Using a Keg as a CO2 Source for Portable Serving!

This technique uses an economical inline secondary regulator to utilize a spare keg as a CO2 source to serve a keg.  I’m not suggesting this setup as a replacement for your kegerator CO2 tank.  You still need a standard CO2 tank.  What this setup could be very useful for is as a replacement for those expensive little regulators or injectors and expensive (considering how much CO2 you get) little CO2 cartridges.  One inexpensive purchase allows you to pressurize and serve your keg on the go for little to… nothing.  Keep reading.

Updated v2 Inline Secondary!

Availability Update… v2 has been sold out for sometime. As of this posting, it’s available again. Check product pages to see current availability.

An updated version of this great gadget is now available.  It’s DuoTight equipped and… has an integrated CO2 gauge.

Although this post features v1, the same concepts apply to a v2 setup with the added benefit of an integrated gauge.  Since v2 is DuoTight compatible, there would be some plumbing differences.

Something like…

More DuoTight!

The Magic Piece of Equipment

Cheap Inline Regulator – via William’s Brewing | via MoreBeer | via Amazon

These inline secondary regulators have been on the scene for a little while now.  At the price I’ve seen them at, sub $10, they are a bargain.  They also add a lot of flexibility to your draft setup, allowing you to easily and cheaply serve using multiple pressures and carbonation levels.

Note that these are inline secondary regulators.  You still need a primary regulator attached to your CO2 tank.  The idea is, you set the primary to the highest pressure you will use (without exceeding specifications of any component of your system) and then use these regulators inline (one per line) to fine tune pressure and carbonation [See: Balancing Your Draft System].  As an example, you could set your primary to 25 PSI for faster force carbonation and set each line to a different pressure based on desired carbonation level.

Setting the Pressure on the Inline Regulator

Since these do not have a gauge, you need some sort of a gauge to use for tuning in the pressure.  The gauge doesn’t have to stay connected, it’s only used while you’re setting the regulator.

This is a perfect application of a Spunding Valve

Re-purposed for converting a spare keg to a CO2 source

I’ve wanted to do this for years.  There used to be a commercial sanke keg that had two chambers.  One was used as a pressure chamber and the other held beer.  I always wanted one of those.  The problem was that it was extremely expensive and I just couldn’t justify the expense.  This magic little secondary regulator makes something similar easily possible.

Here’s the setup.  The left keg is the filled keg, the right keg is the pressure source.  A gas to gas jumper connects the two with the inline secondary regulator in the middle.  Note that the inline does have a flow direction, there’s an arrow on the side to let you know which way this is designed to flow.  Also, although these are ball lock kegs, you could just as well use pin lock kegs.I charged the 2.5 gallon ball lock keg (pressure source) using my kegerator CO2 tank and regulator.  The regulator maxed out at 50 PSI.  Important: Make sure every component in your setup is rated for the amount of pressure you decide to use.

Does it work?  Serving 5 gallons using a 2.5 gallon keg as a CO2 source

Here’s the setup.  The shorter keg is the pressure source.  There are two 5 gallon kegs.  The one with the faucet contains 5 gallons of water, I’ll call that the “beer keg” from now on.  The other 5 gallon keg is there to receive the test liquid.  There’s some extra stuff here.  I have a ball lock carbonation lid on the beer keg with a digital spunding valve on it.  The pressure source keg has a higher PSI gauge on it, connected to a liquid QD.  Those gauges are to monitor the test.At the beginning of the test the beer keg was pressurized to about 10.2 PSI.  Practically speaking, you could use a lower pressure.The test in progressTest completed.  5 gallons served using a spare 2.5 gallon ball lock as a CO2 source.  Whammo.A pressure reading after the serving test was completed.  In order for this to be really valid, the 5 gallon keg should be about the same pressure after as it was before.  It read 10.2 PSI at the beginning of the test and 10.3 PSI when the test was completed.The gauge on the 2.5 gallon pressure source keg went from 50 PSI to about 12 PSI.  50 PSI x 2.5 gallons is about right to serve a 5 gallon keg at a 10 PSI serving pressure,  This could have served a bit more, maybe another gallon.  You could stretch this same amount of CO2 out to serve 2 x 5 gallon kegs if you decreased the serving pressure to 5 PSI, which is a reasonable pressure that you might want to use anyway.  50 PSI x 5 gallons should be able to serve 2 x 5 gallon kegs at 10 PSI serving pressure or about 4 x 5 gallon kegs at 5 PSI serving pressure.

How much CO2 did the pressure keg hold?

From my post, Kegging CO2 Use Estimations and Calculations – We can use the The “weight of CO2 used for serving a keg” formula to estimate this.  That formula is (Serving Pressure + Atmospheric Pressure) x 1/2 x Keg Volume

So, for this 2.5 gallon keg at 50 PSI, those numbers would be…

  • (50 + 14.7) x 1/2 x 2.5 = 80.87 grams or 2.85 ounces

Serving estimates: 1 x 5 gallon keg at 10 PSI serving pressure, 2 x 5 gallon kegs at 5 PSI serving pressure

For a 5 gallon keg at 50 PSI, those numbers would be…

  • (50 + 14.7) x 1/2 x 5 = 161.74 grams or 5.7 ounces

Serving estimates: 2 x 5 gallon keg at 10 PSI serving pressure, 4 x 5 gallon kegs at 5 PSI serving pressure

Cost Savings!

