Gas Phase Measurements Demystified!

The blog is on vacation this week. Here is an earlier post that was popular.

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%.

Frivolous Friday Fun – Plastic Bag Bottle

 

This post caught my eye, because the brewing industry always waits and is careful about introducing new packaging ideas due to oxidation concerns.  Coca-Cola and one of their suppliers have developed a biodegradable bag for dispensing soda in countries where people don’t want to pay the deposits on bottles.  Like PET for stadium and beach sales, will bags ever get high-tech enough for beer?  There is a link to the story and video on the bag here.

Tips for Getting Accurate Portable Dissolved Gas Measurements

 

My last post was about getting accurate in-line gas measurements. There is a link to that post here. Today I’m going to follow-up with some tips on how to achieve accurate portable results.

Portable measurements are a tad more forgiving and easier to accomplish than in-line measurements. You don’t have as many sensor placement restrictions and you can measure in vessels, as opposed to using an in-line probe that needs flow. Here are the main things to remember:

  1. Just as with in-line probes, all of the gasses in your beer — the CO2 and O2 that will be in contact with the sensors — need to be clear and in solution. If there is degassing in the flow chamber then you will probably get lower than expected results.
  2. Even though you are not measuring in-line, you still need to deal with flow in the flow chamber of your instrument. Portable electrochemical oxygen sensors have specified flow rates. If you deliver the sample too slowly or too quickly, the readings will be underestimated. Slow flow will not feed sufficient product to the sensor to satisfy the requirements of the sensor’s electrochemistry. High flow may result in product degassing.
  3. Optical sensors are much more forgiving and require less flow than electrochemical sensors. The instruments used with most optical probes are designed to free you from worry about flow as long as there’s sufficient backpressure on the systems to keep them from degassing.

My final thought is to remember that different analyzers have different functions, and you may need more than one to instrument to get full control over your dissolved gasses. You can’t go wrong with a good electrochemical analyzer, but optical sensors give you a different type of flexibility, and might be the next step in a well-rounded instrument collection.

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