Vacuum Siphon: Better Coffee Through Chemistry

My Wife in her career worked as a Barista and really developed her tastes for coffee and espresso. So our house has some interesting coffee equipment like a Chemex (https://amzn.to/35Gc54b) and the French Press (https://amzn.to/2Z6Y9NX) is my goto in the morning. One of the "apparatus" that always fascinated me was the Vacuum Siphon (https://amzn.to/2Ex8SrI), they are not cheap and have some elaborate setups, but for "coffee snobs" like my wife it is suppose to produce smooth, tasteful, less acidic coffee, similar to a cold brew but hot.

I understand the physics/chemistry behind it and wanted to try making my own setup and see how good it really is. And if you happen to be a Breaking Bad fan, there is speculation that in the coffee scene in the lab what they were using was an elaborate vacuum siphon, with maybe some added Hollywood props, clip is here for reference. So this Instructable is about building a vacuum siphon and a little bit of the science behind it.

Supplies

Materials:

Siphon Cloth Coffee Filters (https://amzn.to/2ZaGvJd)
Thistle Funnel Borosilicate Glass (https://amzn.to/2renM34), I also tried out making my own with a pastry nozzle (https://amzn.to/34ES0cU) and some latex tubing while I waited for this piece to ship.
Boiling Flask, 1000ml - Borosilicate Glass - Flat Bottom (https://amzn.to/34JoRh7)
Flint Glass Tubing (https://amzn.to/2PZ08jk)
Beaker Set (https://amzn.to/2Z4Xf4C)
Rubber Stopper (https://www.lowes.com/pd/Hillman-1-6875-in-Black-Neoprene-Hole-Plug/3013264) this one was a #8 and like an exact fit, didn't allow a lot of wiggle room, would like a slightly larger like a #10 that maybe could be cut down on the sides but allows a tight fit and more material above the rim. (See fail video included in this Instructable).
Latex Tubing (https://amzn.to/2MdE6br)
Heat Source: Camp stove (https://amzn.to/2s29ra8) or Hot Plate (https://amzn.to/2PFcBd0)
Coffee beans with a corse grind, a fine grind can jam up the filter.
Rubber Sealant

Nice to Haves:

Tools Needed:

Propane torch
Drill

Step 1: Plan

Plan is simple, we are going to use two vessels: a beaker and a sealable boiling flask with rubber stopper lid. The two will be connected by a glass tube and on the beaker end of the tube will be a funnel and filter (to keep the grounds from being sucked up).

We'll use some ring stands to hold the two at proper heights, and I'll use a propane camp stove to heat the water, though a hot plate or some other source could easily be used.

For the most part we'll use the equipment as is, but we'll need a hole in the stopper for our tubing and we need to connect our funnel to our tube, and of course we need the bends in the tube at the right heights to make the system work.

Step 2: Science!

The Physics behind the siphon really comes down to the simple ideal gas law: PV=nrT, in layman's terms: Pressure times Volume is equal to the number of moles of gas times the ideal gas constant, times Temperature. In this case the n,R, and V are fixed and we are really playing with the changes in Pressure and Temperature.

Wikipedia also does a good job of explaining the science behind it as well (https://en.wikipedia.org/wiki/Vacuum_coffee_maker)

Walking through the diagrams...

Diagram 1: we start with water at room temperature in our boiling flask, which is sealed up tight with our rubber stopper, our tube is below the water line so our boiling flask is fully sealed even though the other end of the tube is in a beaker and open to the outside world through the filter. The grinds are also in the beaker and the filter on the funnel is positioned low in the beaker, just off the bottom, which is important in later steps.

Diagram 2 & 3: We apply heat to the boiling flask which is going to raise the temperature as well as the pressure in the flask, very quickly water will start being forced up the tube toward the beaker due to that change in pressure and temperature, as the water heats up and the volume in the boiling flask reduces this process starts to accelerate and the water is pushed over to the beaker.

Diagram 4: The heat source is removed, the water is mixed with the grounds and the boiling flask starts to cool with is a drop in Temperature and will cause a drop in Pressure to keep the equation balanced, which will create suction in the boiling flask and start to pull the mixed coffee back in to the boiling flask.

