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%.


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