PART THREE - OPERATION
SECTION 6 - pH CALIBRATION
Model 692P
Rev. 9-898
-38-
6.5 “Two Key”
Table Method
to confirm entry was made and returns to the pH meas-
uring mode.
NOTE:
If display flashes “HI/LO SLOPE ERROR” or
“HI/LO ERROR,” an incorrect buffer value may
have been used or the sensor may be dirty or
defective. If the correct buffer value was used,
calibrate the instrument using the conventional
two-point method (Section 6.3) to check the
sensor slope which is based on the last suc-
cessful calibration. Refer to Part Four, Section
1.3 for details on checking sensor slope.
4. Remove sensor from lower value buffer. Rinse sensor in
clean water and place
in the higher value buffer
.
5. Press
BEGIN CAL
key to initiate calibration of the sec-
ond calibration point.
6. When the pH and
°
C (or
°
F) indicators are both on (not
flashing), press recessed
END CAL
button using a slen-
der tool. See step 3 for additional details.
This completes the “two-key” two-point pH calibration.
Any buffer has some variation of pH value with temperature,
but few operators take the time to measure the buffer tem-
perature with the sensor at equilibrium, and then enter the
pH of the buffer at that temperature. The basis for this “two-
key” table method is that the 692 has the built-in pH value-
versus-temperature curve for a set of buffers and monitors
the temperature and time response when the sensor is put
into a buffer. Using this method provides the most accurate
calibration.
Use the conventional two-point method (Section 6.3) to ini-
tially calibrate the instrument, including when the pH sensor
has been replaced. Thereafter, you can use the “two-key”
table method, but first a qualified person must:
n
Set the instrument for this method.
n
Select and enter one or two pH buffer values from the
buffer table to be used as the calibration point(s).
Thereafter, the instrument can be periodically calibrated
using the procedure described under the “Performing Cali-
bration” subheading beginning on page 40.
