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Cold Drip Coffee Extraction – Idea & Proof of concept

So I’ve been wanting to make a cold drip coffee machine for a long time now. I got hooked when I first tried it at Parallel 49 Coffee Shop.

So as my list of tasks for the week dwindled (now complete, save for side tables), I started my cold brew design with a good old trip to home depot (just to see what they have). It turns out that they have pretty much everything I needed to get started on my proof of concept.

Now, I know this “cold coffee” shit has been done many times. So why a proof of concept? Well from what I read online, Its an “unstable design”. You can’t really get the dripping just right … and even when you do … an hour later, it will be either no longer dripping, or dripping too fast.

So, I decided to start with the basics. Here is my home depot parts list:

1x wine bottle (from home)
1x carafe (from home)
Here’s what it looks like all setup:

Now, lessons learned. Since the wine bottle at the top does not let air back in, the air must come up from the tube … which it does, but not as predictably as I’d hoped. The result is an erratic drip, that needs to be adjusted every few hours. At the end I opened up both valves all the way … it still only dripped a little due to the back flow of air.

So I don’t like the original design … even though it made great coffee.

So today, I took apart an old Halloween Costume from 2011 to make my second “more commonly seen” version of the cold drip coffee maker.

I mostly just wanted to test out a system with no back-flow of air and I wanted to see the adjust-ability and repeat-ability of the 1/4″ needle valve.

So far, I’ve tried as slow as 11 drops/min and it stopped functioning after about 1-hr … (sticktion maybe?) So, I’ve increased it to about 40 dpm.

What I am working towards is a design that I can accurately control the flow (needle valve or Solenoid valve? … maybe even servo controlled needle valve). I’ve toyed with the idea of using a servo controlled valve but I think that would be too noisy.

I’m leaning towards a closed loop servo controlled needle valve (via arduino) with piezo electric drop detection (via sound). This has not yet been attempted by the internet (by my searching).

Once I can get this part working I will try to also control the pressure & oxygen content using NO2 cartridges. Not sure how I would fabricate a container for that though … food for thought.


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