I've been playing with the volumetrics on my Aurelia — I never let the baristas use these for anything except flushing — in an effort to see whether overall shot consistency is helped, and also whether the use of volumetrics speeds dial-in (which is complicated by a timed-dose grinder).
Initially, my tests were going well: shot weights (yes, weighing the shots as they pull on a 0.1g scale) were consistent +/- 1-2g, and taste in the cup good, despite the shots "finishing" in a widely divergent range of times (anywhere from 26-31 seconds, based on what I was trying to get). Things looked promising, though the time differences nagged at me a bit.
But, a second set of tests was very concerning: If I placed an empty vessel beneath an empty (i.e. no coffee) PF and pushed the volumetric button, then weighed the result, I would get DRASTICALLY different amounts (weights) of water out from "shot" to "shot". Like up to 12 grams different.
So, my questions: Has anyone else done this little experiment on a volumetric machine? Did your results resemble mine? Does the absence of coffee, and thus the speed with which water flows out, have a negative effect on the volumetrics' accuracy?
And also, who out there has gone back to volumetrics as a method of more consistently tasty espresso, rather than simple efficiency?
I haven't done that exact test with just water, though I have done quite a bit of measuring water amounts from volumetric machines. It surprises me that you're seeing 12g variation. 12mL is around 3/8oz worth of difference. I haven't seen variations that large.
There might be some aspect of your procedure that is introducing variation when you pull water shots. For example, I don't use a portafilter when measuring water output because it can hold a little water and introduce variation. Try removing the portafilter from the equation and see what happens. There's also that residual water that can hang out briefly in the SIS chamber (see related thread for more detail on this) and drip out over the span of several seconds. Try waiting the same amount of time between water shots as you did between espresso shots and see if the variation disappears.
All that said, even if you did note substantial variation when pulling straight water shots, I'm not sure you really care. You really only need the system to work properly when you are pulling an espresso, and it looks like it is.
The time variation is cause for concern, though I don't think this is due to an issue with the volumetrics. My leading suspect there is the grinder. How consistently is it dosing? Try your test shots again, but this time weigh your portafilter+dose before each shot and see if the time variation tracks with dose variation. I suspect it will.
A barista running a volumetric machine with a consistent dose of ground espresso and consistent prep shouldn't have any trouble delivering shots within a 3 second window.
We ran the programmed buttons for consistency and quality, not convenience.
While I cannot speak to the construction of the Aurelia specifically, I can say that your findings in the second series of tests are correct, but should in no way affect your volumetric's accuracy during brewing.
The flow meters I am familiar with (La Marzocco) are designed to operate within a designed flow rate range. Ideally we want the flow meter to be as accurate as possible in the flow rate range of espresso brewing. The additional cost of a flow meter that would be accurate when such accuracy is not required (periods of group flushing) simply is not needed, which means some variation will occur. If you have any other questions, please feel free to send me a direct message.
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