AW T 210 |
2- W I R E CO N D U C T I V I T Y, P H/O R P, P I O N T R A N SM I T T ER | O I/AW T 2 10 - EN R E V. C
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Appendix A – Temperature compensation
NOTICE
• At power up the AWT210 automatically detects if a valid
temperature sensor is used within the system. If a
temperature sensor is not detected, the
Temperature
Comp . Type defaults to Manual (see page 28) and a
diagnostic alarm (--S010.047--, see page 22) is
displayed.
• The temperature effect on ORP sensors is negligible. The
effect of temperature on pION sensors is difficult to
characterize, except for specific applications. Therefore,
only the solution coefficient option can be used to
compensate for electrode and process changes with
temperature.
Manual and automatic Nernstian
temperature compensation types
When the selected
Temperature Comp . Type is Manual or Auto
(see page 28), manual and automatic Nernstian temperature
compensation types are applied (see below).
These adjust for the thermodynamic properties of
electrochemical half sensors. The Nernstian effect is
characterized by the mathematical equation:
E = E
reference
+(2.3 × R × TK × 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
[ai]
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 AWT210 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.
Solution coefficient compensation type
When the selected
Temperature Comp . Type is Auto Solution
(see page 28), the solution coefficient compensation type is
applied.
This compensates the Nernstian value for pH measurements
and the raw voltage value for ORP or pION measurements by a
fixed value per each 10 °C (50 °F). The temperature
compensation factor is derived from the following equations:
pH
indication
= pH
Nernstian
±COEF × ((T –25 °C)/(10 °C [50 °F]))
m
Vindication
= mV ±COEF × ((T – 25 °C)/(10 °C [50 °F]))
where:
COEF
pH or mV change per 10 °C (50 °F)
pH
Nernstian
Nernstian pH value referenced at 25 °C (77 °F)
after applying the factory and process
calibration values
pH
indication
pH value indicated on the transmitter and
proportional to the current output value
mV
millivolt value of the sensor output after
applying the factory and process calibration
values
mV
indication
mV value indicated on the transmitter and
proportional to the current output value
T
temperature of the solution in °C after
applying the factory and process calibration
values
Examples of solution coefficients for pure water applications
are:
pure water = +0.18 pH/(10°C [50 °F])
pure water with 1 ppm ammonia = +0.31 pH/(10°C [50 °F])
The solution coefficient for the AWT210 transmitter either adds
or subtracts a configured amount of the process variable per
10 °C (50 °F) to the Nernstian compensated process variable.
Thus, an application with a process liquid that decreases in its
pH value as the temperature increases uses a positive solution
coefficient correction factor. Conversely, an application with a
process liquid that increases in its pH value as the temperature
increases uses a negative solution coefficient correction factor.
The solution coefficient affects the uncompensated process
variable for ORP and pION analyzer types in the same manner
as the pH analyzer type.
