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June 2019
4
Calibrate
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.
Quick Start Guide
Quick Start Guide
15
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