Beyond the hardware savings of using this cheap inline regulator for this purpose, this technique should save you money on CO2.

MoreBeer’s CO2 Injector uses 16 gram cartridges.  As of this posting those cost $10 for 6.  That figures to $1.67 each.  You need between 2 to 5 cartridges to serve a 5 gallon keg of beer.  Let’s just use 3 for our estimations, that’s less than the average of those two numbers.

3 x $1.67 = $5.01 for each keg you serve

Versus The keg as a CO2 source method

In my example the 2.5 gallon keg at 50 PSI keg holds approximately 2.85 ounces (the equivalent of just over (5) 16 gram cartridges).  2.85 ounces is about .178 lbs.  I pay $25 for 20 lbs of CO2.  That figures to $1.25 per lb.

.178 lbs x $1.25/lb = 22 cents per 5 gallon keg

Decrease serving pressure to 5 PSI and you should be able to serve 2 x 5 gallon kegs with the same amount of CO2.  That would be 11 cents per 5 gallon keg

Side Note: You need about 29 x 16 gram cartridges to get 1 lb of CO2.  At $1.67 per 16 grams that equates to about $48 per lb of CO2.  That means 20 lbs costs about… $1,200.  I pay about $25 for 20 lbs of CO2 via tank exchange.  This is for the purpose of illustration only, I’m not suggesting anyone uses these in that quantity.

How about serving your kegs for $0 CO2 costs?  Keep reading…

An Upgrade – John Guest Push to Connect to Flare Fitting

This fitting is a John Guest Female Flare to 1/4″ Tube model MI4508F4SLF – I’ve found it difficult to get the right push to connect style fittings.  There are lots and lots of options.  I’ve ordered some that just aren’t compatible.  See links at the end of this post to help find this fitting.This installs on MFL ball or pin lock QDs.  Here it is installed on my ball lock gas QD.  You could just as easily install these on a 1/4″ MFL pin lock QD.This assembly fits directly on the inline secondary regulator, eliminating the need for a section of tubing.Here’s the setup using this fitting

Pros and Cons to using spare keg as a CO2 source for portable serving

Downsides: Size and portability are what come to mind.  Instead of a handheld size setup, you have an extra 2.5 gallon or 5 gallon keg.

Upsides: If you have a spare keg, the costs to implement this are very low.  It should also save money on CO2.  There is also a convenience factor.  Serve an entire keg without swapping out co2 cartridges.

How about $0 in CO2 Costs to Serve Kegs on the Go??

With the right fittings, you can add a Schrader type valve to your keg [See: Adding a Schrader Valve to a Homebrew Keg].  Doing so allows you to use your air compressor to pressurize the pressure keg.

Before you comment, email, tweet, etc [8 ways to connect] to me, PLEASE READ THIS…. This technique involves using your compressor to fill the pressure keg, using atmospheric air to serve your keg.  Yes, this will cause oxidation given enough time.  No, I am not recommending this for any long term arrangement.

I’m suggesting this as a possible technique to use if you’re sure you’re going to go through an entire keg quickly.  Quickly to me means under several hours.  Oxygen will cause oxidation, but those effects should be slim to none in the span of a short period of time.  If you aren’t sure the beer will be consumed quickly, I would suggest using CO2.

Important: It’s very important that you know the pressure ratings of everything you’re using.  Do not exceed stated pressures.  I’ll take it a step further and suggest that you don’t even get close to stated pressures.

This keg has a pressure rating of not to exceed 130 PSI.  Both the lid and the keg are stamped with that rating.  My air compressor has a max pressure of 125 PSI.  Even though the vessel itself is rated for 130 PSI, the PRV started to noticeably vent at about 90 PSI.  The keg eventually leveled off at about 85 PSI.

I’ll use my CO2 estimation formula to estimate the amount of pressurized atmosphere this keg is holding.  Not sure if that is as applicable, but my guess is that it is still a valid estimation.

(85 + 14.7) x 1/2 x 2.5 = 124.63 grams or 4.39 ounces


This economical inline regulator is a great little piece of equipment.  Beyond it’s generally intended use of letting you easily serve multiple pressures and carbonation levels, it let’s you put spare kegs in use for serving on the go and saves you money on CO2.

Grab The Gear:

For a V2 Build:

Although this post features v1, the same concepts apply to a v2 setup with the added benefit of an integrated gauge.  Since v2 is DuoTight compatible, there would be some plumbing differences.

Something like…

John Guest MI4508F4SLF – 1/4″ Female Flare x 1/4″ Tube:

Somewhat difficult to find.  Use the searches below and look for this exact model number and make sure the description reads 1/4″ female flare x 1/4″ tube

Parts for Schrader Valve Conversion:

Related Posts:

Also: Kegerator Tips & Gear | Keg Repair Part #s | Recent Keg Finds

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Make sure the components you use are compatible and rated for your intended application.  Contact manufacturer with questions about suitability or a specific application.  Always read and follow manufacturer directions. hack:kegasco2 tag:tpr

1 thought on “Using a Keg as a CO2 Source for Portable Serving!

  1. Cory

    Such a great idea! I’m going to try this technique for carbonating and serving kegs that I keep outside in the winter when there is no room in my keezer. Now I can have 5 beers on tap inside and another 5 outside without having to buy another tank and regulator.


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