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|
|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|
|Ethylene/Monochlorotrifluoroethylene copolymer (Halar)||0.43|
|Ethylene/Tetrafluoroethylene copolymer (Tefzel)||1.70|
|High density polyethylene (opaque)||2.04|
|High density polyethylene (clear)||3.9|
|Low density polyethylene||8.5|
|Fluorinated ethylene/propylene (FEP)||13|
|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.