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.

Benchmarking dissolved oxygen content: what is the industry standard?

If I had to pick the one question I hear most often from brewers, it would be this: how much dissolved oxygen (dO2) – defined as an absolute quantitative measurement — can I have in my beer before I’ll get premature oxidative flavor changes?  I’m often asked this in the form of a question about successful brewers and their dO2 levels. But since that information is always kept in strict confidence, and we don’t need specific examples anyway, let’s just talk about it from an historical perspective.

I started making Total Package Oxygen (TPO) measurements in 1989.  Back then, many brewers were still using air counter-pressure in their bottle fillers, so it wasn’t unusual to see values between 800 and 1700 ppb. Today, TPO levels in breweries with good oxygen control are below 60 ppb in bottles and 100 ppb in cans.

In mid 1996, my work led me to measure dO2 in the other brewing processes.  Beer after filtration or centrifugation ranged from 30 to 300 ppb, and finished beer ranged from 30 to 500 ppb.  Today, in breweries with good oxygen control, those values range from 5 to 25 ppb.

Flavor changes due to oxidation are one of the few things over which a brewer has control.  By regularly monitoring dO2 throughout your process, you can determine which pumps, valves, filters, centrifuges and other ingress points are contributing unwanted oxygen to your product. If you’re a small brewery, then watch for two culprits: your process during the addition of filter aids, like diatomaceous earth (DE,) and inadequate blowing of storage vessels.

By de-aerating their DE slurry and blanketing the mixer with N2, one brewery reduced their dO2 pickup from 350 ppb to 30 ppb without any other process change.  Monitoring oxygen in a CO2 blanketed tank is a good idea too. And are you worried about an old filler that has no CO2 evacuation?  TPO pickup in the filler can be decreased to almost nothing if a liquid nitrogen doser is used before the packages are filled with beer.

My final thought is about my own beer drinking experiences and how good dO2 control has shaped what I purchase. When I first started drinking craft beer in the late 1980s, the beer quality was erratic. Once I’d been trained in sensory analysis, I learned the differences were mostly due to oxidative flavor changes, and so I’d make a point of actually trying to read the date codes to avoid getting older beer.  As my favorite breweries got their oxygen levels under control, the inconsistency of their product decreased, and I no longer need to worry about squinting at bottles to ensure I was purchasing great beer.

 

 

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