25
pH Calibration
Observe the pH mV readings during calibration to understand the condition and response of the pH sensor. In buffer 7, pH mV
values should be between -50 and +50. In pH 4 buffer, the mV reading should be 165 to 185 mV higher than the reading in pH 7
buffer. In pH 10 buffer, the mV reading should be 165 to 185 mV lower than the reading in pH 7 buffer. The theoretically ideal slope
is -59 mV/pH unit.
1-Point Calibration:
While a 1-point pH calibration is possible, this calibration procedure adjusts only the pH offset and leaves the previously determined
slope unaltered. This should only be performed if you are adjusting a previous 2-point or 3-point calibration.
2-Point Calibration:
Perform a 2-point pH calibration if the pH of the media to be monitored is known to be either basic or acidic. In this procedure, the
pH sensor is calibrated with a pH 7 buffer and a pH 10 or pH 4 buffer depending upon the pH range you anticipate for your water to
be sampled.
3-Point Calibration:
Perform a 3-point pH calibration to assure maximum accuracy when the pH of the environmental water cannot be anticipated or
fluctuates above and below pH 7. In this procedure, the pH sensor is calibrated with pH 7, pH 10, and pH 4 buffer solutions.
Additional Calibration Points:
Up to a 6-point calibration can be completed if the buffer set selected under ISE Sensor Setup is set to
Off
. This allows for any
combination of buffers to be used, but the Calibration value must be manually adjusted.
pH Calibration Procedure
Please note calibration can be accomplished in any buffer order, but pH 7 (USA buffer set) or 6.86 (NIST buffer set) should always be
used regardless of the number of calibration points, as this middle buffer determines the pH offset.
First Calibration Point
From the Calibrate Menu, highlight
ISE (pH)
and press enter.
The
Calibration value
will automatically be adjusted based on the selected buffer
and temperature. Alternatively, the Calibration value can be manually entered.
Observe the
Actual Readings
for stability (no significant change for 40 seconds),
then select
Accept Calibration
. “Ready for cal point 2” will be displayed in the
message area.
To complete only a 1-point calibration, select
Finish Calibration
. “Calibration
successful!” will be displayed in the message area at the bottom of the screen and
the handheld will beep before you are returned to the Calibrate Menu.
Figure 41
Calibrate pH
(First point)
03/03/20 10:34:42 AM
Calibrate ISE1 (pH)
100%
Calibration Value [4.00]
Accept Calibration
Finish Calibration
4.34 pH
P.C.V. 4.00 pH
Ready for cal point 2
Press ESC to Abort
Last Calibrated
03/02/20 07:23:20AM
Actual Readings
22.6 Ref °C 155.9 pH mV
