I got every thing out today to add to the discussion.Milwaukee MW 101 meter. Adjusted temp to 20C (68F).

New General Hydroponics solutions calibrated to 7.00 and 4.00

Aldon corp solutions, maybe 3 years old. 6.85 and 4.08

Milwaukee solutions exp date 9/2016. 6.80 and 4.92

Really old Milwaukee sachets unopened exp 3/2019. 6.83 and 3.99

Recheck on calibration with new General Hydroponics solutions 7.01 and 3.99

This is what i just measured. My conclusion is to buy solutions more often.