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

(ppb/meter)

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.

Wort – How Much Dissolved Air Will It Typically Hold?

I spent some of last week helping brewers troubleshoot issues with their wort aeration, so this seems like a good time to jot a few ideas about the limiting factors of dissolved gasses.

Gasses dissolve into liquids based upon the ability of the liquid to hold a specific gas. For example, there are hydrocarbons that at room temperature can hold more than 200 mg/L of O2. Compare that to pure water, which holds only 45 mg/L of 100% oxygen gas.  Air dissolved in wort will dissolve in about 1/5 the concentration of pure O2.

Barometric pressure can also have an effect. Imagine you’re injecting air into wort and your brewery is in San Francisco at sea level: The air will hold about 16% more oxygen than if your brewery is on the Colorado Front Range at 5,300 feet, even though the relative percentage of oxygen compared to nitrogen is similar.

Cold liquids have a greater ability to hold gas than warm liquids, so liquid temperature is another important factor in determining dO2 content. The graph below shows the relative concentration of air dissolved in water, based on temperature. The solubility of oxygen in wort is slightly less than water, but as far as I’m aware there is no published literature showing dO2 in wort according to temperature, so that’s why we’re using water as our example.

And here’s the way it looks using the raw data used to generate the graph:

Temp (oC)

dO2 (ppm)

N2 (ppm)

0

14.65

23.51

1

14.24

22.92

2

13.85

22.35

3

13.48

21.82

4

13.13

21.30

5

12.79

20.81

6

12.47

20.34

7

12.16

19.89

8

11.86

19.46

9

11.58

19.05

10

11.31

18.66

11

11.05

18.28

12

10.80

17.92

13

10.55

17.58

14

10.32

17.25

15

10.10

16.93

16

9.98

16.63

17

9.68

16.34

18

9.48

16.06

My final thought comes back to the importance of controlling and closely monitoring those process parameters over which you have control. You may not be able to control barometric pressure, but you can be aware of it. And you do have control over temperature, so that’s another help toward meeting your goal of consistent dissolved gas levels.

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