IT’S ALMOST A BLIND BASKET
A FIRST STEP TO BETTER UNDERSTAND A PRESSURE CURVE DURING
ESPRESSO
1
STÉPHANE RIBES –JUNE 2019
•Compression of the air pocket trapped above the coffee puck drives the pressure evolution during an
espresso shot
•Air compression is driven by 3 main parameters:
1. Initial available air volume above the puck (headspace)
2. Flow rate of input water (from the pump)
3. Exit water flow rate penetrating the coffee puck
IT’S ALMOST A BLIND BASKET
SUMMARY (1/3)
1
2
3
2
Coffee
Headspace
12 3
•Considering a perfect gas model, the following formula can be established:
•Experiments with a blind basket and the DE1PRO machine show acceptable validity of this formula 3
•P: absolute pressure (in bar) –P0: 1 bar = atmospheric pressure
•t: time (in seconds) –t0: the system becomes air tight due to the
presence of a continuous water bed above the puck
•HS 1: initial headspace (in cm3)
IT’S ALMOST A BLIND BASKET
SUMMARY (2/3)
3
•Vinput
2: total volume of water dispensed by the pumps
after t0(in cm3/s)
•Vexit: total volume of water that has flown through the
top surface of the puck
1 mL/s
2 mL/s
4 mL/s
1Headspace is often described as the distance from the
top of the puck to the shower screen –in this document it
represents the total air volume trapped above the coffee
puck, including between the screen and the brass diffuser
2For liquid water we assume 1 cm3= 1 mL
With a constant water flow, Vinput = Flow rate * (t - t0)
3For this experiment, the trigger to switch from the initial
flow rate to 0 mL/s was set to 7 bar
Input water flow rate:
•Main observation results:
1. The lower the initial headspace the sooner and quicker the pressure increases
2. The higher the input water flow rate the sooner and quicker the pressure increases
3. Penetration of water into the coffee puck delays the pressure rise; with slow preinfusions
(e.g. 2.5 mL/s) this effect is barely influenced by the grind size
IT’S ALMOST A BLIND BASKET
SUMMARY (3/3)
1
2
3
0 s
5 s
10 s
15 s
20 s
25 s
30 s
35 s
0 5 10 15 20 25 30 35 40 45
HEADSPACE (cm3)
EFFECT OF HEADSPACE
Time to reach 7 bar (s) @ 1 mL/s
0 s
5 s
10 s
15 s
20 s
25 s
30 s
35 s
0 0 , 5 1 1 , 5 2 2 , 5 3 3 , 5 4 4 , 5
WATER FLOW RATE (mL/s)
EFFECT OF WATER FLOW
Time to reach 7 bar (s)
0 s
2 s
4 s
6 s
8 s
10 s
0 b
2 b
4 b
6 b
8 b
10 b
12 b
14 b
0 , 9 0 1 , 0 0 1 , 1 0 1 , 2 0
EK43 SETTING
EFFECT OF GRIND SIZE
Max extr. P (bar) Add. time to 3 bar (vs blind basket)
1 2 3
DE1PRO v1.1 Decent Espresso machine & experiments with a blind basket 12 grams dose in a 15g VST basket
1 2 3
4
IT’S ALMOST A BLIND BASKET
EFFECT OF HEADSPACE
5
Blind basket
est. headspace: 36 cm3
Blind basket + 10 mL of water
est. headspace: 26 cm3
Blind basket + 26 mL of water
est. headspace: 10 cm3
Blind basket + 20 mL of water
est. headspace: 16 cm3
DE1PRO v1.1 –Blind basket
Lower headspace values obtained with
additions of water into the blind basket
0 s
5 s
10 s
15 s
20 s
25 s
30 s
35 s
0 5 10 15 20 25 30 35 40 45
HEADSPACE (cm3)
EFFECT OF HEADSPACE
Time to reach 7 bar (s) @ 1 mL/s
1
•More pressure overshoot as the
input water flow rate increases
IT’S ALMOST A BLIND BASKET
EFFECT OF WATER FLOW RATE
2
6
1
1.0 mL/s 1.5 mL/s
4.0 mL/s2.0 mL/s
1
DE1PRO v1.1 –Blind basket
For this experiment, the trigger to switch
from the initial flow rate to 0 mL/s was set
to 7 bar
0 s
5 s
10 s
15 s
20 s
25 s
30 s
35 s
0 0 , 5 1 1 , 5 2 2 , 5 3 3 , 5 4 4 , 5
WATER FLOW RATE (mL/s)
EFFECT OF WATER FLOW
Time to reach 7 bar (s)
•With an overly fine grind the
pressure curve looks similar to the
blind basket curve (light green) with
the same headspace –the delay
before the pressure rise (in this
example 4.5 s to reach 3 bar) is
caused by the “exit” water flow that
penetrates the coffee puck
•Changing the grind setting to cover
a wide range of extraction
pressures does not significantly
modify the delay before the
pressure rise
IT’S ALMOST A BLIND BASKET
EFFECT OF GRIND SIZE
7
1
2
EK0.9 EK1.0
EK1.2EK1.1
+4.5 seconds
+5.0 seconds
1
2
DE1PRO v1.1 –IMS SI 200 IM shower screen
12 grams dose in a 15g VST basket
No paper filter, no hog tool
3
•With the same coffee dose and
grind setting, an increase in the
basket size (20g to 22g = +2 mm
height) creates more headspace
that induces additional delay to
start building up pressure
•The later pressure rise induces a
lower maximum extraction pressure
IT’S ALMOST A BLIND BASKET
EXAMPLE 1: BASKET SIZE
8
1
2
VST 22g
(EK1.8)
VST 20g
(EK1.8)
DE1PRO v1.1 –IMS SI 200 IM shower screen
No paper filter, no hog tool
Lower extraction
pressure 2
+5.0 seconds +7.5 seconds
1
18g dose 18g dose
VST 20g VST 22g
EY 19.9% EY 19.9%
•The change of shower screen has
almost no impact on headspace,
thus with the same dose the
additional delay to build pressure is
roughly unchanged
•The gentler pressure decrease with
the SI 200 screen may be related
to the shorter distance of the
shower screen to the top surface of
the puck (additional tests required
to confirm this hypothesis)
IT’S ALMOST A BLIND BASKET
EXAMPLE 2: SHOWER SCREEN (1/2)
9
1
2
IMS SI 200
EK1.5
IMS CI 200
EK1.4
DE1PRO v1.1
15 grams dose in a 15g VST basket
Grind setting adapted to reach the same
extraction pressure
No paper filter, no hog tool
15g dose
IMS CI 200 IM IMS SI 200 IM
15g dose
+6.0 seconds +5.5 seconds
1
EY 20.4% EY 21.3%
2
•Switch to the SI 200 screen and
increase the basket size to maintain
an almost constant distance from the
screen to the puck surface induces a
headspace increase that adds more
delay to the pressure rise
•The drop in the maximum pressure
reached during extraction (with
same dose and grind size) is the
result of both higher headspace
and larger hole size of the
VST 20g basket
IT’S ALMOST A BLIND BASKET
EXAMPLE 3: SHOWER SCREEN (2/2)
10
1
IMS SI 200
EK1.8
IMS CI 200
EK1.8
18g dose
IMS SI 200 IM –VST 20gIMS CI 200 IM –VST 15g
18g dose
DE1PRO v1.1
No paper filter, no hog tool
14 seconds to
reach 2 bar
1
16 seconds to
reach 2 bar
2
2
