donderdag 20 maart 2014

Stephen Sweeney's TPF - Transparent PortaFilter

Stephen Sweeney from Seattle enjoys some fame in the espresso world for a number of custom made modifications he has made: transparent acrylic super-magnetic bases for baskets and portafilter holders for the HG One grinder to fit the portafilters for Londinium, La Pavoni and other machines, for instance. And, for the HG One and other machines, beautiful wooden handles.

Fewer people know that he is also a designer and builder of canons. Real working scale models like this 1867 Whitworth 40mm rifled breech loader:

He has taken this canon and others to competitions where the builders got a chance to demonstrate the functional precision of their cannons, hitting objects far away in the distance. When he was on his way to these competitions, tailgaters (people following too closely) were pretty shocked to find themselves staring down the barrel of a large cannon!

He also builds custom kayaks:

So you will not be surprised I placed my hopes on Stephen to make me a unique tool to study the invisible: whatever is happening inside the metal filter locked in the portafilter during espresso extraction.

I got the idea from two short and crude clips published by La Spaziale:

It surprised me that so far there are just these two clips, but after corresponding with Stephen and getting the updates on his first TPF model and the next one, I got a better understanding of the problems that arise when building such a one-off tool.

One needs to find a way to replace both the wall of the portafilter and the basket itself with something transparent, but it needs to hold the metal bottom of a normal filter basket. Also, the top should lock in securely to the brew group. Then, the combination of very different materials (metal, acrylic, sealant) must be able to all hold together in temperatures rising and falling very quickly from room temperature to temperatures close to the temperature of boiling water, and huge pressure differences from the pressure in the room (on the bottom) to 8 or 9 bar (at the top). Plus, a coffee puck tamped inside, air and water flowing around...

I can now imagine how the Spaziale team would maybe have felt the urge to push further and use their TPF more, but it's possible that it didn't hold that long and the manager, looking at the cost in time and materials, likely veto'd any further exploits.

Stephens final TPF held out during a few tests in his workshop and it stayed in one piece during fifteen tests Roemer Overdiep and I did in Amsterdam on the Londinium I lever machine, but then it too began to give. One side is starting to pop open and from what I heard from Stephen, you do not want to be standing very close, peering at the looking glass, when it pops open en explodes hot water and coffee grinds in your face.

Before we started documenting these TPF experiments, we rented a Sony 4K High Speed camcorder. It fits the Canon prime lenses we have available so we would not miss any detail.

Roemer is working on the footage and although editing it all will take some time, he expects to be able to send me a first clip within a day or two.

From what I have seen on the small monitor I can promise you it's mighty spectacular. The slow storm raging inside the TPF is beautiful to behold.

We also learned something we would maybe not have found out without the TPF: when using a tamper that fits very tight, it's best to tamp lightly. After finishing a firm tamp and pulling back the tamper, the puck can be pulled loose along the sides and this results in the puck jumping up and being slammed down again when the spring lever is being pulled. In a pump machine the puck will not jump up of course, but the seal along the outside of the puck will allow some initial flow.

The "puck jump" happens very fast but is very clearly seen when we use the super slow motion feature of the camera. It hardly causes a problem but to get a nice and even flow when using a very tight fitting tamper, tamping lightly seems advised.

When we used the tamper that fits my millennium La Pavoni Europiccola, that proved a very nice fit for Stephens TPF and the puck remained still, allowing the raised piston to suck in air through the puck instead of along the sides. Sadly, by the time we figured that out, the life cycle of the TPF was nearing the end and we couldn't risk damaging the $10.000 camera...

For a short moment (elongated in the footage) there's a lot more coffee flying around in the TPF than I imagined there would be, but very swiftly, once the pressure is applied, all clears up and you can see the tiniest stream of water along the grains of coffee grinds and air and CO2 struggling to get out of the way.

We cannot thank Stephen Sweeney enough for making this little project possible!

Watch this space for more.

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