With one week of use I retest the calibration of the HM Digital pH-80, during this week used the meter to adjust 12 jars of 6 liters for flushes...
Retest with calibration solution 7.0 -> 7.1 (before calibrating to 7.0 wanted to test with another solution)
Retest with calibration solution 4.0 -> 4.0 (went back to test with 7.0 solution and 7.1... went back to test with 4.0 solution and was right, 4.0...
... after adjusted the pH of a 6L jar from 7.6 to 6.5 and went back to test with the calibration solution.
Retest with calibration solution 7.0 -> 7.0
Retest with calibration solution 4.0 -> 4.0
I think the problem is in the 7.0 solution, was done with the powder that came with the meter, joined the powder to 100ml of distilled water, the brand says it can be stored for 2/4 weeks and used to make 5 tests, each test with 20ml.
I'll be back in one week to retest the meter with the calibration solutions and to update the topic with the results.