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[Photos: Kevin Liu]

A few months ago, I wrote about the science of carbonation. In that article, I mentioned it's possible to rig a cheap, powerful, and versatile carbonation system using readily available parts.

Today, I'll show you exactly how to do it and all the benefits as well as a few potential downsides.

Why not just buy a carbonating system?

There are plenty of great commercial options out there, but there are also some pretty good reasons to rig your own custom device.

The popular SodaStream series of carbonators are attractive and easy to use, but they produce seltzer that's a little weaker than store-bought options. Plus, the machines and their proprietary refill cartridges don't come cheap. And if you carbonate anything but water, you'll void the warranty.

The Mastrad Purefizz gets good reviews and lets you carbonate anything you want, but relies on single-use carbon dioxide cartridges that both can get pricey and aren't as eco-friendly as a reusable tank.

If you're in the market for a carbonation tool that uses cartridges, I'd go for the far more versatile ISI Whip. For $30 more than the Mastrad, you get a true kitchen multitasker that can make sodas, whip creams, and even make rapid infusions.

But cartridges are pricey. 100 CO2 chargers cost around $30—so carbonation is 30 cents per liter, assuming a single charge per liter. Using the DIY rig, I was able to carbonate for about 5 cents a liter. Ka-ching!

For me, the true draw of a DIY system boils down to cost, convenience, and customization. I got tired of constantly buying new cartridges, especially since I had no idea when one of them would up and quit on me. Refilling carbon dioxide into a standard tank is ridiculously cheap. And, as I found out, it's really helpful to be able to carefully control carbonation levels.

Parts You'll Need:

Total = $131

How to Build Your Carbonation Rig

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Step 1: Buy and fill a tank of carbon dioxide

I bought a standard 24-ounce paintball tank that cost $35 filled at a paintball place down the street from my house. (More on carbon dioxide sources and safety below)

Many folks prefer larger, more expensive tanks . I went with the paintball ones because they can be filled at any sporting goods store (though this might not be ideal—see the section on safety, below). The larger tanks can be filled at a welding supply store or other stores that supply carbon dioxide.

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Step 1.5: Connect the tank to an adapter

If you do end up buying a tank made for paintball, know that you will need a special adapter (another $17) to connect it to the regulator.

I also recommend picking up some teflon plumber's tape from the hardware store to use on the connection points between the tank, adapter, and regulator.

In the above image, you can see the adapter I used in blue and a little plumber's tape sticking out where I made the connection to the regulator.

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Step 2: Connect the tank to a pressure regulator

The regulator is what makes the whole carbonation rig awesome. I bought a regulator used for pumping draft beer out of a keg. The knob on the front controls your output pressure. Two gauges show both your output pressure and the remaining pressure left in your tank.

These gauges are great because one lets you control exactly how carbonated your soda is, while the other one warns you before you run out of gas.

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You'll notice that the regulator has a pressure relief valve. I accidentally triggered this one time and freaked out as gas started spewing from the device. This valve is designed to release gas if your output level goes above 60 PSI (pounds per square inch). If you do trigger it, quickly turn down the output pressure on the regulator, then gently pop the mechanism back into its housing and the gas should stop.

Step 3: Connect the regulator to a ball lock gas line assembly

To connect the regulator to a plastic soda bottle, you'll want a ball lock assembly with plenty of extra hose. As we'll discuss in a moment, you'll be shaking your carbonated bottle a good bit, so you want enough hose so that you don't run the risk of jerking the regulator around as you shake.

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Step 4: Connect the gas line to a carbonator cap

The ball lock attaches to a carbonator cap that can be screwed onto a standard 1 or 2-liter plastic soda bottle. There's also a stainless steel version if you want something more durable.

Step 5: Screw the carbonator cap on the bottle and...

Carbonate! To carbonate water, I set my regulator to 40 PSI. For cocktails, I try to go as high as possible without triggering the release valve, which ends up being just under 60 PSI for me. But first...keep reading.

How to get the best results carbonating water or cocktails

Follow these tips.

Chill the liquid. Carbon dioxide dissolves better in colder liquids.

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Leave plenty of headspace. A good rule of thumb is to leave the bottle 1/4 empty. Before charging the bottle, squeeze out as much air from the headspace as possible. Air competes with carbon dioxide for room in water. The bottle will inflate once you turn the gas on and the headspace will give you room to shake the carbon dioxide into the liquid.

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Leave room for foam. Some liquids, like wine or beer, will produce a lot of foam during carbonation. For these situations, try filling your bottle just half or a quarter full to leave room for the foam to grow. Also, don't forget to release the pressure from the bottle as slowly as possible.

Shake it like a bartender. Carbon dioxide doesn't just magically dissolve into water: you have to shake the bottle while you're adding gas (don't detach it!) to encourage the gas to get into the liquid and stay there. Pretend like you're shaking a cocktail and give it a good thirty seconds to a minute. You'll hear the regulator slowly release more gas with every shake—a good indicator that you're doing it right.

