Evaluating what Michael's spacer and various shower screens do to coffee

1. Introduction

In November 2019, Stéphane Ribes conducted several experiments to evaluate the impact Michael's 3.8mm brass spacer has on coffee made with a DE1PRO v1.1 espresso machine from Decent Espresso. A spacer is a device installed between the group-head shower and the water-diffusion screen, usually used to reduce head space (the distance between the upper surface of the puck and the shower screen).

The study, which can be downloaded here, examined whether changing the position of the shower screen—and therefore reducing head space—would influence pressure behavior, extraction uniformity, extraction yield, and sensory performance.

The spacer was not tested in isolation. Instead, Ribes compared four shower-screen configurations representing different degrees of protrusion into the basket and different puck-to-screen distances.

The four configurations were as follows:

All configurations were tested with a 15 g dose. The IMS CI 35 WM plus spacer created the greatest intrusion into the basket and therefore the most restricted headspace.

The central hypothesis is that reducing the puck-to-screen distance (aka headspace) might change the hydraulic behavior above the puck, produce a gentler pressure decline following peak pressure, and potentially improve extraction uniformity and cup quality.

2. Questions asked

The following questions can be inferred from Stéphane's presentation:

1. Does installing the 3.8 mm brass spacer—or using a shower screen that similarly reduces headspace—change the pressure decline after the maximum extraction-pressure peak?

2. Does reducing head space improve extraction uniformity, as judged visually from the bottomless-portafilter extraction and the spent puck?

3. How does the spacer or altered shower-screen geometry affect extraction yield?

4. How do the different spacer/screen configurations affect sensory attributes and overall taste score at approximately 6–7 bar and 9–10 bar extraction pressures?

5. Which of the four tested configurations appears to provide the best overall compromise between hydraulic behavior, extraction uniformity, extraction yield, and taste?

3. Method

The tests were carried out on a Decent Espresso Machine DE1PRO v1.1 fitted with a red Cafelat 8.0 mm silicone gasket.

A 15 g VST basket was used, each time with a 15 g dose.

Coffee was ground with a Mahlkönig EK43 S fitted with SSP “High Uniformity” burrs with Silver Knight coating.

The coffee used was Finca Los Cotuzos, a naturally processed bean from El Salvador, which was roasted on 30 October 2019.

The experiment used Montille water from Le Mont-Dore, France, adjusted to 50 ppm as CaCO₃ alkalinity and 100 ppm as CaCO₃ total hardness. The mineral composition was adjusted using sodium carbonate and Epsom salts.

Before every shot, both the basket and shower screen were thoroughly dried with a clean tissue.

The beans were ground into a double-wall stainless-steel cup. Distribution consisted of WDT in the basket with a Londinium tool and a Decent funnel, with no tapping afterward.

Tamping was performed using The Force Tamper with a 58.5 mm flat base, applied twice consecutively.

The profile was described as a mixture of a Londinium profile and a blooming shot. Its low-pressure bloom was specifically intended to maintain a relatively constant air-pocket volume above the puck and thereby limit additional damage to the puck surface when water flow subsequently increased.

Here's what the four programmed stages look like:

Thus, the profile intentionally separates rapid initial wetting from a pressure-controlled bloom and then raises flow progressively before maintaining a fixed flow.

TDS was then measured with an Atago PAL refractometer zeroed with SCAA water. Coffee samples were not additionally filtered. Samples were measured at room temperature after thorough agitation.

4. Proceeding

The four configurations of shower screens and spacers tested were as follows:

1. standard IMS CI 200 IM with no spacer;

2. IMS SI 200 IM with no spacer;

3. IMS CI 200 IM plus the 3.8 mm brass spacer;

4. IMS CI 35 WM plus the spacer.

The IMS CI 200 IM screen used was the standard screen included with the Decent Espresso machine. So it being tested without the spacer provided the baseline. The latter three configurations progressively moved the shower screen toward the puck, with the CI 35 WM plus spacer creating the smallest available head space.

