The Struggle for a Steady Kettle Flow

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Recently, I started attempting to modify my Brewista Artisan 0.6L gooseneck kettle to reduce its maximum flow rate and make it easier to keep a constant kettle flow of about 5 grams per second during my brews.

The Brewista Artisan 0.6L has a gooseneck with an inner opening diameter of 5.5 mm at the exit spout, which results in a maximum flow rate of about 24 g/s. This is a bit more than I would like, as I rarely ever need to pour that fast, and this makes it a bit harder to pour consistently at only 20% of the maximum flow rate. I asked someone on Instagram who has a Fellow Stagg EKG 0.6L to measure its maximum flow rate, and it is apparently a bit lower, at 20 g/s. This is quite nice, and adds another reason why I would have chosen the latter kettle if I was to do this again. Other reasons include not having to press re-heat again every time I pick up the kettle; the nice counter-weight in the handle which makes it easier to pour consistently; and the much better and more responsive customer service of Fellow versus Brewista.

If you wonder why I’m worrying about kettle flow speed, the consistency of my swirls and my accuracy in measuring TDS, it’s all because I’m trying to make every aspect of my brewing as consistent as I can from one brew to the next. There are a lot of things I would like to investigate which will require my brews to be very stable, including but not limited to:

• The effect of filters on flow and clogging.
• The effect of coffee aging and hardness on flow (through its shattering properties during grind).
• The effects of bed preparation (nest shape, etc.) on extraction evenness.

Those are just a few examples in a very long list; it’s possible that getting a Decent espresso machine in pour over mode may allow me to do this, but saving up Patreon funds for it will take a long time, so I think it’s worth improving my manual repeatability in the meantime. I want to improve my repeatability not only in terms of extraction yield, but also brew time and flow of coffee coming out of the V60.

The first way in which I attempted to reduce my kettle flow was to obtain a Mandritech Hand Drip Water Valve, which you may have seen on my Instagram or Telegram channel. This product is only sold in Asia, but Edo Rezaprasga, a reader of my blog, was kind enough to mail me one so that I could test it.

The device ended up working very well in terms of getting a more constant kettle flow, as can be seen on this graph where I tried to maintain my flow at 5 g/s with and without the valve:

There is however one big drawback in using the Mandritech valve; it ended up making the flow of water much less cohesive, which resulted in less agitation, and lower average extraction yields. You can see how the flow breaks up after a very small vertical distance at 5 g/s in this video:

I could even see that the layer of water on top of my coffee bed was super clean, which is a telltale sign that very little agitation is being applied on the slurry:

For example, brewing Passenger’s Los Yoyos 2017 Harvest coffee without the Mandritech with my usual recipe (22 g dose with 1:17 ratio, Rao/Perger boiling water, 5 g/s kettle flow) yielded 1.49% TDS with a 320 g beverage weight for an average extraction yield of 21.7%, whereas brewing the same coffee in the same way with the Mandritech yielded a 328 g beverage weight at 1.28% TDS for an average extraction yield of 19.1%. This is quite a heavy loss !

Fancy gooseneck kettles are slightly twisted near the tip, and that makes their flow much more cohesive. As a result, the stream of water falling from the kettle can penetrate deeper in the slurry, and can introduce turbulence all the way down to the coffee bed instead of having all its kinetic energy dispersed near the surface of the water. This is an important aspect of gooseneck kettles that I had not appreciated before.

This does not mean it’s impossible to brew well with the Mandritech, but optimal pour over recipes for its use would resemble those that work well with the Melodrip, and this is yet another rabbit hole I have not explored well. Judging from Ray Murakawa’s brews, it seems that Melodrip-style pour overs work better with smaller doses, much finer grind size and more agressive blooms to get rid of dry pockets despite the finer grind size.

