Volumetrics and Gravimetrics Explained
What are Volumetrics?
Volumetrics are a programmable system for creating predictable yield outcomes in espresso extraction. Internal flow meters manage water sent to the dispersion screen in pulses. Each pulse is a consistent volume of water. Once programmed, the espresso machine will send that volume of water into the ground and tamped espresso every time that button is activated. This programming is typically done with coffee in the portafilter in order to create more consistent outcomes.
So what are Gravimetrics?
Gravimetrics are also a programmable system for creating predictable yield outcomes - but instead of it being controlled by a flow meter, a built in scale on the drip tray triggers the computer to turn off the water flow when it gets to a programmed weight of yield. This system requires less programming - but it certainly does require certain workflow considerations. With every shot that is pulled, the drip tray scale must auto-tare (or reset to zero once the shot glass or cup is placed on it) so the timing of the cup delivery needs to be just right.
Both of these models create predictable yields. In doing so, they eliminate one variable from the Espresso Recipe. Instead of using Dose (grams) and Grind Size to control flow rate to hit a goal Yield (grams) in a desired Time (seconds), the Barista can use Dose and Grind Size to control a set Yield in a desired Elapsed Time.
If you’ve read up on my Extraction Models blog post or have spent any time in coffee in the last ten years, this chart will look very familiar to you.
In Volumetrics and Gravimetrics, we see a linear progression of flavor along this timeline when using a single dose. This allows a Barista to triangulate flavor at a single dose by adjusting grind size for time.
So when shopping for an espresso machine, there a lot of things to take into consideration. Here are a few to consider about Volumetric or Gravimetric machines.
By removing Yield as a variable, Baristas dial in faster.
When using the same Dose, finer grinds slow flow rate and extend Elapsed Time. Grinding coarser increases flow rate and reduces Elapsed time. When you throw most any coffee into the following model, you can see how much easier it is to create predictable tasting espressos all shift long.
Yields are consistent, so most beverages served have a very similar amount of espresso in them. This allows the Cafe to create idyllic ratios of coffee to milk, coffee to water, or espresso as a more consistent signature drink ingredient.
Also, adjusting for flavor development in a Volumetric or Gravimetric model is more linear. When using the same Dose, grinding finer slows flow rate and extends Elapsed Time. Grinding coarser increases flow rate and reduces Elapsed time. When you throw most any coffee into the following model, you can see how much easier it is to create predictable tasting espressos all shift long.
By removing Yield as a variable, there is some unlocked flavor potential that is ignored in this extraction model.
When Yield is no longer a variable, the Barista is limited by RATIO (yield:dose) when dialing in their coffee. This restriction is minor but it does reduce some of the opportunity of coffee flavor to develop.
The linear quality of this model assumes that the Barista will dial in to a single dose. When Dose is changed, it changes the ratio of yield:dose which has an affect on extraction. Lower ratios (ex: 1:1) tend to exhibit lower extraction percentage than higher ratios (ex: 2:1). So if a Barista expects to develop more flavor by up-dosing in order to slow flow rate and increase elapsed time - they often witness a frustrating blend of Ratio and Flow Rate cancelling out one another.
While I do believe all of the above, I think the biggest selling point of these models are that they take one variable out of the Barista’s work flow so that the Barista can invest their time being a host. When I don’t have to look down to turn off the water flow at a set Time or goal Yield, I am free to do the work of creating and keeping regulars. I’m convinced that there are multiple paths to success - this is but one path.
Yes, QSQ teaches Volumetric and Gravimetric modeling in the Espresso Flavor Modeling Courses. ;)