Coffee was brewed in all configurations and the resulting espresso, basket/puck surface, shower screen, and knocked-out puck were documented.

First pressure comparison: similar maximum pressure

In the first round of experiments, the dose, extraction parameters, and workflow were held constant at 15 g. When the IMS CI 200 IM screen was used with the spacer, the grinder was tightened slightly, from EK1.3 to EK1.25, so that a similar maximum extraction pressure could be reached.

The primary measurement was the percentage pressure decline following the maximum pressure peak.

Second pressure comparison: identical grind setting

In the second round of experiments, the grinder was fixed at EK1.4, while maintaining the same extraction parameters and workflow.

The exception was the IMS CI 35 WM screen plus spacer. Because this arrangement placed the shower screen especially close to the puck, the dose had to be reduced from 15.0 g to 14.5 g to prevent direct puck-to-screen contact.

Pressure-level comparison

For each of the configurations, shots were brewed and compared at:

6–7 bar

9–10 bar

The following sensory and visual comparisons were then made:

visual extraction uniformity from the bottomless portafilter;

visual uniformity of the spent puck;

extraction yield;

body;

acidity;

sweetness;

flavor intensity;

bitterness;

astringency;

overall taste score.

Results

5.1. Pressure decline after peak pressure

The first round of experiments found that with either the spacer or the IMS SI 200 IM screen installed, the pressure decline after maximum pressure was gentler.

The first comparison, in which maximum pressure was approximately matched, produced the following results:

The standard CI 200 IM arrangement therefore produced the steepest decline, losing roughly half of its peak pressure. Adding the spacer reduced that decline by approximately six percentage points.

The IMS SI 200 IM without a spacer behaved similarly to the CI 200 IM plus spacer, supporting the idea that screen position and head space—not merely the presence of a brass component—were responsible for much of the change.

The most extreme geometry, CI 35 WM plus spacer, produced by far the gentlest decline at approximately −30%, although its extraction yield in this test was also the lowest at 18.5%.

5.2. Same-grind comparison at EK1.4

At an identical EK1.4 grind setting, the same pattern appeared:

Again, the standard CI 200 IM produced the greatest pressure loss. The brass-spacer version and protruding IMS SI 200 IM screen both moderated the pressure decline substantially, while the CI 35 WM plus spacer once again produced the smallest drop.

Ribes thus noted again that less headspace, be it through a spacer or a shower screen, lead to a gentler pressure decline.

5.3. Experiments comparing the configurations at 6-7 bar vs 9-10 bar

Extraction yield at 6–7 and 9–10 bar

The combined extraction-yield chart gives the following values:

The results therefore do not show a general extraction-yield increase from adding the spacer. The conventional CI 200 IM arrangement actually produced the highest measured EY, approximately 19.5%, in the high-pressure condition.

The spacer's clearest benefit was instead in pressure behavior and visual extraction characteristics.

The CI 35 WM plus spacer is particularly revealing: despite producing the gentlest pressure decline, its extraction yield did not increase and actually fell to approximately 18.5% at 9–10 bar.

Visual extraction uniformity

Ribes separates visual uniformity into observations from the bottomless portafilter and the spent puck. Here's the charts showing his assessments:

And here's what the spent pucks looked like:

The standard IMS CI 200 IM configuration showed reasonable visual uniformity from the bottomless portafilter—particularly at 6–7 bar—but very poor visual puck uniformity at both pressure levels.

Adding the spacer to the CI 200 IM changed this markedly. Both the bottomless-portafilter assessment and the puck assessment moved toward the high-uniformity end of the graphical scales, and the 6–7 and 9–10 bar results were relatively close together.

The IMS SI 200 IM configuration also displayed comparatively high puck uniformity, although bottomless-portafilter uniformity was better at 6–7 bar than at 9–10 bar.

