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Calibration and maintenance
4.1
Two point buffer calibration
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 7.0 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 7.0. 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.
Immerse sensor in the first buffer solution. Allow sensor to equilibrate to the buffer
temperature (to avoid errors due to temperature differences between the buffer
solution and sensor temperature) and wait for reading to stabilize.
The value of the buffer can now be acknowledged by the transmitter.
2.
Once the first buffer has been acknowledged by the transmitter, rinse the buffer
solution off of the sensor with distilled or deionized water.
3.
Repeat steps 1 and 2 using the second buffer solution.
The theoretical slope value, according to the Nernst equation for calculating pH, is
approximately 59.17 mV/pH. Over time the sensor will age, both in the process and
in storage, and this will result in reduced slope values. To ensure accurate readings,
Emerson recommends replacing the electrode when the slope value falls below 47
to 49 mV/pH.
4.2
Recommended pH sensor standardization
For maximum accuracy, the sensor can be standardized on-line or with a process grab
sample after a buffer calibration has been performed and the sensor has been conditioned
to the process. Standardization accounts for the sensor junction potential and other
interferences. Standardization will not change the sensor’s slope, but will simply adjust the
transmitter’s reading to match that of the known process pH.
Calibration and maintenance
Rosemount Hx338+ pH Sensor
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