How the Purity of Sparged Carbon Dioxide Affects the Oxygen Concentration of Beer

 

My last blog post discussed the importance of carbon dioxide purity when using injected CO2 to increase the CO2 concentration of your beer. The thing we learned is that the purity of CO2 must be very high (99.99% or better) when using injection, or you will at the same time significantly increase your dissolved oxygen levels. However, injection isn’t the only method for adding CO2 to beer. Sparging, in which CO2 is bubbled through beer (usually in a tank with slight over-pressure) is another common practice for boosting CO2. So in this post we will explore how sparging a finished beer tank with carbon dioxide impacts the final oxygen concentration of your product.

First, let’s do a quick review of what happens when you inject CO2. When you inject gas (usually into a pipe,) you are forcing a given weight of gas into the liquid under pressure. All of the carbon dioxide, plus any trace oxygen and nitrogen, gets pushed into the beer and dissolves completely, allowing you to calculate the weight of gas used and extrapolate from there your various gas concentrations.

On the other hand, when you sparge gas into a liquid the dissolved concentration of gas will be bound by Henry’s Law. Henry’s Law tells us that the amount of gas that will dissolve in a liquid will be proportional – at a constant temperature – to the partial pressure of gas in equilibrium with the liquid.  This means that the gasses dissolved in your beer will never be more concentrated that the partial pressure of the gas you are using to sparge.

The consequence is that, with any given CO2 concentration outcome desired, you will have significantly lower oxygen concentrations for sparged beer then for injected beer. For example, in injected beer the oxygen pickup from injecting one volume of 99.95% CO2 (at 0oC) into the beer when the oxygen concentration is 0.01% is 143 ppb. But a theoretical sparging of that same CO2 into the beer at atmospheric pressure would follow Henry’s Law, and your oxygen pickup would be about 7 ppb.

In real brewing situations, however, most brewers use tank overpressure to help get sparged CO2 into solution, so you would probably be picking up about 2 times the above amount, or 14 ppb. The table below shows the expected oxygen pickup given varied percentages of O2 traces (in your CO2) when measured at sea level and at 0oC:

Sparged CO2 at 1 V/V

0.001% O2

0.005% O2

0.01% O2

<1

3

7

My final thought is that CO2 purity isn’t nearly as important if you are sparging rather than injecting, since the amount of gas that will dissolve into your liquid is much lower. This also applies to the purity of the gas you use to flush air from tanks before filling.

Advertisements

Gas Phase Measurement Units

In the past couple of weeks I’ve received several questions about measurement units and how they differ from one another. Have you ever tried to keep bar, mbar, atm, Kpa, %Vbar, %, torr, ppm and ppb straight? If you’re listening to someone in speed mode (I plead guilty) it can be a challenge to follow.

So let’s start by looking at the way different units present at 1 bar, the unit of pressure sometimes also referred to as “atmosphere.”

1 bar =

  • 1000 mbar
  • 750.1 Torr
  • 750.1 mm Hg
  • 29.53 inches Hg
  • 0.987 Atm
  • 14.50 psia
  • 100 kPa

As a brewer you probably won’t see much of units like Torr or mm of mercury (mm Hg), but there’s a unit called %Vbar or ppmVbar that may be helpful. I use them a lot and they can easily be interchanged with percent, but there is a specific distinction in that it is tied to atmospheric pressure and thus stands for “Percent Volume Barometric and “PPM Volume Barometric”. “

So why use Vbar instead of just percent? If you’re at a high elevation and want to specify that that the percent of the gas you are measuring is being measured at atmospheric pressure, then Vbar is your unit. For example, Denver Colorado is roughly 5280 feet. At that elevation there are about 15 percent fewer atmospheric gas molecules —  855 mbar – versus the 1013 mbar you would find at sea level in San Francisco. The Vbar units confirm that the instrument is at atmospheric pressure while the sample is being measured.

This table compares different gas percentages using some of the most common units you may encounter:

Unit

mbar

Bar

Atm

Percent (absolute)

%Vbar

PPM

100% gas

(at sea level)

1013

1.013

1.000

100.0

100.0

1,000,000

100% gas  (atmospheric at 5280 feet)

855

0.855

0.844

84.4%

100

1,000,000

1.000 % gas

10

0.010

0.010

1

1

10,000

0.100 % gas

1

0.001

0.001

0.1

0.1

1,000

0.010 % gas

0.1

0.0001

0.0001

0.01

0.01

100

0.001% gas

0.01

0.00001

0.00001

0.001

0.001

10

0.0001 % gas

0.001

0.000001

0.000001

0.0001

0.0001

1

My final thought is to understand the units available to you. If you are purging down a tank with CO2 and want a specific percentage of CO2 purity, use the units that will equate back to what could dissolve in your beer if the purge didn’t exhaust all of the contaminating gas in the tank.

The Units of Gas Phase Measurements Demystified!

I don’t think there is anything more confusing than the units used when measuring gases dissolved in liquid compared to the units used when measuring in the gas phase.  Here is one more post on CO2 before I completely exhaust the topic – pun intended. Let’s see if I can make it clear.

Dissolved gas is always expressed in terms of weight per volume. When someone says that they have 10 ppb or 0.01 ppm of dissolved gas in their beer, what they really mean is that they have 10 micro grams per liter or 0.01 milligrams per liter dO2 content.  Again: this is gas expressed as weight of gas per liter.

Gas phase measurements, on the other hand, are always expressed as a volume of gas per volume of total gas. This can be in units of percent, bar (volume barometric,) mbar, atmosphere, or — the two most confusing units of all – ppm and “%CO2 purity.” Lets talk about those two:

  • Gas phase ppm is a comparison of the gas being measured to all of the gasses in the sample. If you have 100% O2, then you will have 1,000,000 ppm O2. Since it is cumbersome to talk about 1% or more as ten thousand parts per million, we use units of percent. But when we get to trace levels below 0.1%, then we start throwing around units of ppm.
  • Percent CO2 purity is all of the gas being measured that is only CO2. Here’s a table to help you sort it out.
Unit of Measurement O2 Content

 

O2 Content

 

O2 Content

 

O2 Content

 

O2 Content

 

Percent 100 1.0 0.1 0.01 0.001
bar 1.013 0.010 0.001 0.0001 0.00001
mbar 1013 10.13 1.01 0.10 0.01
ppm 1,000,000 10,000 1000 100 10
Atmosphere (ATM) 1.0 0.01 0.001 0.0001 0.00001
% CO2 Purity 0 95.0 99.5 99.95 99.995

Did you follow? Just in case I lost you, %CO2 purity assumes that all of the impurity in CO2 is air.  Since nitrogen comprises 4/5th of air, you have to take the oxygen content, add back in the nitrogen, and subtract all the air from the 100%.

My final thought is to add just a tad more data to compare ppm dissolved to ppm gas phase. Here’s a quick test: if you measure CO2 gas with an instrument reading, in the liquid phase, 0.001 ppm or 1 ppb dO2 at 20 deg C, what would be the equivalent gas phase reading?  At 20 deg C, the O2 content in the gas would be about 22 ppm. In terms of %CO2 purity, that would be about 99.99%.

%d bloggers like this: