WotB wrote:
They have something to do with temperature stability I believe. Reduce the wild fluctuations in temperature with increased flow.
Not quite WOTB - but close. Thermal stability is a design and control area. The restrictor is fitted in the group head, so it's after the boiler and H/E. A flow restrictor is a small "jet" generally made of teflon (so scale won't stick to it) and it sits above the group head to govern the flow of hot water through the group. If you are looking down on top of the group head, you will notice a big bolt. The flow restrictor sits inside that area, e.g. the bolt must be removed for access - there is also a filter/screen unit in there as well. WARNING !!!!!!! Please be very careful if you embark upon removal and refit of flow restrictors. They are very small in diameter (like under a mm) and when you attempt to remove the bolt, all sorts of debris MAY fall into the opening and clog your group and/or block the jet completely. Yes, that means it could end in tears. It's a job for clean hands, clean working area and advice is to run the water at low pressure whilst you are removing and reinserting the jet, that way if any foreign debris happens to fall in the area, the water will push it out and avoid being lodged in there causing extra problems. Now, the purpose of the flow restrictor is to slow down the saturation and extraction of the coffee, creating a situation whereby additional (or extra) flavour can be extracted from the shot. Think of it this way, as the water passes through the puck, it can pickup more "stuff". Some balancing needs to be done to grind and dose, but overall I've experienced improved results from the machines I've had the mod applied to. Another experience is that not all "jets" are created equal. As they are mostly made of teflon, the jets can have even a minor difference in diameter (and we are talking here about thousands of an inch, or mm) this can have a dramatic effect on shot time/volume as this is controlled by turns of the flow meter, not amount of water passing through. As an example, 2 group machine with the same flow restrictors fitted, one was delivering 30ml and the other 40ml. A quick resize of the small jet with jewellers drill managed to bring tham back a little closer.
