Stratos Multi E401N
99
6.2 Process Variable pH Calibration/Adjustment
Note:
During calibration, the function check (HOLD) operating state is active for each of the calibrated
measuring channels. The outputs respond as configured.
• Calibration: Detect deviations without readjustment of calibration data
• Adjustment: Detect deviations with readjustment of calibration data
NOTICE!
When using analog sensors, adjustment is required following sensor replacement.
pH Calibration/Adjustment Explanations
Each pH sensor has a separate zero point and a separate slope. Both values change as a result of aging
and wear. The voltage supplied by the pH sensor is corrected by Stratos Multi for the zero point and
the electrode slope of the pH sensor, and displayed as the pH value.
During calibration, the sensor’s deviation is initially determined (zero point, slope). The sensor is
immersed in buffer solutions with a precisely known pH value for this purpose. Stratos Multi measures
the voltages of the sensors and the temperature of the buffer solution, using this information to
calculate the sensor’s zero point and slope.
Calibration values determined during calibration
Zero Point
The pH value at which the pH sensor supplies the voltage 0 mV. The zero point is different for each
sensor and changes with age and wear.
Slope
The slope of a sensor is the voltage change per pH unit. With an ideal sensor, this is -59.2 mV/pH.
Temperature
The temperature of the process solution must be logged, since the pH measurement is tempera-
ture-dependent. Many sensors feature an integrated temperature probe.
There are limits that are calculated during calibration when monitoring glass and reference imped-
ances. The following limits apply to standard glass electrodes:
• Temperature range: 0 ... 80 °C/32 ... 176 °F
• Impedance range: 50 ... 250 MΩ (at 25 °C/77 °F)