Carbonate at least twice. Water or cocktails will have some amount of air dissolved in them by default. After your first shake with carbon dioxide, vent the bottle by unscrewing it from the carbonator cap quickly. The rapid release of pressure causes lots of bubbles to form, releasing both CO2 and other dissolved gases from the water. Sure, most of the carbonation will boil off, but so will any other gases dissolved in the liquid. Once the bubbles settle, recharge the bottle and carbonate again. This is another reason why I don't like carbonators that use cartridges: to do it right, you should be using two cartridges every time you carbonate, which can get expensive.

Avoid using cloudy juices. Or really anything that isn't crystal clear, like a homemade syrup. Particles in your liquid act as nucleation points (spots where bubbles are likely to form). This means it's more likely that dissolved carbon dioxide will turn into bubbles and bubble off from the liquid, making your carbonation less strong. If you have to use a cloudy syrup to make a drink, I recommend making seltzer water and mixing it with the syrup just before serving the drink.

Rebottle for storage. Although you'll want to leave plenty of headroom to actually carbonate your water or cocktail, you won't want to store the finished product with that much headspace. Instead, either top off your bottle with a second batch of carbonated drink, or carbonate in a larger container and then store in smaller bottles.

Safety concerns

Let's address the elephant in the room: is rigging your own soda-carbonation system safe?

To look into the issue of whether the carbon dioxide you can buy is safe to consume, I scoured a bunch of homebrew and SodaStream-hacking forums, and most people's arguments fell into two basic camps:

  1. There's no such thing as "food-grade carbon dioxide" or...
  2. The FDA regulates food-grade carbon-dioxide and it has to be 10 times more pure than industrial grade.

Neither side is entirely correct.

The FDA does not, in fact, regulate the quality of carbon dioxide in foods. Instead, the FDA simply requires that any gas use in food be of "suitable purity" as defined by "good manufacturing practice."1

But, there is a definition for food-grade carbon dioxide. This PDF from the International Society of Beverage Technologists defines food-grade carbon dioxide as a 99.9% pure gas. It should also be tested for a number of other impurities.

After checking at a few bulk suppliers of carbon dioxide, it appears that "industrial grade" gas is anywhere from 99.5% to 99.8%, "high purity" gas is supplied at 99.9% pure and medical and research gases are even higher purity.

Many of the impurities that could contaminate carbon dioxide gas are harmless—stuff like plain old air or water vapor. But there are a few that could cause real harm.2 In fact, this document from the Coca-Cola company specifically warns against using industrial-grade carbon dioxide, such as the carbon dioxide used in welding.

So what can you do?

I got in touch with Dave Arnold, a carbonated-cocktail pioneer, and he told me that although he gets his carbon dioxide from a 'welding supply' place, that same source also supplies carbon dioxide to the food industry and medical industry. As long as you tell the supplier you intend to use the gas for food applications, they should be able to give you the right type.

I tried Dave's tip out by calling my local homebrew shop. I asked them where I could purchase food-grade carbon dioxide. They immediately knew what I was talking about and pointed me toward two carbon dioxide supply options. If your homebrew shop can't help you out, check with a local restaurant and try to find out where they get the tanks for their fountain soda machines filled.

As for the rest of the components in this rig being food-grade, you'll notice that all of the pieces I bought are designed for homebrewers. The tank itself is often used as a hack for refilling SodaStream machines, and I haven't any read any reports of the tank itself corroding or leeching hazardous chemicals, which would be the primary concern.

In summary: before you go in on a tank of gas, check out whether there are good places to get it filled near you. If they don't know what you're talking about, stay away and find a different supplier who understands that you need everything you use to be food-safe. But ultimately, this is a DIY job and you (the DIY-er) need to be ok with assuming the responsibility of ensuring all your components and ingredients are safe for whoever you plan to serve your drinks to. Double-check as you go.

What about exploding bottles?

Standard soda bottles should withstand up to 150 psi without exploding, but you would never want to get anywhere near that at home. The biggest risks come from using bottles that have been left out in the sun, run through the dishwasher, or crumpled so much that creases have formed in the plastic. Aim to use a new-ish bottle. If you don't get a regulator that caps out at 60 PSI, I would recommend staying around 60 PSI anyway to reduce your risk.

And don't forget to clean up afterward. One of the risks of carbonating cocktails at home is that if you do end up foaming sugary liquid all over the place, the mess can get all over your equipment and eventually promote the growth of harmful microbes. Make sure to clean your rig if it gets dirty in any way.

1. I also checked all the other FDA documents I could find on carbon dioxide and food-grade definitions in general, and couldn't find anything more on carbon dioxide. I also referenced Ruth Winter's food additives book, which simply said carbon dioxide was generally recognized as safe.
2. See the second figure in this article. It lists common contaminants and their sources.

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