31
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 4.7).
