wtw inoLab pH/Oxi Level 1 Operating Manual page 35

Probe evaluation After the calibration, the instrument automatically evaluates
the current status of the probe using the relative slope. The
relative slope does not affect the measuring accuracy. Low
values indicate that the electrolytic filling will soon be
depleted and the probe will have to be regenerated.
Display
E3
Perform error elimination
according to chapter 6
W ...
HAT TO DO IF
Relative slope
S = 0.8 ... 1.25
S = 0.7 ... 0.8
S = 0.6 ... 0.7
S < 0.6 or S > 1.25
Operation
99