Sam Plett

Espresso is brewed somewhere around 9 bar (1 bar is equal to atmospheric pressure).  Consumer and most prosumer espresso machines use vibratory pumps (often called 'vibe' pumps) to achieve the brew pressure.  The problem is that vibe machines create approximately 15 bars of pressure so in order to reduce the pressure consumer machines in the medium price range have over pressure valves (OPVs) installed that limit the brew pressure to either 9 bar or 11 bar (if the machine is rated for E.S.E. pods) in more expensive machines variable over pressure valves are installed that allow the user to change the brew pressure to their liking.  However, the cheapest espresso machines are generally not equipped with any OPVs but some cheap espresso machines achieve pressure reductions through gizmos that use springs, pressurized portafilters, etc.

There has been quite a bit of discussion in the past on both home-barista and coffee geek about using electrical resistance to reduce the voltage that goes to the vibe pump and specifically the use of a home lightbulb dimmer switch to achieve it. By reducing the voltage you reduce the amount of pressure the pump will create.  The use of a dimmer switch means that you can change the vibe pumps pressure 'on the fly' or you can leave it set at one spot if you find that is what works best for your machine.  Despite all the discussion there was never a write up on how the mod is performed so I decided to record the process when I did it on my Starbucks Barista.

Necessary Parts

1. Dimmer switch.  I used the kind that is used to add the dimming feature to a lamp cord.  Cost was about $14.00
2. Electrical Wire.  I bought approximately 2 meters of lamp wire.  Cost was about $2.00
3. Connectors.  You'll need 1 male and 1 female.  Cost of each pack was about $2.00 / each.

The Process

1. Follow the directions on the dimmer for hooking up the lamp wire to it and then add a female end to one side and a male end to the other side.
   
2. Unplug the espresso machine, take off the cover of the espresso machine and unplug the wire that goes to the positive side of the vibe pump.  On my machine there is a convenient spot where I could insert the dimmer switch (circled in the picture).  When I pulled those two cables apart I just plugged one cable into one end of the dimmer switch and the other cable into the other end of the dimmer switch.
   
3. I then attached the dimmer to the side of my espresso machine.  I still have to figure out a way to make it look nicer, but for now I just wanted to put it somewhere that was outside the case and in an area that won't get wet.
   
4. Test it and close up the case.

The Results

Since I performed the mod I have only made 3 double shots with a slightly reduced brew pressure but I can taste that the Bali coffee I am using tastes much sweeter, smoother and more fruity (it had quite an earthy taste before).

Really this mod is more of a hack to reducing pressure, as ideally a variable OPV would be used.  I have also noticed I can't adjust the pressure very accurately as a small movement in the dimmer switch results in large changes to the voltage received by the pump.  There is also a lot of discussion on home-barista.com and coffeegeek.com that reducing the voltage to the vibe pump could damage it and that vibe pumps do not really work at lower voltages due to how the internal mechanism of the pump work (this is probably why I am seeing large variations with small changes on the dimmer).

On the plus side, the mod is totally reversible, which is nice should I ever change my mind.