The CI 35 WM plus spacer behaved similarly: puck uniformity appeared strong at both pressures, while the bottomless-portafilter assessment was better at 6–7 bar than at 9–10 bar.

Taken together, these visual assessments support the hypothesis that reducing the distance between the screen and puck can improve the apparent uniformity and structural condition of the puck, even when extraction yield itself does not rise.

Taste assessment and taste scoring

Ribes then went ahead and evaluated the coffees made with each shower screen and spacer configuration for their body, acidity, sweetness, flavor, bitterness, astringency, and he also gave them an overall taste score. Here's the results:

Sensory results: standard IMS CI 200 IM

With the standard screen and no spacer, the sensory plot shows clear pressure-dependent differences.

At approximately 6–7 bar, the cup retained more acidity and flavor and exhibited somewhat less bitterness.

At 9–10 bar, the profile showed:

more body;

much lower acidity;

similar sweetness;

lower flavor intensity;

substantially more bitterness;

somewhat more astringency.

The overall taste score appears to be only about 2–3/10, with the lower-pressure version scoring slightly better than the higher-pressure shot.

Thus, the baseline screen configuration performed relatively poorly sensorially in this test.

Sensory results: IMS CI 200 IM plus spacer

Adding the brass spacer produced a substantially more balanced sensory pattern.

The differences between the 6–7 and 9–10 bar shots became comparatively small:

body remained moderate;

acidity remained moderate;

sweetness was low to moderate;

flavor was moderate;

bitterness remained low;

astringency was particularly low at 6–7 bar.

The total taste scores rise to approximately 4–4.5/10, noticeably above the unmodified CI 200 IM configuration.

This suggests that even though extraction yield was slightly reduced, cup balance improved.

Sensory results: IMS SI 200 IM

The protruding IMS SI 200 IM screen produced one of the strongest sensory outcomes in the experiment.

Its cups showed:

relatively high acidity;

relatively high sweetness, especially at 6–7 bar;

moderate flavor intensity;

extremely low bitterness;

extremely low astringency.

The overall taste score is approximately 5–5.5/10, substantially higher than the standard CI 200 IM and slightly higher than the CI 200 IM with spacer.

This reinforces the idea that the observed improvements were connected to screen geometry and reduced headspace, rather than being unique to the brass spacer itself.

Sensory results: IMS CI 35 WM plus spacer

The CI 35 WM plus spacer produced the greatest reduction in puck-to-screen distance and also yielded the highest plotted overall taste score.

At 6–7 bar, the cup shows approximately:

low-to-moderate body;

high acidity;

high sweetness;

relatively low flavor intensity;

almost no bitterness;

almost no astringency.

Its taste score is approximately 6/10, the highest individual result shown on the page.

At 9–10 bar, however, acidity and sweetness drop, while the overall taste score falls to approximately 4–4.5/10.

The configuration therefore appears to work particularly well at the lower 6–7 bar pressure range, rather than benefiting from a higher peak extraction pressure.

5.4. Overall comparison of all four shower screen and spacer configurations

The results reveal several important distinctions:

The experiment therefore separates two concepts that are often treated as equivalent: higher extraction yield and better extraction quality were not the same thing here.

The standard shower screen without space (baseline) generated the highest measured extraction yield, yet it showed the steepest post-peak pressure decline, the weakest puck-uniformity assessment, and the poorest taste results.

Conversely, configurations that reduced headspace produced smoother pressure behavior and generally better sensory scores even when their extraction yields were slightly lower.

6. Conclusion

Taken as a whole, Ribes's experiments suggest that reducing headspace can materially change the hydraulic and sensory behavior of a Decent espresso extraction, but the benefits are expressed more clearly in pressure stability, puck uniformity, and cup quality than in extraction yield alone.

Based on the results, we can now answer the questions brought up in chapter 2 as follows:

1: Does installing the 3.8 mm brass spacer—or using a shower screen that similarly reduces headspace—change the pressure decline after the maximum extraction-pressure peak?

