WEF Discussion Forums
Laboratory Management and Technical Issues
BOD blank depletion: 0.2 or 0.20?
We were having a discussion in our municipal wastewater lab about this...
The 18th and 20th Editions of Standard Methods use 0.2 mg/L as the limit for blank depletion in a BOD or CBOD test.
The 21st Edition uses 0.20 mg/L as the limit.
Perry's wonderful book A Bug's Eye View also uses 0.2 mg/L.
I was sure this had been discussed before -- perhaps it was in the old forum -- but I'm wondering how many sig figs folks use when assessing blank performance in the BOD/CBOD test. Did SM mean to provide clarification with the additional sig fig in the 21st edition?
Thanks in advance for the feedback!
Right... with the extra digit your blanks would have to be 0.20 or less.
Is a blank of 0.24 acceptable to auditors for a lab using the 18th Edition?
It is Perry's conclusion that the Standard Methods BOD committee intended that two significant figures be used when they specified the 0.20 limit and that it wasn't an accident that it changed from 0.2 to 0.20. The next edition of his book will use 0.20 rather than 0.2.
That extra significant doesn't have much do with protecting the environment, but it can have an impact on the lab's ability to check analytical performance. Every little thing you do to minimize variables, and accepting 0.204 as your limit rather than 0.24 is little, will make your overall precision better, resulting in a lower standard deviation for the GGA test, something inspectors like to see, and something you should strive for.
Hey thanks so much for the replies!