Dear all,
I have recently renovated a '80ties (true) HX Aurora Brugnetti. The machine works like a charm, but I noticed an ugly crater in the puck after each subsequent pull. It appears that the crater is caused by a strong jet of water exiting the seal once the piston is moving upward. I have a video illustrating the situation (see below). Line pressure is set at 2.7 bar.
My question to the Aurora Brugnetti experts: since this machine is a true HX the pressure on the cylinder inlet port will be 2.7 bar. Since the inlet port is a small hole (3-4 mm?) I can understand that this produces a vigorous jet of water. It seems the machine is lacking a flow restrictor?
What do you think and is any of you experiencing something similar?
Cheers, TY

ps. similar issue, although with a dipper machine.
Lever group flow restrictor?

Have you taken the screen off or is it missing?

Hi Paul,
No, I have taken the screen off for illustrative purposes. With the screen on the jet blows through the shower killing my puck.
Your assistance is greatly appreciated.
Thanks, TY

Another HB member made this modification to correct that issue I believe.

Thanks, I have been looking at that mod. Interesting, but something to consider of all other options fail.
Cheers, TY

Hi TY,

Nice restoration!

Yes, I had the same jet/crater problem initially which requires a hard tamp. Anything softer and the puck will be 'trashed'. Thus the mod was done.

As you might have noticed, there is a stainless ball in the rear of the group head. That is a one-way valve to prevent back-flow and does nothing to regulate the flow. Without it, the lever catches at a higher angle - as a result, you get lower brew pressure and smaller shot volume.

I've tried adding a flow regulator(the type that you use for tap water) upstream of the inlet tap , but it alone doesn't work as well as the added dispersion screen. With both (dispersion screen+flow regulator), the flow is much better than even from a GS3. Can't see why that should be a last option.

Don't think it is possible to get better flow than this In the thread(linked by Dominico), I've recycled old materials I had on hand so it doesn't look that good (works wonder though). Yours is an as-new machine and so deserves an equally elegant solution. Andyone used one of these I believe and it should work well too.

http://www.cafeparts.com/Product.asp?ItemID=13137


https://www.kaffee-netz.de/threads/auro ... ti.100072/

Oh, another solution you can try since you have such a nice work shop.

Behind the group head, there is an unused hole for dipper tube (the thread may be 1/4" but not very sure). You can insert a blind cap there, with a drilled orifice (maybe around 0.6mm?). Even with this, I still think the dispersion screen will give you the best result.

Hi Sam,
Thanks for your response. Greatly appreciated. I have been admiring your thread and found it very inspirational.
In the case of the Aurora there is the matter of flow and pressure.

I think I do understand why a flow restrictor alone does not work. If the flow is reduced, the small brew chamber (welded on the boiler) is slowly filled with water. However, water can not escape since the pistonseals close the cylinder, thus pressure is builing up amounting to line pressure let's say 3 bar. Hence, the pressure at the inlet port is also 3 bar. When the piston moves up -exposing the inlet port-, a short powerfull (3 bar) jet of water will exit (killing the puck). I believe the solution is to reduce line pressure to approx 1 bar (similar as boiler pressure) and subsequently play with flow restriction.
What do you think?

Regards, TY

Hi Sam,
This is a great in-depth discussion. Moreover, I appreciate such a discussion taking into account the vast distance that is in between us (16.545,94 km).
Prior to my last post I have been considering the flow restriction in terms of a drilled orifice as well. However, giving this more thought, it will reduce flow but not pressure...thus not solving the problem.
What do you think?
Cheers, TY

Thanks for the kind words. It was nothing inspirational nor spectacular like yours over at koffiepraat.

The solution you proposed will definitely reduce the jetting effect, but I am not sure if it's reduced enough to completely eliminate the puck crater effect. Worth a try for sure.

As you have noted, it is a complex interaction between the flow rate, velocity, flow restrictor location, water pressure and pressure drop (can be a good thesis title,anyone? ). If you can place a flow restrictor in the water inlet hole in the group cylinder (where the jet is formed/observed), I am sure it will work. But it is not easy to carry out such mod.

I have had a long history dealing/battling with this type of water dispersion issue few years back. Olympia Cremina's coffee puck has a dent after replacing the seal?!

The Olympia Cremina had the same type of screen, the same type of inlet hole, and it had exactly the same jetting issue. The Cremina, like any other dipper levers, operates at circa 1 bar (0.8bar for my case if I recall correctly) but still had the jetting issue. So pressure is not the sole reason for craters. Though, the flow rate was unrestricted on the Cremina so a direct comparison can't be made.

So, I would say give your method a try (I would be curious about the result too). If you want a quick guaranteed solution, you will always have the option.

Looking past the jetting issue, I think it comes down to what preinfusion pressure you would like to achieve. The pressure in itself (3 bar) is not an issue if water dispersion is well thought out (clearly not on a stock HX Aurora).

I have tried first hand, and many has reported in a similar manner, that a Londinium 1-P makes an arguably better shot compared to a Londinium 1. The major difference between the two being the Londinium 1 preinfuses at boiler pressure (1bar) and the Londinium 1-P preinfuses at line pressure (3 bar). Even Reiss (of Londinium) is now updating the new Londinium 1 with capability to preinfuse at 3 bar.

So I think 3-bar preinfusion may and likely has its own merits. Higher shot volume being one (due to more saturated puck during preinfusion), requiring slightly finer grind also allows for a higher extraction rate especially for lighter roasts. Shot quality aside, I imagine too low of a line pressure (<1.2 bar) and you might encounter boiler-fill issue.