If for some reason you were required to work with an existing piston, like the Peppina's, with a single o-ring at the end of it, and yet you wanted to implement this dual o-ring design, you could place an o-ring in a channel running around the inside wall of the piston housing, as the Caravel does. This seal prevents water from entering the piston housing above the ingress ports. It is under significantly less pressure than the o-ring at the business end of the piston head.
Regards
Timo


P.S. I'm superimposing several 'views' in this quick-and-dirty drawing. You're seeing a cross-section at the upper o-ring and a frontal view on the bottom o-ring. The bottom o-ring is made of a new see-through material Remember, the port-holes are in the wall of the piston-housing.

Peppina has a canal connecting piston cylinder to group. A pliable one-way washer on the group prevents backflow from the puck into the canal. If the puck is too dense (too fine a grind/too heavy a tamp) the piston will be unable to push the water through the puck and the water will be under pressure. Just as blocking a running faucet with your thumb gets your shirt wet, if the Peppina is choked in this manner the PF will spew or sneeze if unlocked before the pressure dissipates. It's easier, of course, to choke a machine with an oily dark roast than with a lighter northern-Italian-style roast. There is no wait between shots when the grind and tamp are in the groove, and you can remove the PF immediately from the Peppina without a 'sneeze'.

ADDENDUM:
But this 'sneeze' (when it happens) is in the nature of things when pressure backs up. It all comes down to the force of the water versus the degree of impermeability of the puck-in-basket. The flow-rate through the puck-in-basket is the place to look, and the basket itself is implicated: water flows through some baskets more quickly than through others. If you have two baskets with the same diameter at the top and one of them tapers conically, water will flow through the conical more slowly than through the cylindrical basket because there are fewer egress holes in the base of the conical basket. A plugged cylindrical basket could present with similar symptoms. Then you'd need an espresso acupuncturist.

The sneeze does not affect the Peppina alone. Again, however, if you're not pushing the puck to extremes of density/dose, it shouldn't happen very often with the Peppina. Of the three machines I am using now, the one that is hardest to stall (i.e. the least likely to stall) is the Peppina. The Arrarex Caravel is the easiest to stall. The Cremina occupies the middle ground.

ADDENDUM2:
I've heard of people pumping the Peppina's lever up and down repeatedly as though it were attached to a car jack or air mattress. On a properly functioning Peppina, you don't have to pump with multiple shallow motions, although you could do so as alternative way to get the preinfusion going. But doing so you run the risk of turning the Peppina into a sneeze machine. If you pull the lever all the way down in one smooth motion, you should get a full draught of water from the kettle. Allow the lever to rise and water should be pushed to the group. For preinfusion, you simply guide the lever half-way or two-thirds of the way on its first ascent, count off the duration, and then allow it to continue under its own power. A double takes two pulls. The Peppina's cylindrical basket is a double. The single basket is conical.

...
Regards
Timo

timo888 wrote:Is there a functional reason behind chrome-plating brass? Or is that purely aesthetic? If the latter, I'd like the group to be naked brass please.
Regards
Timo

Hello Tim,
That is a darn good question. Brass and copper by nature are ok for use in food processing. "However" if they become dirty or are left unused and damp they corrode, the chemical reaction forms salts of the copper and those may be toxic. ( Copper salts are used to make timber rot and bug proof ) Silver or gold are nice, gold does not tarnish. Chrome works and is highly wear resistant but is expensive. Nickle is a toxic substance and is no longer used for food service items since 1923 or so. Tin is the common copper plating for food service. It is applied modernly by electroplating but historically by hot painting or swabbing at its melting temperature; like soldering the whole piece. It is suggested that the "tinning" extend outward about a ¼" from the food contact area. Tin is also impervious to almost all chemicals used in the cleaning of food service items. It does wear over time but has been in use for centuries.
So, in answer to your question, yes you may have a lovely shiny all brass group if you keep it spotless{ or tin plate the inside and top}. Many of the brass groups were "tinned" on the inside surfaces. ( See: http://www.home-barista.com/espresso-machines/cutaway-of-la-marzocco-saturated-grouphead-t700.html the cutaway of the Marzocco . That shows internal tin plating. )
Regards
Espressme

timo888 wrote:Mathias,

In the design I have in mind, the one-way valve assembly (I call it the 'group cap') is a threaded cylinder with a plate which contains the array of egress ports and the valve(s). The group cap connects the bottom of the piston cylinder to the top of the group cylinder, which protrudes into the boiler. The group-cap is a fairly extensible design and it could accommodate other kinds of one-way valves. It's not clear to me where the one-way valve fits in your diagram and I'm not familiar with the All-Clad one-way valve. Could you describe it?

I am certainly a proponent of simplicity. The group should be very simple. It needs a cylinder that projects up into the boiler, and collar to bolt it to the underside of the boiler, and a cylinder on the bottom into which the PF locks. The water will be at the correct temperature and so we don't need anything fancy in the group. No thermosyphons etc etc. I would like to be able to make use of readily available OEM portafilters.

