AW T4 20 |
U N I V ER S A L 4 - W I R E , D UA L- I N P U T T R A N SM I T T ER | O I/AW T4 2 0 - EN R E V. B
97
Appendix A pH solution coefficient
The solution coefficient compensates the Nernstian value for
pH measurements, and the raw voltage value for ORP
measurements, by a fixed value per each 10 °C (18 °F).
The temperature compensation factor is derived from the
following equations:
pH
indication
= pH
n
ernstian
±COEF ×
((T –25 °C)/(10 °C [18 °F]))
mV
indication
= mV ±COEF ×
((T – 25 °C)/(10 °C [18 °F]))
where:
COEF
pH or mV change per 10 °C (18 °F).
pH
n
ernstian
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 [18 °F])
pure water with 1 ppm ammonia = +0.31 pH/(10 °C [18 °F])
The solution coefficient for the AWT420 transmitter either adds
or subtracts a configured amount of the process variable per
10 °C (18 °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 analyzer types in the same manner as the pH
analyzer type.
