Emerson Rosemount 3900, Quick Start Manual

Page: 22 / 40

2 Calibrate and maintain
2.1 Calibrate pH two point buffer
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).
NOTICE
A pH 7 buffer solution reads an mV value of approximately zero, and pH
buffers read approximately ±59.1 mV for each 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.
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 transmitter can now acknowledge the value of the buffer.
2. Once the transmitter has acknowledged the first buffer, rinse the
buffer solution off the sensor with distilled or deionized water.
3. Repeat Step 1 and Step 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, Emerson recommends that you replace the electrode
when the slope value falls below 47 to 49 mV/pH.
2.2 Recommended pH standardization
For maximum accuracy, standardize the sensor on-line or with a process
grab sample after performing a buffer calibration and conditioning the
sensor 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 November 2020
22 Emerson.com/Rosemount