HomeRoast Digest


Topic: Silvia question - the pressure mod wothout one key (52 lines)
1) From: Greg Scace
Chris Beck,
Actually, I measured around 165 psi at the beginning of a 2 1/2 oz double, 
falling somewhat over the course of the pour.  You're right that the 
unregulated system finds its own equilibrium.
I can describe the mod to you and you can experiment if you like.  I don't 
know one key dimension yet, but really there's no reason you can't mess 
around with it.  Look at the over-pressure protection valve that is at the 
entrance to Silvia's boiler.  Notice that there is a return line going back 
to the water supply tank.  The valve design is really simple.  A spring 
loaded plunger is forced against a seat, preventing water from flowing thru 
the return line.  After sufficient boiler pressure is reached, the plunger 
is forced off of the seat, allowing flow back to the reservoir.  By 
reducing the spring pressure, you can drop the "cracking" pressure to 
something useful.  You can imagine, by looking at the valve body, that the 
current spring force is developed by screwing the "cap" onto the valve 
body, which compresses the spring.  If you install extra sealing gaskets, 
you'll reduce the spring pressure.  I got a bunch of gaskets from 1st Line, 
but I just haven't had time to open up the valve body, install some gaskets 
and measure how big the stack has to be to get 9 bars.
It's really a simple thing to do.  And I bet you really don't need a 
pressure gauge to improve on things.  Just listen to the pump when you're 
pouring a 2 1/2 oz double.  You know pretty well what the pump sounds like, 
I bet.  Then add some gaskets to the valve, lowering the spring seating 
pressure.  Stick a blind insert in your pf and turn on the 
machine.  Instead of stalling your pump, the valve should open and the pump 
should sound maybe a little less labored than it does when you're pulling a 
double shot.  Use your taste buds to decide what's right.  In my limited 
diddling with pressure control, I concluded that there was definitely a 
benefit to doing the control bit over the uncontrolled Silvia, but I wanted 
the complete picture so I could publish something more substantial on 
alt.coffee.  I haven't yet decided that I think 9 bars is the holy grail, 
and I don't yet know if brew pressure is a parameter that should be 
adjusted from blend to blend.  That is why I am waiting for the blind test 
bit to completely spew on this.
Feel free to experiment with this.  I think you can go a long way just by 
stacking gaskets in the over-pressure protection valve.
-Greg
PS - In case anyone's wondering, 9 bars is way bigger than the pressure 
developed when Silvia is steaming.  So you won't compromise steaming by 
performing this particular mutilation of your machine.
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