Regards
Timo

Hello Tim,
Here is a high tech/ low tech home-made "one-way" washer/valve used by modelers to cover valve ports in pulse jet engines. It has the advantage of "washer"simplicity and may be configured to valve multiple ports. It could be etched, or laser or water-jet cut in multiples for a very low cost. Please be sure to read the "Etch your own valves" link! Don't forget to think outside the box. Could there be: flappers inside an outer ring?, inner and outer ring with radial flappers like a wind mill? Two rings with flappers; inside and outside?
http://www.aardvark.co.nz/pjet/valvelife.shtml
Have fun!
Regards
Espressme

mathias wrote:Timo, do you know how to design the o-ring seating (diameter and tolerances)? I never found information on that.

Hello Mathias,
Here is a company's info I've used for years:
http://www.precisionassoc.com/contribute/tcom.htm#
go to O-Ring Design Dimensions and click the links there. You will get printable Adode Reader documents that explain the factors needed for design of the grooves &c, also you will find on the link page info for deciding which "O" ring compounds. Go to their home page and find which compounds for food and medical use.
Have fun
regards
espressme

For the material of the piston o-rings, I would use EPDM NSF61-compliant for drinking water. The same material could be used for the one-way washer. It's good to 300 degrees F.

If NSF61-compliant washers would be acceptable nowadays to the various regulatory agencies that monitor food service equipment for food-safety compliancy, then they present the least expensive and simplest alternative. There are also bushing type valves like the one pictured below which comply with the European Pressure Equipment Directive of 2002 (not sure if that directive applies to domestic/commercial food/beverage equipment and appliances):



Are there advantages to the finned valves used for rockets other than inexpensive manufacture? Can they be made of stainless steel to minimize the galvanic corrosion issue? Would they be better and/or cheaper than NSF61-compliant EPDM (assuming it would pass regulatory muster)?

Could the group be made of stainless steel? What are the heat properties of stainless steel? What manufacturing processes would be required to make a simple cylindrical group, with a flange so it can be bolted to the bottom of the boiler, and with the requisite collar and grooves to lock a portafilter in? The group could be sophisticated in its utter simplicity rather than in its ingenious complexity. Nothing like an E61.

Regards
Timo

Nice find in that valve!! Keep it MR. Tim

On a different subject yet related to the choice of metals, we want to make sure the heating elements for the two boilers and their electrical connections cannot corrode or 'erode':

10 June 2006
http://www.conformity.com/current_newsbreaks.html

... Eugster/Frismag of Switzerland has recalled about 28,000 automatic espresso coffee makers. The company says that the electrical connectors in the espresso machine can erode and pose a fire hazard. Eugster/Frismag has received four reports of fires started inside the coffee maker, but no reports of injuries or property damage. The espresso machines were sold through department stores and independent specialty stores nationwide from July 1999 through October 2005 for about $800.

http://www.cpsc.gov/cpscpub/prerel/prhtml06/06106.html

timo888 wrote:Are there advantages to the finned valves used for rockets other than inexpensive manufacture? Can they be made of stainless steel to minimize the galvanic corrosion issue? Would they be better and/or cheaper than NSF61-compliant EPDM (assuming it would pass regulatory muster)?

Hi Tim,
All the usual disclaimers and IMHO
Yes, the valves could be made of shim ( spring temper) .010 ( .2mm ) 18-10 ( 304 in the USA.) Using water jet or etching could leave them absolutely flat. 18-8 and 18-10 are ok'd for food service. Don't lose that bushing valve tho!

If NSF61-compliant washers would be acceptable nowadays to the various regulatory agencies that monitor food service equipment for food-safety compliance, then they present the least expensive and simplest alternative. There are also bushing type valves like the one pictured below which comply with the European Pressure Equipment Directive of 2002 (not sure if that directive applies to domestic/commercial food/beverage equipment and appliances):

And, yes the washers could also be readily made from an acceptable silicone material that would take the heat. Many newer stove top gaskets are made from these materials and it is easy to cast into a simple one sided mold or a two part one. Also there is a injection moldable material available. They could even be stiffened by the stainless steel shim cast into them or, the elastomer ( rubbery material ) valve seals could be cast into the SS shim plates.

Could the group be made of stainless steel? What are the heat properties of stainless steel?

Hi Tim,
For the simplistic answer, conductivity of Stainless steel is about .17, and that of Aluminum is about .344, that of brass/copper is about 1.041 and that of silver is about 1.1. That means the metal will conduct/lose heat at that speed/rate. So, a metal is chosen for its ability to hold or lose heat for each application. For food service Stainless steel is very good for holding heat and real poor for a quick start-up to a stable temperature.

Enough for this post, All the usual disclaimers and IMHO
regards
Espressme