Section 3: Calibration And Maintenance; Ph Two Point Buffer Calibration; Recommended Ph Sensor Standardization; Ph Electrode Maintenance - Emerson Rosemount 3900 Instruction Manual

Instruction Manual
LIQ-MAN-3900

Section 3: Calibration and Maintenance

3.1

pH Two Point Buffer Calibration

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).
NOTICE
A pH 7 buffer solution reads a mV value of approx. 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 since it may affect the actual value of the buffer solution mV/pH value.
1.
2.
3.
4.
3.2

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 sensors slope, but will simply adjust the transmitter's reading
to match that of the known process pH.
3.3

pH Electrode Maintenance

Electrodes should respond rapidly. Sluggishness, offsets, and erratic readings are indicators that
the electrodes may need cleaning or replacement.
1.
2.
3.
Calibration and Maintenance
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. Value of buffer can now be
acknowledged by transmitter.
Once the first buffer has been acknowledged by the transmitter, rinse the buffer solution
off the sensor with distilled or deionized water.
Repeat the steps 1 and 2 using the second buffer solution.
The theoretical slope value, according to the Nernst equation for calculating pH, is
approximately 59.1 mV/pH. Over time the sensor will age, both in the process and in
storage, which will result in reduced slope values. To ensure accurate readings, it is
recommended that the electrode be replaced when the slope value falls below
47 to 49 mV/pH.
To remove oil deposit, clean the electrode with a mild non-abrasive detergent.
To remove scale deposits, soak electrodes for 1 to 5 minutes in a 5% hydrochloric acid
solution.
Temperature effect on life expectancy: If glass electrode life expectancy is 100% at
77 °F (25 °C), then it will be approximately 25% at 176 °F (80 °C), and approximately 10%
at 212 °F (100°C).
Section 3: Calibration and Maintenance
April 2017
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