01 02 03 DropBOB Designs: Cold Drip Coffee Extraction - Idea & Proof of concept 04 05 15 16 19 20 21 22 23 24 25 26 27 28 31 32 33

Cold Drip Coffee Extraction - Idea & Proof of concept

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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 Y-filter
2x 3/4" ball valve
2x 3/4" npt to 1/2" barb fittings
length of 3/4" tubing
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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