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Master Brewers Association of the Americas > BREWING RESOURCES > Ask the Brewmaster > Posts > Calibrating DO meters
August 18
Calibrating DO meters

Q: ​How important is the BBT ppb readings on the D.O. sensors? I have been using 99.999% nitrogen which was given to me by Hamilton to zero the sensors. When I look at the specs of the gas, it says that it can have < than 2ppm o2. I am not sure that is good enough if we are trying to zero and read ppb? Even the 99.9999% can still have .5ppm O2? I have a hard time understanding the correlation of O2 in nitrogen (ppm) and O2 in beer (ppb). Is there a calculation I can use to prove this out? Also the “0” calibration and the span “210000ppm 0r 21% O2”  seems like a huge span to then try and read accurately in the ppb range. If I could find it, I would love to get a “standard” D.O in solution of 25ppb to verify calibration.

A: Extremely important - decreasing DO in the BBT is absolutely necessary to produce the best possible beer. Most DO meters for this application are accurate to about +/-1 ppb.

For both gas phase and liquid phase (DO), the O2 content can be expressed in the unit ppm (which is very confusing). However, the unit for gas is ppm vol/vol and the unit for liquid (DO) ppm m/m, so these are completely different units. As rule of the thumb, the conversion of ppm vol/vol and ppm m/m is 20,000:1

Taking your specification of the class 5.0 nitrogen with a purity of 99.999% which still can have < 2ppm O2. These are 2 ppm vol/vol and equals 2/20,000 is 0,0001 ppm m/m = 0,1 ppb m/m DO. This gas is perfectly suited for performing a zero point calibration/verification. There are procedures for creating "liquid zero" standards, but this is less accurate than using the gas standard that you already have.

It may be challenging to create or purchase a 25ppb liquid standard. An option could be requesting a certified (or even better) accredited gas mixture of N2 with an O2 content of 50 ppm vol/vol which equals approx. 25ppb m/m DO (check its availability). During the O2 verification it's important that the gas and O2 meter have a very similar temperature and the gas flow is low, so you do not have pressure build up in the O2 meter.

If you want a more practical, lo-tech opton, many breweries keep an aged beer standard of known DO in house to use as a quick, rough calibration check.

Many thanks to Frank Verkoelen and Jeff Tocio (Pentair Haffmans), as well as Kevin Sudderth (Hach) for their invaluable contributions to this response.

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