gscace wrote: ... In these experiments I purposely applied a great deal of pressure to the coffee cake early in the extraction (11.5 bar) in an attempt to extract more from the coffee earlier in the extraction than later. The effect of this was to produce bitter crema, compared to profiles in which the pressure was adjusted to lower values. ...

Way back, when I "discovered" that the Tea's OPV was adjustable, I had the same experience. It had been set to 11 bar max (the Italian standard for vibes), and produced around 10.5 on the gauge for ristretto or singles. These were uniformly bitter. The bitterness went away when I dialed it down to 9.25 bar blind, which gave about 8.5 to 9 bar during brewing. (ymmv depending on the quality of the OPV)

The effect is stable and repeatable for any given machine, basket and dose. It does change if one changes dose or basket to give lots of headroom to the puck. The extra headroom acts similar to preinfusion, and somewhat ameliorates the harsh tastes one gets from too much pressure. How dramatic this is depends on the machine -- not as big E61s, which preinfuse in any case, and where the effect is to go from decent to good shots. On machines with low dwell times like the Silvia or Elektra, head space is a huge variable, taking shots from undrinkable to very tasty. If you want to use 18 or 19 gram doses with these machines, the taste will be a lot better if you use the extra deep LM triple rather than a standard double.

Mr. Schulman,

I can relate to your remarks in that during my own (limited) experience I have seemed to enjoy the under dosed shots in my Gaggia basket more (which seems to leave a medium level of headroom) over those that I dosed to the point they contacted the screen after swelling.

Of course to get more headroom I have to use a light dose, I'm very tempted now to find a nice triple basket (recommend one?) and try my standard 14g dose in it. Thank you very much!

TimEggers wrote:Mr. Schulman,

I can relate to your remarks in that during my own (limited) experience I have seemed to enjoy the under dosed shots in my Gaggia basket more (which seems to leave a medium level of headroom) over those that I dosed to the point they contacted the screen after swelling.

Of course to get more headroom I have to use a light dose, I'm very tempted now to find a nice triple basket (recommend one?) and try my standard 14g dose in it. Thank you very much!

The problem with triple baskets is that they're too deep to fit in non-bottomless PFs. So you'll need both. Espressoparts sells every basket under the sun, including the triples. You should try an LM double while you are making the order.

With a triple, you should be able to get in up to around 17 to 18 grams and still have lots of headspace.

Hi Again Tim -

Should you get around to measuring the brew pressure on your Gaggia, be SURE and do it with some controlled flow from the portafilter either by means of a needle valve (preferred) a la Bob Roseman or leakage from the portafilter itself.

The Gaggia Coffee is equipped with a spring loaded ball-check valve in the grouphead which also serves to substantially drop the pressure to the puck. See the parts illustration of your machine on this site and you will see the valve assembly I am referring to:

http://www.partsguru.com/default.html

This spring loaded ball-check valve is necessary to hold boiler pressure while in the steaming mode and prevent any leakage through the group. Silvia gets up in the range of 3.0 bar (steam mode) and I ASSUME the Gaggia does ABOUT the same.

So after reading this thread it confirms what I have been wondering all along. I do think the pressure varies without the OPV and its been a tossup for the last year trying to dial in the procon pump

My machine did not come with an OPV, I was told this is how the "italian" version came and an OPV is merely for safety reasons, however when trying to measure with my portafilter gauge (which has the needle valve) it fluctuates all over the place unless the machine is cold (not very long)

Can someone help me locate a OPV and connecting T/tube to drip tray for a rancilio S26, the newer S-24/26/27s all have this fitting. Also I really dont mind buying used equipment or trying other solutions as long as the parts are good.

Here is a pic of the OPV (circled) and directly to the right the connecting 3-way "T", Directly to the Left of the circled OPV is the copper tube that runs to the drip tray

Here is a pic of my machine without the parts

bjax wrote:Can someone help me locate a OPV and connecting T/tube to drip tray for a rancilio S26, the newer S-24/26/27s all have this fitting.

This thread only applies to vibratory pumps, not rotary pumps. Rotary pumps do have an over-pressure valve, but it isn't used to regulate brew pressure, it's used as a safety in the event the pump is set too high or for hot water expansion (hence why it's often referred to as an "expansion valve" in this context, although it's the same over-pressure valve).

The pressure adjusting screw for a Procon rotary pump is shown below:


From Procon exploded view; also see Adjusting espresso machine's rotary pump brew pressure?

If you are seeing varying brew pressure, check the inlet pressure. If the inlet pressure varies, the brew pressure will vary too. You will need a pressure regulator if the inlet pressure is too high (20 to 25 PSI is typically recommended).

so is the opv needed then, why didnt my machine come with one?

Err, I already explained why in my prior post, but I will try again...

I don't know the hydraulics schematics of your machine, but it's certain to have a pressure release mechanism. All espresso machines need some form of water expansion relief, otherwise the pipes would split or the boiler would crack the first time it was powered up. As I said before, a rotary pump has no need for an over-pressure valve for regulating the pump output pressure since that's done by the pump's bypass valve.