My last couples of posts have dealt with sensor validation. I recently did some coaching on inline oxygen sensor placement, so I’d like to continue that discussion with those examples. One placement was pre-and-post centrifuge in a regional brewery and the other was in a process pipe flowing at about 500 gallons-per-minute. While the flow rate may make a difference in how far a sensor needs to be from pumps and other interferences, the same techniques can still be used to determine sensor placement.
The key to great dissolved gas measurement is making sure to measure in places where all of the gas is in solution. The same validation tips used for double-checking instrumentation can help you decide where to place an inline analyzer. In general we can say that if you measure close to a known – or at least probable — source of air ingress and yet can’t detect the full impact, but measure farther down the process and find higher values, then you probably want to place your sensor further away from that source.
Let’s talk first about the pre-centrifuge application. Since a centrifuge creates a small amount of vacuum, I don’t recommend putting a sensor immediately before it. Where there is any type of a vacuum, there is the possibility of a false low reading, especially if the vacuum can cause any foaming or bubbles forming in the beer due to degassing. If possible, I’d place a sensor where I’m fairly sure there is no suction on the beer.
You can get a false low reading post centrifuge as well, but for different reasons. The first is that any dO2 that ingresses via the centrifuge may not be fully dissolved in the beer in a short run to a sensor. The second is that any CO2 forced out of solution in the centrifuge may lower the dissolved O2. (The CO2 will carry O2 out with it, but then they will both reabsorb farther down the line.) In both cases, you can do better by placing the sensor at a distance post centrifuge.
Validation of inline sensor placement is easy if you use a portable instrument to help, even if you are measuring over a long distance. First take readings with both the inline and the portable at the farthest reasonable sample port from your Bright Beer Tank. If you get similar readings on both, you’ll know that your inline is correct and that you can trust both instruments. Then use your portable to take another sample at a port just before your bright beer tank. If there is more dO2 than you found at your inline placement, then you may have an ingress problem that needs correcting. (You may be tempted to test right in your BBT, but don’t. Tank filling nearly always results in pickup or loss of DO2, so measuring for that variable is a whole different matter.)
The advice I gave to the customer who was evaluating the 500 gallons-per-minute process pipe was similar, and can likewise be applied to most placement situations. Here’s a summary of my basic guidelines for the best inline probe locations:
– As far as possible from the outlet of pumps
– At least five pipe diameters from bends
– As far as possible from CO2 or O2 injection
– Never in a descending pipe
My final thought is to always validate an inline sensor location if possible. Remember to look for the places in your process where all of the gas in your beer is in solution: that’s usually the best place to put a sensor.