I did it last week when it was reading 28ppt and afterwards the same tank read 36 ppt.
It slowly starts dropping and today it was at 28 ppt again so I re calibrated it just now and it reads 37 ppt.
I've noticed that it has a heavy deposit of scaling on it.
This is probably due to the fact that the sump (where the probe is located) is where my ATO water (mixed with Kalkwasser) is distributed.
Could the scaling be effecting the probe reading?
Can I soak the probe in vinegar & water to remove the scaling or will the vinegar effect the probe's sensitivity?
Or could there be another reason for the drifting reading?

You can see at the end of the chart it was low (28) then I pulled the probe out re-calibrated and then it jumped up (37)