Validation Techniques for Optical Dissolved Oxygen Sensors in Beer

I’ve recently spent a few posts discussing measurement validation, mostly with a focus on situations where instruments were reading properly and there was a problem somewhere in the brewing process. Now I’d like to continue the discussion with an example of the reverse: the beer was actually okay, but sensors were in need of validation and calibration.

Last week I visited a brewery where they were getting unusually high portable dO2 measurements on beer that had been fermenting for a week. After a day of sitting in a conical fermenter, beer usually has a dissolved oxygen concentration of less than 5 ppb, but in this case their optical oxygen analyzer was reading 30 ppb. The brewer was wondering if there was an issue with his fermentation, so here’s what we did:

  • We measured the beer in the fermenter with a second instrument that we could easily calibrate and validate, should the reading fall outside the expected range.
  • Then we measured 99.999% N2 calibration gas on both instruments.

Here’s the reasoning behind these steps:

  • Optical oxygen analyzers will drift upward over time due to photo bleaching of the fluorescent matrix.
  • Using a 99.999% N2 or CO2 calibration gas to check the zero of the instrument is a way to validate low-level (less than 200 ppb) measurements. It is important that the pressure of the gas flowing through the instrument be as low as possible. (Do this by controlling the gas flow at the inlet to the instrument and opening the analyzer flow valve completely. Ideal gas flow should produce about 2-5 bubbles per second when the outlet tube from the analyzer is placed in a few inches of water.)
  • If instrument readings using calibration gas are above 3 to 5 ppb, then it’s probably time to calibrate the instrument.

In this case we discovered that both instruments needed calibration. On the beer, one was reading 8 ppb and the other 30 ppb. On the calibration gas, the first instrument read 3.5 ppb and the second read 16 ppb. We didn’t proceed to calibrate right then, but knew that once they were calibrated they would agree.

My final thought is to validate whenever a reading does not seem logical. In this case, the brewer was wondering if there was an issue with his fermentation, but it wasn’t his process, it was the need for instrument maintenance.

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