Endura APA592
pH / Redox (ORP) transmitter
Appendix A
98
OI/APA592–EN Rev. E
A.3 Nernstian Temperature Compensation
Manual and automatic Nernstian temperature compensation types adjust for the thermodynamic properties
of electrochemical half sensors.
The Nernstian effect is characterized by the mathematical equation:
E=E
reference
+(2.3 × R × T
K
× LOG[ai] /n × F)
where:
E
Overall sensor output
E
reference
Reference half sensor output
(typically a constant)
R Constant
TK
Absolute temperature (Kelvin)
n Ion
charge
F Constant
[a
i
]
Ion activity
The ion activity is nearly equal to the ion concentration for weak solutions containing that particular ion. The
Nernstian equation is used to adjust the output of an electrochemical sensor to a reference temperature
that is typically 25 °C (77 °F).
Temperature effects of pH sensors are well behaved and are characterized by the Nernst equation. The
APA592–PH transmitter applies Nernstian compensation to all three temperature compensation options
when the transmitter is configured as a pH analyzer. If interested in the uncompensated value, set the
transmitter to manual temperature compensation and calibrate the temperature to 25 °C (77 °F). This
enables monitoring of the uncompensated value.
Automatic Nernstian temperature compensation provides the most useful information and is recommended
in most cases. Since ion dissociation is affected by temperature, the pH value can also be affected. If these
processes behave in a repeatable manner, the dissociation can be characterized and a solution coefficient
can be used to compensate for these effects.
