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3
Calibration and maintenance
3.1
Sensor Preparation
1.
2.
3.2
Rosemount 389/389VP pH Calibration
1.
2.
3.2.1
Recommended Two Point Buffer Calibration Procedure
Prerequisites
Select two stable buffer solutions, preferably pH 4.0 and 7.0. (pH buffers other than pH 4.0
and pH 7.0 can be used as long as the pH values are at least two pH units apart.
Note
A pH 7 buffer solution reads an mV value of approximately zero, and pH buffers read approximately
±59.1 mV for each pH unit above or below pH 7. Check the pH buffer manufacturer specifications for
millivolt values at various temperatures, as it may affect the actual value of the buffer solution
mV/pH value.
Procedure
1.
2.
3.
Instruction Manual
Shake down the sensor to remove any air bubbles that may be present at the tip of
the pH glass bulb.
In most cases, the pH sensor can simply be installed as shipped, and you can obtain
readings with an accuracy of ±0.2 pH. To obtain greater accuracy or to verify proper
operation, calibrate the sensor as a loop with its compatible transmitter.
After a temporary connection is established between the sensor and the instrument,
you may perform a buffer calibration.
Consult the appropriate pH/ORP transmitter instruction manual for specific
calibration and standardization procedures, or see below for the recommended two
point buffer calibration procedure.
Immerse the sensor in the first buffer solution. Allow the sensor to adjust to the
buffer temperature (to avoid errors due to temperature differences between the
buffer solution and sensor temperature) and wait for readings to stabilize.
The value of the buffer can now be acknowledged by the transmitter.
Once the first buffer has been acknowledged by the transmitter, rinse the buffer
solution off the sensor with distilled or deionized water.
Repeat steps 1 and 2 using the second buffer solution.
Calibration and maintenance
23
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