What is a volumetric espresso machine?

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Joco

#1: Post by Joco »

Recently I have come across some articles and studies that are saying that volumetric machines are "more consistent". But this got me thinking... what exactly is a volumetric machine?

http://www.lamarzoccousa.com/blog/ben-k ... lumetrics/
http://mattperger.com/Man-vs-Volumetrics

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Peppersass
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#2: Post by Peppersass »

A volumetric machine is one that can dose a programmed measure of water when pulling a shot. The barista pushes a programmed brew button, and the shot runs until the machine detects that the programmed amount of water has been dispensed. Then the brew cycle is terminated.

The volumetric feature utilizes a flow meter that monitors the amount of water being dispensed. To program it, you run a test shot while the machine monitors the flow. You can then just push a button to duplicate the amount of water dispensed for subsequent shots.

Lots of commercial machines, such as the La Marzocco Linea PB, have volumetric capability. Among pro-sumer machines, La Marzocco has a volumetric version of the GS/3 (the MP or paddle version is not volumetric.) The La Spaziale Vivaldi I & II and the Dream T are capable of volumetric operation, too.

Kaminsky found that in cafe settings, target brew ratios and extraction yields were more consistent when the shot was cut automatically after a certain volume of water was dispensed, versus cutting the shot by time and/or by by eyeing the consistency and color of the stream. Although the flow meters used in espresso machines aren't super accurate or consistent, evidently they're more consistent than the average barista.

Personally, I believe the best method is to use a scale to weigh the beverage and to eyeball the flow. Kaminsky didn't test that method, most likely because it isn't practical in a cafe setting. I'm still waiting for the equipment folks to give us an espresso machine with a built-in shot scale that can stop the brew cycle when the target weight is reached!

Joco (original poster)

#3: Post by Joco (original poster) »

Thanks! But one question. Why can't the barista just watch the extraction and cut it off at the proper volume? Ya know like using a measured shot glass or something. Wouldn't that be the same thing?

malling

#4: Post by malling »

That's impossible to do precisely in a ceramic espresso cup, and even harder to do In a bigger cappuccino cup.

Volume isn't the way forward, shooting by weight is.
I'm still waiting for the equipment folks to give us an espresso machine with a built-in shot scale that can stop the brew cycle when the target weight is reached!
If I remember correctly, such a machine has already been on show a year or so ago.

jonr

#5: Post by jonr »

Peppersass wrote: I'm still waiting for the equipment folks to give us an espresso machine with a built-in shot scale that can stop the brew cycle when the target weight is reached!
My modified Gaggia Classic does this. It not only stops at the right weight but it also adjusts flow during the shot so time also comes out about right (it also does preinfusion and tapers off flow near the end of the shot).

It works well; not sure why it isn't more common.

Joco (original poster)

#6: Post by Joco (original poster) »

Ah gotcha. Yea obviously shooting by weight would be better (I actually have seen some cafe's do this with a scale on the drip tray), but I was thinking perhaps eyeballing volume would work too.

Nate42

#7: Post by Nate42 »

I would think that provided a volumetric system measures flow of water going in to the puck (as opposed to flow of espresso out) then the end result would be roughly the same as a system that stopped by weight. The density of water at espresso temps is a known thing, and the coffee solids are going to add all of about 3%, basically noise.

Weighing shots is a useful thing for us home baristas to do, especially when dialing something in for the first time, because the presence of crema makes the volume of the finished shot an unreliable measure. But provided you're accurately measuring the volume of water going in, there shouldn't be anything unreliable about that.

jonr

#8: Post by jonr »

The coffee absorbs a considerable amount of water, so the water volume before and after the portafilter aren't at all the same. But if coffee dose and compaction remain the same, the difference might be pretty constant. And it's not clear to me which one (volume in or weight out) is actually the more consistent determinant of taste.

malling

#9: Post by malling »

coffee do more then just absorbe the water, it restricts the flow in such a way that it is also functioning as a backflush disk with a tiny hole, so a small portion of the water goes back into the driptray.

In fact the Volumetric dosing machine I have been using are measuring a whole latte cup just to brew 28g Of espresso!

DanoM

#10: Post by DanoM »

Actually, when you set up your volumetric dose of espresso you should do it using a mini scale on the drip tray to cut your extraction at the right weight. Once you've done that if the coffee is dosed the same, tamped similar, temps are similar you should be getting the same weight into the cup everytime (within an acceptable margin). The backpressure of the puck isn't causing a dump of any liquid into the drip tray until the shot is complete and the 3-way valve gets opened, so that should be MOSTLY irrelevant.

I've set these before and even months later with a totally different coffee the espresso was the same weight, within 0.2g, of the original setting. Whether your system is as accurate will depend on it's parts and controllers, but I think the commercial volumetric machines should be pretty accurate to the cup.
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