Just when I was about to leave the possibility of flow restriction aside, Edo wrote back to me and told me he found a set of flow restrictor plugs small enough to fit my Artisan 0.6L model ! Those are very similar to the Brewista-branded ones that only work with the larger 0.9L kettles, except with way more choices in terms of outer and inner diameter sizes. This is yet again another piece of geeky coffee gear that is only available in Asia, so I could only get my hands on one thanks to Edo again ! I asked him to get the model that has an inner diameter closest to 4.1 mm, as I calculated that this would result in a reduced flow of about 10 grams per second according to Sampson’s law. Edo found one with an inner diameter of 4.0 mm (the Bonavita number 5) and sent it to me.

We were not sure if the outer diameter of the restrictor would correspond to whatever the inner diameter of my kettle is at the base of the gooseneck, but I couldn’t verify whether it was 5.5 mm wide with my digital caliper anyway, because it doesn’t fit inside the kettle. We thus decided to try ordering it anyway and see what would happen.

When I received it, I measured its outer and inner diameters with my caliper and found that the inner diameter was 4.0 mm as expected, but the outer diameter was 6.9 mm, a bit larger than the exit spout of the Brewista Artisan gooseneck. Fortunately, 6.9 mm seems to be exactly the size of the gooseneck entrance opening at the base, because the flow restrictor fitted perfectly ! I have no idea why they are called Bonavita flow restrictors; I am not aware of a Bonavita kettle with such a small gooseneck diameter.

The maximum flow rate with the restrictor installed turned out to be about 9.5 g/s, very close to the expected value ! When I saw that, I went ahead and tried brewing with it, which resulted in a total mess. I realized mid brew that the boiling water seemed to create some bubbles near the smaller entrance of the flow restrictor, resulting in a very uneven flow. The kettle flow kept interrupting, or spitting high-velocity drops of water which created bad splattering. Here’s an example of such uneven flow in this video:

In order to test whether the boiling water was really the cause of this messy flow, I tried pouring with water at 210°F instead of 212°F, and the result was much better ! Here’s what this looked like:

So I went ahead and made another brew at 210°F, and this time the kettle flow went super well. Here’s a video of that brew:

I was brewing Heart’s Colombia Decaf Platino which is a washed mix of Caturra & Castillo decaffeinated with the ethyl acetate process. I used a grind setting of 7.3 at 700 RPM on the EG-1 with a 22 g dose, Rao/Perger water, a kettle flow of 5 g/s with a 1:17 ratio that resulted in a brew time of 4:50, a 323 g beverage weight and a 1.31% TDS concentration. This corresponds to a 19.2% average extraction yield. Decaffeinated coffee always extracts much lower than usual so that number is quite normal. Decaf coffee beans are also very brittle and therefore generate a lot of fines. This is why I used a coarser grind size than usual, and still ended up with a long drawdown time of 4:50; I should have ground even coarser than that. The brew tasted quite good; Heart are one of the rare roasters that roast their decaf coffee well. It was not astringent despite the long drawdown time, but I actually never tasted an astringent decaf, so I would not be surprised if the ethyl acetate process also removes whatever is responsible for astringency. Note that both of my swirls were quite bad in this brew.

You might think that it’s not worth losing 2 degrees Fahrenheit to get a more reliable kettle flow, but I actually think it’s worth it especially in my case; the loss of temperature in the slurry will be smaller than 2 degrees because water temperature drops faster from 212°F than it does from 210°F, but more importantly it will make my brews more repeatable which is very useful for science !

Here’s a comparison of how steadily I was able to pour with the non-upgraded Brewista Artisan kettle, with the Mandritech valve, and with the flow restrictor:

You can see that either the Mandritech or the flow restrictor allowed me to reach a much better stability, but only the flow restrictor allowed me to do this without losing the cohesion of my kettle flow ! To quantify this a bit better, here are the averages and standard deviations of the kettle flows I obtained during these practice pours:

• Default kettle: 4.6 ± 0.6 g/s
• Mandritech: 5.0 ± 0.2 g/s
• Flow restrictor: 5.1 ± 0.2 g/s

Thanks again to Edo Rezaprasga for making all of this possible ! I’m super happy I finally found a solution to make my kettle flow more stable.