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7.5 AUTO CALIBRATION
7.5.1 Purpose
1. New sensors must be calibrated before use. Regular recalibration is also necessary.
2. The use of auto calibration instead of manual calibration is strongly recommended. Auto calibra-
tion avoids common pitfalls and reduces errors.
3. For more information about calibration in pH measurements and the use of buffers, refer to
Section 13.7, Buffers and Calibration.
7.5.2 What Happens During Auto Calibration?
1. The transmitter displays prompts that guide the user through a two-point buffer calibration.
2. The transmitter recognizes the buffers and uses the temperature-corrected pH value in the cali-
bration. The transmitter also measures noise and drift and does not accept calibration data until
readings are stable. Stability limits are user-programmable. See Section 9.7, Buffer Calibration
Parameters.
7.5.3 Use of Calibration Standards (buffers)
1. A pH measurement is only as good as the calibration, and the calibration is only as good as the
buffers used. A careful buffer calibration is the first step in making an accurate pH measurement.
2. Calibrate with buffers having pH values that bracket the pH of the process. For example, if the pH
is between 8 and 9, calibrate with pH 7 and 10 buffers. Commercial buffers for intermediate range
pH are readily available. Buffers outside the range pH 3.0 to pH 10.0 may not be readily available
and must be prepared by the user. Tables 8-2 and 8-3 in Section 8.7, Buffer Calibration
Parameters, list the buffers that the transmitter recognizes.
3. Allow time for the sensor and buffers to reach the same temperature. If the sensor was just
removed from a process having a temperature more than 10°C different from the buffer, allow at
least 20 minutes.
4. For best results, calibrate with buffers having the same temperature as the process. If the buffer
and process temperature differ by more than about 15°C an error as great as 0.1pH may result.
5. Be careful using buffers at high temperatures. Protect the solution from evaporation. Evaporation
changes the concentration of the buffer and its pH. Be sure the pH of the buffer is defined at high
temperatures. The pH of many buffers is undefined above 60°C. Finally, no matter what the tem-
perature is, allow the entire measurement cell, sensor and solution, to reach constant temperature
before calibrating.
6. The pH of a buffer changes with temperature. Equations relating pH to temperature for common
buffers have been programmed into the Model 3081 pH transmitter. During auto calibration, the
transmitter calculates the correct buffer value and uses it in the calibration.
7. Buffers have limited shelf lives. Do not use a buffer if the expiration date has passed. Store buffers
at controlled room temperature.
8. Do not return used buffer to the stock bottle. Discard it.
9. Protect buffers from excessive exposure to air. Atmospheric carbon dioxide lowers the pH of alka-
line buffers. Other trace gases commonly found in industrial environments, for example, ammonia
and hydrogen chloride, also affect the pH of buffers. Molds, from airborne spores, grow readily in
neutral and slightly acidic buffers. Mold growth can substantially alter the pH of a buffer.
10. Rinse the sensor with deionized water before placing it in a buffer. Remove excess water from the
sensor by gently daubing it with a clean tissue. Do not wipe the sensor. Wiping may generate a
static charge, leading to noisy readings. The static charge may take hours to dissipate. A few drops
of deionized water carried with the sensor into the buffer will not appreciably alter the pH.
MODEL 3081 pH/ORP SECTION 7.0
CALIBRATION OF pH MEASUREMENTS