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4.4 SINGLE-POINT pH CALIBRATION
MODEL 54e pH/ORP SECTION 4.0
CALIBRATION
A single-point calibration (standardization) should always be performed
following a two-point calibration. It may have to be performed as often as
once a day to ensure that the sensor is accurately reading pH. As the
glass electrode or the porous reference plug fouls and ages, the pH read-
ing will drift. The single-point calibration re-establishes the exact pH read-
ing. Experience with your process and it's acceptable range of pH will dic-
tate how often the single-point calibration will have to be performed. The
procedure is most easily performed on-line by determining the pH of a
process grab sample with a laboratory instrument. This value can then be
entered into the Model 54e pH/ORP to make it agree with the lab instru-
ment.
Many processes have effects on on-line sensors that cause them to read
slightly different than grab samples measured with laboratory electrodes.
Standardizing the controller allows both readings to agree. This proce-
dure does not calculate the sensor slope.
1. Take a grab sample that is as close to the sensor as possible.
2. Using a calibrated pH instrument with automatic temperature com-
pensation, determine the pH of the process or grab sample (as close
to actual process temperature as possible). Continue with this pro-
cedure if an adjustment is needed.
3. From the main display, press any key to obtain the main menu. With
the cursor on "Calibrate", press Enter (F4).
NOTE
The Hold Mode screen may appear if the feature was enabled in
section 5.6. Changing the Hold Mode to ON holds the outputs in
a fixed state, and avoids process upsets during calibration.
Remember to change the Hold Mode back to OFF when calibra-
tion is completed.
4. Move the cursor (using the down arrow key) from "Buffer Calibration"
(or 2-pt calibration) to "Standardize" and press Enter (F4).
5. The pH reading in large numbers is the live process pH reading. The
next line displays the pH reading when this screen was first
accessed. Press Edit (F4) to perform the standardize.
Use the arrow keys to change the second line standardize value to
the correct pH and press Save (F4) to complete the procedure. Esc
(F3) will cancel.
The pH reading in the large display will change to the new value and
the zero offset number will be recalculated.
NOTE
Before exiting the calibration mode, remember to change the
hold mode setting to OFF (if it was turned on in step 3).
Buffer calibration
Standardize
Adjust temperature
Exit Enter
4.01 pH
Standardize: +07.00 pH
Esc Save
7.01 pH
Standardize: 7.01 pH
Exit Enter
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