter except not as deep. There's no basket taper or reduced number of holes to help the single filter constrain the flow. Tried cramming in more coffee, finer grinding and fierce tamping, but the single shot time was not impacted much.

Next plan was to reduce pump power with series resistance. I think a resistor in series moves the pressure-flow curve of the pump towards the origin. Inspiration from Click Here (www.home-barista.com) "Is anyone using voltage to control Vibe pump pressure instead of OPV?"

The junk box contained a DPDT center-off switch and a couple of 7.5 ohm/10W resistors. I wired them up to the switch so that zero, one resistor (7.5 ohms), or two resistors (15 ohms) are in series with the pump, depending on switch setting.

The Low Power setting (2 resistors in circuit) seems to be the best for a single, yielding around 25 seconds shot time, with good crema and flavour. The Medium Power setting (1 resistor in cirtuit) is good for a double restrait. And the High Power setting (no resistors in circuit) remains fine for a double.

I realize that some prefer non-pressurized baskets, but with this frugal mod, my rinky-dink pressurized machine is almost respectable.

GeorgeRYoung: 2ResistorsSeries.png
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