Section 5: Calibration; General Information; Use Of Calibration Buffers (Standards) - Emerson Rosemount 398R Reference Manual

Reference Manual
00809-0100-3098

Section 5: Calibration

5.1

General Information

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5.2

Use of Calibration Buffers (Standards)

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Calibration
New sensors must be calibrated before use.
Regular recalibration is also necessary and is determined by the user.
The use of a two-point buffer calibration is always recommended.
Refer to the transmitter Reference Manual for more specific calibration procedures.
Good buffers lead to good calibrations: 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.
Use appropriate buffers: 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.
Sensor and buffers must be at the same temperature: 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.
Buffers must be at process temperature: 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.
Be careful using buffers at very 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 temperature is, allow the entire measurement cell, sensor and
solution, to reach constant temperature before calibrating.
The pH of a buffer changes with temperature: The pH of a buffer is a function of
temperature. The pH of alkaline buffers depends more strongly on temperature than
the pH of acidic or neutral buffers. Most process pH instruments, including those
manufactured by Rosemount, have an auto calibration feature. The instrument
recognizes the buffer being used and automatically corrects for the change in buffer pH
with temperature. If the instrument does not perform the correction, the user must
enter the appropriate value. Buffer manufacturers usually list the temperature
dependence of the buffer on the label.
Buffers have limited shelf lives: Do not use a buffer if the expiration date has passed. Store
buffers at controlled room temperature.
Do not reuse buffers: Do not return used buffer to the stock bottle. Discard it.
Protect buffers from excessive exposure to air: Atmospheric carbon dioxide lowers the pH
of alkaline 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.
Calibration
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