Two Point Buffer Calibration; Ph Standardization - Emerson Rosemount 398R Reference Manual

Calibration
February 2019
5.3

Two Point Buffer Calibration

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5.4

pH Standardization

Standardization means making the process instrument match the reading from a second pH meter.
The second pH reading is usually made on a grab sample.
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Remove the protective vinyl cap from the sensor tip.
Rinse the sensor and immerse it in the first buffer. Ensure that the glass bulb and the
temperature element are completely submerged by keeping the sensor tip about 3 inches
below the liquid level. Swirl the sensor to dislodge trapped air bubbles. Do not allow the
weight of the sensor to rest on the glass bulb.
Once the reading is stable, enter the buffer value in the analyzer. If the analyzer does not
correct for changes in buffer pH with temperature, be sure to enter the temperature-
corrected value.
Remove the sensor from the first buffer. Rinse the sensor and place it in the second buffer.
Follow the same precautions given in step 2.
Once the reading is stable, enter the buffer value in the analyzer. If the analyzer does not
correct for changes in buffer pH with temperature, be sure to enter the temperature-
corrected value.
After calibration, note the sensor slope. Slope has units of mV per unit change in pH. An
ideal sensor has a slope of 59 mV/pH at 25°C. Slope decreases as the sensor ages. Once
the slope drops to between 47 and 49 mV/pH, the sensor should be replaced.
Remove the sensor from buffer 2 and return it to the process liquid.
Take the sample from a point as close as possible to the process sensor. To avoid starving
the process sensor, use a downstream sample point.
Wait until the process pH is constant or, at worst, slowly drifting before taking the grab
sample.
To ensure that measured pH is truly the pH of the process liquid, determine the pH of the
grab sample immediately. pH is a function of temperature. If the temperature of the
process differs from ambient, measure the pH of the grab sample before its temperature
changes. Some process liquids are poorly buffered. The pH of the sample may change
significantly upon exposure to air or to the sample container. To avoid deterioration of the
sample, measure the pH immediately.
Following the instructions in the instrument manual, adjust the process reading to the
value measured on the grab sample.
Reference Manual
00809-0100-3098
Calibration