Diagram 5: As the Temperature continues to cool the remainder of the coffee will be pulled over. The filter and the position of the funnel will leave the grounds and a little liquid remaining in the beaker.

The end result is a brew of coffee at the right temperature that allows for less acidity and a smoother better flavor, and some of that is up to the user in terms of water and amount of grounds.

If this didn't fully make sense to you there is a video at the end of the instructable that highlights these steps as the vacuum siphon is operating.

Step 3: Drilling Holes in the Stopper

To get our apparatus ready we first start with drilling a hole in our rubber stopper for our glass tube to travel through. Since our stopper is a little flexible the hole can be the same size of the tube, which in this case is 1/4".

Step 4: Bending the Glass & Testing for Leaks

Using a single piece of tube, starting at our boiling flask we need to go up and out of the boiling flask over toward our beaker and then down into the beaker, which means we'll need two right angles in the tube.

Bending the glass is quite fun and something I learned in high school chemistry class:

Just apply a good heat source, I'll use a propane torch on low, and rotate the tube to try and get it to heat evenly.
Very gently, do not force it, try to bend the glass, once the glass heats up and you feel it start to move you want to remove it from the heat source.
Continue to bend until you get to the right angle, as the glass cools it'll start giving you some resistance, just place it back in the heat and rotate to soften it up again.
Once you get to the desired 90 degrees remove it and place it on something fireproof (like a ceramic tile) to cool.
If you overheat the glass the tube walls might collapse and you may end up with a kink at the bend or a hole in the side, so it's safer to underheat and reapply the heat to get to the right angle.

Once the glass has completely cooled (completely!) you can take it to the sink and fill it with water to see if there are any leaks, don't do that to hot glass or the rapid change in temperature will likely cause it to shatter.

Here's a video I made showing the process: https://youtu.be/59iyb5AuMPU

Step 5: Attaching the Pieces Using Tubing & Sealing

We need to feed one end into our rubber stopper and down a length so that the opening of the tube sits just above the boiling flask's bottom. To do this, first off don't force it, the glass may break and cut you (learned this one the hard way). Apply a little cooking oil to the tube, which will make it slide through the rubber more easily, then slowly feed the tube through the rubber stopper, rotating seems to help move it along as well.

Apply the rubber sealant to the top, and also the bottom of the stopper, this will seal it up but will also help keep the stopper from moving once the pressure forces are applied to it. I also learned this one the hard way and have a fail video that shows the rubber stopper get sucked in to the boiling flask.

To attach the funnel to the tube we use a piece of the latex tubing and some zip ties. Apply a little rubber sealant then slide the glass into the latex tubing. You want about two inches of overlap which gives room for the sealant and also plenty of room to apply two or three zip ties.

If you break your bent glass, like I did, you can also use the latex tubing, sealant, and zip ties to put it back together without having to bend another one.

Step 6: Assembling the Filter

If you choose not to get the Thistle Funnel or want to try making your own with a pastry nozzle, just apply some sealant to the nozzle and jam it as far as you can into one end of a length of latex tube and let it dry. Be sure to use enough latex tubing so you can attach it to the glass tubing easily.

Then place it in the center of one of the filters and draw the strings tight so that the filter wraps around the funnel and won't allow any grounds in.

Step 7: Operation

Measure out how much coffee grounds you need and place it in the beaker.
Measure out the amount of water for those grounds and place it in the boiling flask.
Put the filter on the funnel if you haven't done so already.
Place the rubber stopper firmly into the boiling flask sealing it up.
Apply heat to the boiling flask. When the water has all left the flask, remove the heat source.
Stir the grounds and water mixture in the beaker.
Let cool, as it cools, the coffee will be pulled back to the boiling flask.
The whole process takes about 15 mins with the majority of the time being the cooling part.

Here's a video of the siphon in action: https://youtu.be/2lZTTTzohBs

Here's also a video of the siphon failing because I did not securely attach the rubber stopper and the pressure was too much: https://youtu.be/Olxi1JuEm3I