How to determine basket (espresso portafilter) volume?
The dosing of an espresso basket is a fundamental part of espresso preparation. With so many espresso baskets on the market, manufactured in varying shapes and sizes, knowing what these baskets should hold can only be calculated in conjunction with the machine which it is to be used. Even though some brands are pre-stamped with their weight, the truth is these measurements are of no value to us. The reason why, is due to coffee density.
When coffee is roasted it loses weight through both carbonization and moisture loss. Much like a piece of wood placed on a campfire, whilst heavy initially, the wood decreases in weight as it burns, until eventually it becomes coals. Obviously, the resulting coals will have a lower density than the logs initial weight, before being added to the fire.
At risk of using a crude simplistic analogy, the same can be applied to coffee during the roasting process. The initial green bean will weigh more than the roasted coffee bean, and just like the piece of wood, the longer coffee is roasted, the more it decreases in weight and density.
Because of varying coffee densities, it is impossible to say how much a basket holds unless the predetermined roasted coffee density (roast degree) is known. Ie. A full porta filter basket may hold 24 grams of a light roasted coffee, but the same basket filled with a dark roasted coffee may only weigh 20 grams. This illustrates that branding baskets by what grams they hold is not only misleading, but in truth, irrelevant. But if we don’t brand baskets with their weight how do we classify what a basket holds?
Basket WEIGHT vs Basket VOLUME
When preparing an espresso basket for dosing, our primary concern is to not exceed the clear space level. Exceeding this level will disturb the Tamped surface when fitting the porta filter and result in a poor extraction. Our aim is to sufficiently fill the basket leaving a void for the incoming water to occupy while it equalizes the airspace to pump pressure then starts to infiltrate to the coffee. As the water slowly infiltrates the prepared coffee and the Mailard reaction commences the coffee expands to inhabit the complete void, known as the extraction chamber.
While we will observe the weight of the coffee that we are loading to the basket for extraction, this is for calculative analysis in the retrospective. For ease of process, we may use weight as a reference point for repetitive process and to realize any sudden changes to process when applying this volume to the basket. But strictly speaking, we are not dosing by weight we are dosing by volume. And whilst you may think I’m splitting hairs here, I’m not. If your focus is on weight all your future studies will be based around weight. (and coffee weight can change from batch to batch) If instead you dose by volume, your future investigations will be based on volume. The difference in ideology is quite dramatic. (This will be discussed further in future articles)
So, if a manufacturer can’t tell you your basket volume, (and they can’t) how then do we determine what this volume is?
6 Steps to Determining Extraction Chamber (Basket) Volume:
1. Take your basket and stretch some cling wrap around the OUTSIDE of the basket and secure with an elastic band.
Step 1. Cling wrap around the outside of basket.
2. Put the basket on a scale, tare it off to zero and fill with water to the top. Do this quickly to avoid any leakage and therefore an incorrect measurement. Record this value as “Total Basket Volume”. In this example it’s 68.5ml.
Step 2. Measuring basket volume with water.
3. Discard the cling wrap, and make a coffee with the basket in your machine as per your normal method, being careful not to over or under pack the basket.
4. Take the basket out with the coffee puck still intact and place a film of cling wrap across the top of it and using the weight of your coffee tamper to lightly push the cling wrap into the negative space, across the puck surface. Do not disturb the puck. You may have to wait for puck to cool, before doing this.
Step 4. Preparation to determining head space volume.
5. Put basket on scale, tare it off and fill with water. This weighted water amount is your head space volume. It is the volume that the group head protrudes into your basket. Record this value as “Headspace Volume”. In this example, it’s 21.2ml
Step 5. Measuring Head Space Volume.
6. Take this headspace volume from the total basket volume you determined earlier. The amount you’re left with is the total extraction chamber volume. In our example 68.5ml – 21.2ml equals 47.3ml. Therefor Your extraction chamber volume is 47.3ml. Note; This volume can change depending on what machine the basket is used on. Another reason why stamped baskets are not relevant.
By using this basic process, you can check various baskets regardless of shape, and determine their extraction volume.
“But why use water?
Water is typically used as a calculation standard and is generally the reference for specific gravity calculations. With a Specific Gravity of 1 it becomes a simple calculation of 1gm = 1ml.
The conversion could not be simpler.”
For our next article, we shall be looking at basket shape and their influences.
Cheers,
Bush and Bush Coffee Systems
Glossary:
Clear Space: This is the space in the top of the prepared basket that allows clearance to the shower assembly when attaching the portafilter to the group head. Clearance is essential to prevent the disturbance of the tamped coffee surface. It is not a calculated space.
Total Basket Volume is the volume of a basket completely filled to the top lip.
Headspace is always referred to in Volume. It is the measured volume of fixed machinery (Shower Screen assembly and associated equipment) which inhabits the total basket volume. The Headspace Volume is measured once only and once known, remains a constant for that particular machine.
Extraction Chamber Volume is the Total Basket Volume less the Headspace Volume.