Yes. This is the clearest result of the experiment. With the standard IMS CI 200 IM screen, pressure declined by roughly 44–51% following the peak. Adding the spacer reduced this to approximately 37–45%. The protruding IMS SI 200 IM produced a similar improvement without a spacer, with declines around 36–45%, while the IMS CI 35 WM plus spacer reduced the decline to approximately 30%.

The result therefore suggests that reduced headspace is a major explanatory variable, rather than the brass spacer itself having a unique effect.

2: Does reducing headspace improve extraction uniformity, as judged visually from the bottomless-portafilter extraction and the spent puck?

The visual evidence indicates that it does.

The standard CI 200 IM screen showed particularly weak puck uniformity. In contrast, the CI 200 IM plus spacer, IMS SI 200 IM, and CI 35 WM plus spacer all showed substantially stronger puck-uniformity assessments.

The photographs also show a noticeably fragmented knocked-out puck with the standard CI 200 IM configuration, while the other configurations produced more intact puck discs. Because this was evaluated visually rather than through a quantitative imaging method, the result should be regarded as suggestive rather than definitive.

3: How does the spacer or altered shower-screen geometry affect extraction yield?

It does not systematically increase extraction yield.

The highest individual result was approximately 19.5% EY with the standard CI 200 IM screen at 9–10 bar. The spacer version reached approximately 19.2%, the IMS SI 200 IM about 19.3%, and the CI 35 WM plus spacer ranged from approximately 18.5% to 19.0%.

The experiment therefore provides no evidence that reducing headspace automatically raises extraction yield.

Instead, it demonstrates that a slightly lower extraction yield can coexist with better pressure behavior and better sensory performance.

4: How do the different spacer/screen configurations affect sensory attributes and overall taste score at approximately 6–7 bar and 9–10 bar extraction pressures?

The standard CI 200 IM configuration produced the weakest overall taste scores and displayed substantially more bitterness at high pressure.

Adding the spacer improved balance and raised taste score to roughly 4–4.5/10.

The IMS SI 200 IM performed better still, reaching approximately 5–5.5/10, with high acidity and sweetness and very little bitterness or astringency.

The strongest individual result was the IMS CI 35 WM plus spacer at 6–7 bar, at approximately 6/10. It combined high acidity and sweetness with almost no bitterness or astringency.

Increasing that configuration to 9–10 bar did not improve the cup and instead reduced its sensory score.

5: Which of the four tested configurations appears to provide the best overall compromise between hydraulic behavior, extraction uniformity, extraction yield, and taste?

There is no single winner on every metric.

The IMS CI 35 WM plus spacer produced the gentlest pressure decline and the highest individual sensory score, particularly at 6–7 bar, but required a reduced 14.5 g dose to avoid puck contact and did not provide the highest extraction yield.

The IMS SI 200 IM without a spacer appears to offer the strongest practical compromise: it produced a similarly gentler pressure decline, good visual puck uniformity, extraction yield close to the baseline, low bitterness and astringency, and one of the highest taste scores.

The CI 200 IM plus brass spacer nevertheless demonstrated the spacer's effectiveness clearly: compared with the otherwise similar standard CI 200 IM arrangement, it reduced the post-peak pressure decline, improved visual puck behavior, reduced undesirable sensory intensity, and increased the overall taste score.

The central lesson of the experiments is therefore that shower-screen geometry and headspace can significantly influence espresso behavior even when conventional extraction-yield measurements change only modestly. The experiments suggest that reducing the air space above the puck can stabilize the extraction after peak pressure and potentially produce a more uniform and better-tasting espresso, but reducing headspace too far introduces practical constraints such as puck-screen contact and the need to lower the dose.

#research #Ribes #coffee #headspace #screen #spacer #documentation

Older:

mirjam created 2026/08/25, mirjam updated 2026/08/26