Emerson Rosemount FCL-01 Reference Manual page 66

Calibration
October 2023
Standardization
User entered
slope and offset
Stability setting
5.4.3 Autocalibrate pH sensor
Procedure
1. Obtain two buffer solutions.
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The pH measured by the transmitter can be changed to match
the reading from a second or referee instrument. The process of
making the two readings agree is called standardization. During
standardization, the difference between the two pH values is converted
to the equivalent voltage. The voltage, called the reference offset, is
added to all subsequent measured sensor voltages before they are
converted to pH. If a pH sensor is buffered, then standardized and
placed back in the buffer solution, the measured pH will differ from the
buffer pH by an amount equivalent to the standardization offset.
If the slope and offset are known from other measurements, they can
be directly entered into the transmitter. Enter the slope as a positive
number corrected to77 °F (25 °C). To calculate the slope at 25 °C from
the slope at temperature t °C, use the equation:
To calculate the offset, use the following equation. The offset can be
either positive or negative.
During automatic calibration, the transmitter measures noise and
drift and does not accept calibration data until readings are stable.
Calibration data will be accepted as soon as the pH reading is constant
to within the factory-set limits of less than 0.02 pH change in 10
seconds. The stability settings are programmable.
Ideally, the buffer pH values should bracket the range of pH to be measured.
Figure 5-2: Calibration slope and offset
A. Slope = △ mV/ △ pH
B. Buffer 1
C. Buffer 2
D. Offset
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