ADVANCED STRADA HUSBANDRY - RETURN TO THE LAND OF VST'S
We've been living with the Strada for 6 months now, so it seemed like a good time to revisit my previous post, The practical guide to Strada EP Husbandry.
On the whole, using the machine is a rewarding experience. Its still has the same niggly issues as before, but they still don't effect the quality of the coffee we serve.
what we have found, is that we've changed profile a few times, for various reasons, and recently gone back to VST baskets.
Playing with Profiles
The last post ended with us running this profile:
15s ramp up from 0-12 bar, 15s ramp down from 12-0 bar
Pretty simple, and we found that it really emphasised the sweetness in overly bright and acidic coffees. At some point during the following months, we found that this really wasn't working for less bright, sweeter coffees, and moved to the previous profile with a longer pre-infusion and lower max pressure.
0-10 secs 3bar pre-wet,10-12 secs ramp to 8 bar, 12-32 secs 8 bar, 32-34 secs ramp to 0
so far this has been producing stable and consistent shots, but why is this easier now than it was before?
We've definitely found 8 bar easier to work with than 9 bar, that slight drop in max pressure seems to mitigate the acceleration at the end of the shot to some degree. I also believe that we are just more consistent, we are used now to the foibles of the machine and grinder, making them both easier to use.
I'd like to investigate as profile with a very short window of max pressure, with an elongated ramp down as the shot progresses. This should reduce the pressure as the puck density decreases.
VST - The Return.
Yup, we're back on VST baskets, and so far, so good. The shots are more consistent than before, with our wastage being on a par with the synesso baskets. Why is this, seeing as we had so much trouble last time?
I think its down to precision. Being precision baskets, this dictates that everything else you use must be equally precise in order to make the most of them.
Firstly I made sure we had a better tamper. Our previous RB tamp had a 57.9mm C-Flat base, our new one is a 58.15mm Flat base. The shape and the diameter both work better with the straight side of the VST basket, as opposed to the curved bottom of the Synesso.
Secondly, we changed the shower screen. This had a much, much bigger impact in my opinion.
We moved to IMS competition screens with 200um holes which give a vastly different dispersion pattern from the standard LM screens, take a look; IMS vs LM Shower screens
The 1st screen in the video is the IMS, notice how the construction is of one piece with holes that are kept away from the very edge. The flow is more uniform across the whole screen.
The 2nd screen is the standard LM, here the two piece construction make for a vastly different water flow. notice how the vast majority of the water flows centrally, but there is also a noticeable amount at the very edges of the screen.
Thorough flow modelling would be the only way to know for sure, but here is what I think is happening. Firstly the large volume of flow centrally is causing a higher than normal mount of channels in the middle of the puck. Used pucks regularly had a set of small channel holes right in the middle. At the same time the flow from the edge of the screen is creating extra channels around the edge of the basket, amplifying the problem.
It's my contention that both these causes of channelling are reduced using the IMS screens.
Why? my best guess would be that the more even water distribution is aiding an even expansion of the puck into the headspace, thus reducing the instance of channelling.
What I know is that the VST baskets have been significantly easier to use on the Strada since making this switch.
Random noise of the day!
The Strada make a lot of odd noises, let's focus on one of them.
That popping/grinding sound you here when you first engage the pump? That's cavitation, it's quite literally small air bubble stuck in the gears bursting and releasing their energy as sound and heat.
It's nothing to worry about, but it will reduce the life of the pump a little. It can be reduced by adjusting the mix valve for the water being fed into the pumps, but I'd advice a qualified LM engineer do this for you. Or you could move to a colder climate.