23 Glossary
Slope of the pH sensor
The slope or sensitivity of a pH sensor is
defined as the quotient of the voltage
emitted in mV per pH level. In theory, a
pH sensor should generate a voltage of
+59.16 mV per pH level at 25 °C. In time,
the slope flattens off, initially slowly but
later more quickly. That is why it is impor‐
tant that this change is compensated for
during calibration. As with zero point com‐
pensation, slope compensation has to be
repeated at specific intervals depending
on the application.
Zero point of the pH sensor
The zero point of the pH sensor is the pH-
value at which the sensor potential is
0 mV.
Asymmetric potential of a pH sensor
The asymmetric potential of a pH sensor
is the potential difference that occurs
when the pH sensor is immersed in a sol‐
ution, which corresponds to the internal
electrolytes. Ideally the potential differ‐
ence is 0 mV.
Glass break detection
[ON]
/
[OFF]
: Switches the pH sensor's
glass break detection
[ON]
or
[OFF]
. The
factory setting is
[OFF]
. The controller
shows a fault message when a fault is
detected when the setting is
[ON]
.
The glass break detection monitors the pH
sensor to detect whether the pH sensor's
pH-sensitive glass membrane is broken.
In the event of the glass membrane
breaking, the pH sensor's resistance
becomes smaller, approx. 2 mega-Ω . The
controller can analyse this change in
resistance. The controller emits a fault
message and control is stopped. This fault
cannot be acknowledged.
The pH sensor's glass membrane also
has low resistance when the process tem‐
perature increases. If the process tem‐
perature is approximately > 60°C, the
detection threshold of 2 mega-Ω has been
reached. At process temperatures of
> 60°C, glass breakage is detected
although no glass has broken. In order to
avoid a false alarm, switch off glass break
detection at process temperatures of
> 60 °C .
Glossary
169
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