27
O&M Manual
Rev-B (7/15)
Part 5 – Calibration
5.1
Overview and Methods
Since the sensor slope (mV/pH output) will degrade over time, the instrument must be calibrated
periodically to maintain a high degree of measurement accuracy. Frequency of calibration must
be determined by the application. High temperature applications or applications involving
extreme pH operating conditions may require more frequent calibration than those that operate at
more neutral pH levels and ambient level temperatures. It is important for the user to establish a
periodic cleaning and calibration schedule for sensor maintenance to maintain high system
accuracy.
Before calibrating the instrument for the very first time after initial installation, it is important to
select the proper operating parameters in the configuration menus for items like Sensor Type and
Auto Buffers.
If Auto Buffers is not enabled, select buffers with values that are close to the normal operating pH
of the process. For example, if the process is operating normally at 8 pH, buffer values of 9.18
pH and 7.00 pH are preferred over buffers of 4.00 pH and 7.00 pH. If possible, select one of the
buffers to be near 7.00 pH.
NOTE:
Buffers must be at least 2 pH units apart to ensure accurate
calibration
.
The system provides two methods of pH calibration: 2-point and 1-point. These two methods are
significantly different. See Sections 5.13 and 5.14 for a brief description of their uses.
5.11 Sensor Slope
The sensor slope is a number (expressed as a percentage) which represents the current
condition of the sensor electrodes. The slope display is updated after every calibration. When
new, the sensor slope should be between 95% and 105%. A 100% slope represents an ideal
sensor output of 59.16 mV/pH, from standardization (7.00 pH at 25°C). Over time, the glass
electrodes in the sensor will age with use. This results in a reduction of the slope (mV/pH output)
of the sensor. Thus a sensor slope of 85% is equivalent to an output of 50.29 mV/pH from
standardization. The instrument will not allow calibrations on a sensor with a slope less than
80%. The slope information from the most recent calibration can be viewed at any time in the
Measure Menu (see Section 4.62).
5.12 Sensor Offset
Sensor offset is a number that indicates sensor output (expressed in mV) in 7.00 pH buffer at 25
ºC. Ideally, the sensor will output 0 mV under these conditions. A sensor offset reading of +10
mV indicates that the sensor will 10 mV when placed into a perfect 7.00 pH buffer at 25
ºC. In other words, sensor offset shifts the entire mV/pH curve up or down. Sensor offset is
generally produced by a small voltage drop at the sensor reference junction. Large offsets are
most typically the result of foulants on the reference junction, an aged reference junction, or a
weak reference fill solution. The instrument does not allow calibrations on a sensor with an offset
greater than +90 mV or less than
–90 mV. Sensor offset information from the most recent
calibration can be viewed at any time in the Measure Menu (See Section 4.62).
