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The choice of the second buffer depends on your application: if you measure values below pH 7, use pH 4 as a second buffer. If you measure values above pH 7, use pH 10 as your second buffer. Some of our pH meter use 3 points calibration, keep in mind to always start with pH 7 first and others after.
I set off to find out why. I discovered why. I also discovered why meter accuracy is both more, and less, critical than we think. Four different meters ranged from 121 mg/dl (6.7 mmol/l) to 158 mg/dl (8.7 mmol/l) in one test taken at the same time. (Ignore dates on meters, I rarely use the date function.)