PART FOUR- OPERATING AIDS
SECTION 1 - PRESERVING MEASUREMENT ACCURACY
Rev. 9-898
Model 692P
-45-
PART FOUR - OPERATING AIDS
SECTION 1 - PRESERVING MEASUREMENT ACCURACY
1.1 Keeping Sensor Clean
1.2 Keeping Instrument
Calibrated
1.3 Checking Sensor
Slope
1.4 Avoiding Ground Loop
Errors
1.5 Avoiding Electrical
Interference
1.6 Checking Buffers for
“Two-Key” Table
Method of pH
Calibration
Clean the sensor as required using the recommended proce-
dure described in the sensor operating instruction manual.
Calibrate the instrument using one of the methods de-
scribed in Part Three, Section 6. Measurement errors may
be caused by calibrating with old, contaminated, or diluted
pH buffers. Do not reuse buffers. Never pour used buffer
back into its bottle -- always discard it. Note that the pH
value of a buffer will change slightly as its temperature
changes. Therefore, when calibrating always allow the tem-
peratures of the sensor and buffer to equalize.
The “slope” of a pH sensor is an indicator of its performance.
After completing any two-point calibration, you can display pH
sensor slope with the SLOPE function. The 0-100% slope
reading corresponds to 0-59.2 mV/pH. A new sensor typically
has a “slope” near 100%. When it decreases below 80%, the
sensor probably requires cleaning or replacement. For best
slope reading accuracy, calibrate the millivolt input using the
two-point method (Part Four, Section 2.2).
The instrument may be affected by a “ground loop” electri-
cal problem, typically caused by moisture in a junction box.
This condition provides a conductive pathway from the sen-
sor connections to earth ground. Keep connections dry and
corrosion-free to prevent a ground loop from occurring.
Do not run the sensor cable in the same conduit with line
power. Excess cable should not be coiled near motors or
other equipment that may generate electric or magnetic
fields. Cut cable to proper length during installation to avoid
unnecessary inductive pickup (“electrical noise” may inter-
fere with sensor signal).
When using the “two-key” table method for pH calibration,
make sure the buffers you use are of the same formulation
as those selected from Table E. This provides best accu-
racy. To verify this, find the corresponding buffer value table
on the following page and compare its pH-versus-
temperature data to the data on the buffer bottle. Matching
data confirms that the buffer is of the same formulation.
