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4.2 AUTOMATIC TWO-POINT CALIBRATION
MODEL 54e pH/ORP SECTION 4.0
CALIBRATION
The two-point calibration is performed when the controller is initially
installed and whenever elements of the sensor assembly or controller are
replaced. The two-point calibration re-establishes the slope of the elec-
trode. This is necessary because the slope (mV/pH change) decreases
as the glass pH electrode ages.
Buffer calibration uses measurements of two solutions to calculate the
slope (efficiency) and the zero offset of the pH sensor.
Obtain two pH buffer solutions with different pH values. They should be
at least 2 pH units apart. Unopened buffers have a shelf life of about a
year and should generally not be reused because of possible contamina-
tion.
Before taking readings, clean and rinse the sensor (if necessary). Shake
the sensor down to remove air bubbles from the glass electrode tip. Place
the sensor into the first buffer solution. Verify and adjust the temperature
(Section 4.1), if necessary.
The controller is set at the factory for automatic calibration. If this feature
has been disabled (Section 5.9), see Section 4.3 for manual calibration.
Auto calibration includes automatic buffer recognition (factory set buffers
are 4.01, 7.00, and 10.01 pH) and a stabilization check. The stabilization
feature eliminates errors caused by changes in temperature and
response time of the glass electrode. To change these settings, refer to
Section 5.9.
1. From the main display, press any key to obtain the main menu. With
the cursor on "Calibrate", press Enter (F4). With the cursor on "Buffer
calibration", press Enter (F4) again.
NOTE
The hold mode screen (top left) will appear if the hold mode was
enabled in Section 5.6. Activate hold mode by pressing Edit (F4),
using the arrow key to change Off to On, and then pressing
Save(F4). The hold mode holds the outputs and relays in a fixed
state to avoid process upsets to a control system. To leave the
hold mode in it's current state, press Cont (F3).
2. With the sensor in the first buffer, press Cont (F3). To cancel the cal-
ibration, press Abort (F1).)
The word "Wait" flashes until the sensor is stabilized.
The controller is waiting for the pH reading to stabilize within the
parameters established in Section 5.9. If the controller appears to be
locked at this stage, the reading is not stable enough. Increase the
stabilize pH and/or decrease the stabilize time and retry the calibra-
tion.
Press Abort (F1) to cancel the calibration if it appears to be taking too
long.
7.00 pH
Hold Mode: Off
Exit Cont Edit
4.02 pH
Buffer 1 -- “Wait”
Abort Cont
4.02 pH
Autocal buffer 1
Abort Cont
Buffer calibration
Standardize pH
Adjust temperature
Exit Enter