Emerson Rosemount 389, Reference Manual

Page: 27 / 46

3 Calibration
3.1 Sensor preparation
Procedure
1. Shake down the sensor to remove any air bubbles that may be present at the tip of
the pH glass bulb.
2. In most cases, the pH sensor can simply be installed as shipped, and readings may
be obtained with an accuracy of ±0.2 pH. To obtain greater accuracy or to verify
proper operation, calibrate the sensor as a loop with its compatible transmitter.
3.2 pH calibration
1. After a temporary connection is established between the sensor and the
instrument, a buffer calibration may be performed.
2. Consult the appropriate pH/ORP transmitter instruction manual for specific
calibration and standardization procedures, or see below for the recommended
two-point buffer calibration procedure.
3.2.1 Recommended two point buffer calibration procedure
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 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. 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 the sensor in the first buffer solution. Allow the sensor to adjust to the
buffer temperature (to avoid errors due to temperature differences between the
buffer solution and sensor temperature) and wait for readings 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 the sensor with distilled or deionized water.
3. Repeat steps 1 and 2 using the second buffer solution.
Once the transmitter has acknowledged both buffer solutions, a sensor slope
(mV/pH) is established (the slope value can be found within the transmitter). The
slope value should read about 59.1 mV/pH for a new sensor and will decrease over
Reference Manual Calibration
00809-0100-3389 February 2019
Reference Manual 27