Correlating Caustic “Air” Measurements with TPO – Theory vs. Practicality

This is the second of a two-part post on caustic “air” measurements.

In my last post I showed how it is possible to have two similar “air” readings in packages, even though the Total Package Oxygen (TPO) was wildly different. It this post we will examine the theoretical basis for the differences and dive a bit more deeply into why the correlation between “air” and TPO is hit-and-miss.

A couple of years ago I worked with a soft drink manufacturer. They wanted to see if they could correlate the dissolved oxygen content of their packages with “air” testing. They collected hundreds of individual data points, taking two different packages off a canning line at a time and running an “air” test on one while measuring dissolved oxygen concentration on the other.  Their results were confusing, so I thought that maybe by plotting the pairs of data – with the “air” results on one axis and the dO2 value on the other — things might make some sense.  What shaped up instead was a fan-shaped blob.

At first I wanted to blame measurement technique or instrumentation malfunction. But the more I tried to make sense of the data, the more I decided there had to be something real I just didn’t understand. It then dawned on me that what the data represented was a combination of “air” trapped in the headspace and “air” coming from the liquid. My next move was to plot how the data would look if 100% of the “air” were originating in the headspace, versus 100% of the “air” originating in the liquid.

The calculations for the headspace were simple. We know that “headspace air” is about 20% O2 and 80% N2. The liquid calculations required a bit more consideration, because if 100% of “air” originates in liquid, then solubility laws will result in air that has about 37% O2 and 63% N2.

Once I had calculated the numbers, I arrived at the following conclusions. At 20 oC, every mL of “headspace air” will contribute about 0.78 ppm (780 ppb) of TPO to a package. At the same temperature, every mL of “dissolved air” will contribute about 1.57 ppm (1570 ppb) of TPO to a package. (Note: although we were measuring “headspace air” and “dissolved oxygen,” at this point I calculated out to TPO because doing that always gives a more accurate picture.) I then plotted this theoretical difference to see how the two lines would fall. Here is the graph:

The area of the graph between the two lines shows all of the theoretical possibilities for valid TPO concentrations based on a given air concentration.  For instance, if the “air” volume is 1.0 mL, the TPO will range between 0.78 ppm and 1.57 ppm.

I wish the reality of these theoretical values could be easily applied to actual package situations, but in my experience the “air” tester rarely picks up all the gases, so it almost always underestimates TPO. The difference seems to depend whether the majority of the “air” is trapped in headspace or is dissolved in the liquid.

My final thought is that “air” measurements can be helpful in certain situations and are always better than not measuring at all, but to seriously zero-in on problems, dissolved O2 measurements (calculated to TPO) are the way to go.

Frivolous Friday Fun – The History of the Pull Tab

If you remember the days of pull-tab necklaces, then you might like this follow-up to last week’s link about a revival of the flat topped can: It’s a link to the history of the pull-tab can end. Thanks to my colleague JP for this idea and last Friday’s fun as well!

Sample Oxygen Contamination from Leaking Valves & Fittings

I occasionally get questions about high O2 values that point back to ingress through fittings, valves or tubing. Just this week I spoke to two brewers with these issues. One involved a sample valve on a fermenter that was leaking air into the sample stream. The other had to do with fittings on a package piercer that were leaking air during package O2 measurements.

The issue with the valve on the fermenter was a pretty common one, so let’s talk about how we sorted out the problem. This brewer had a fermentation vessel that wouldn’t go below 200 ppb, no mater what they tried. The first line of thought was that there might be something wrong with the O2 monitor, but it was working well on their other fermenters and bright tanks, so it was easy to rule it out. Next thought was that there might be a problem with the fermenter itself, but based on the level of CO2 leaving the tank it seemed that fermentation was proceeding nicely. So by process of elimination, we zeroed in on the valve.

Checking air leaks between a source of beer and a portable analyzer is fairly simple.  First run the sample at your usual flow rate. Then increase the flow and check to see if the oxygen reading decreases.  If it does then the issue is most likely with a valve, fitting or the tubing used to take the measurement.

As liquid moves through tubing it will pull a small vacuum – “the venture effect” — on any tiny opening that might not be large enough to leak liquid, but will still leak gas. The same thing holds true for liquid flowing through pipes: A small hole will pull air into the liquid. Since the amount of gas that gets pulled into the liquid is not proportional to the flow rate, increasing the flow will pull in less air per volume of liquid and the concentration of gas migrating into the sample will decrease. So if you increase your flow and your dO2 concentration drops, then there’s a good chance that oxygen may be migrating through the fittings or valves.

There is one important thing to watch out for when you try this: You don’t want to increase the flow too much, or you’ll get degassing in the flow chamber of the instrument around the sensor, and that will also show a decrease in O2. To prevent this, first flow your beer at the minimum recommended flow rate of the instrument and then don’t increase the flow beyond the maximum flow rate.  If you are unsure, ask your instrumentation manufacturer.

There is one other thing that can cause similar issues, and that’s the polymer tubing used to deliver the sample to the instrument.  Plastics like Teflon®, otherwise known as PTFE or PFA, have very high oxygen transmission rates through the walls of the tubing.  If you don’t use the tubing that’s supplied with your instrument, then check the tubing specs or you may wind up with O2 ingress. The table below shows the oxygen transmission rate into different polymers.

Polymer Material O2 Pickup in water


Polyvinylidene chloride (Saran) 0.02
Nylon 0.03
Polychloro trifluoroethylene (Kel-F) 0.05
Polyvinyl fluoride (Tedlar) 0.05
Polyvinylidene fluoride (Kynar) 0.1
Polyethylene Terephthalate (Mylar) 0.12
Polyvinyl chloride (non-plasticized) 0.14
Polyacetal (Delrin) 0.2
Ethylene/Monochlorotrifluoroethylene copolymer (Halar) 0.43
Ethylene/Tetrafluoroethylene copolymer (Tefzel) 1.70
High density polyethylene (opaque) 2.04
Polypropylene 5.3
High density polyethylene (clear) 3.9
Polycarbonate (Lexan) 5.1
Polystyrene 5.3
Low density polyethylene 8.5
Fluorinated ethylene/propylene (FEP) 13
Tetrafluoroethylene (PTFE) 19
Natural rubber (Latex) 60
Silicone rubber (Silastic) 1700

Saran is a registered trademark of Dow Chemical. Kel-F is a registered trademark of 3M. Delrin, Mylar, Tedlar, and Tefzel are registered trademarks of DuPont. Kynar is a registered trademark of The Pennwalt Corporation. Halar is a registered trademark of Ausimont U.S.A., Inc. Lexan is a registered trademark of General Electric.

My final thought is to always be on the lookout for problems and run through some testing when things don’t add up. A few simple tests can help you sort out whether you have issues in your system or if it’s time for monitor maintenance.

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