Emerson Rosemount5081, Инструкция по эксплуатации, Страница: 51 / 152

7 Measurements
7.1 Calibrating pH sensors
7.1.1 Calibration overview
The calibration menu allows you to calibrate the pH and temperature response of the
sensor. Calibrations can be performed using the manual or the auto-calibration option. In
both cases, the transmitter does a two-point calibration for pH and one-point
standardization against a reference thermometer for temperature. In auto-calibration, the
transmitter automatically stores the temperature-corrected calibration data. In manual
calibration, you have to enter buffer values. The values are only registered when the
readings are stable (automatically determined by the transmitter). All calibration
procedures are guided by prompts on the display.
7.1.2 Calibration standards (buffer solutions)
• Calibrations are critical to make accurate pH measurements.
• It is recommended that the value of one of the pH buffer solutions is lower than the
pH of the process stream and the value of the other pH buffer solution is higher than
the pH of the process stream.
• For best results, make sure that the temperature of the solution is the same
temperature as that of the sensor. Allow the entire measurement cell, sensor, and
solution to reach relatively constant temperature.
• Using buffers at high temperatures can cause evaporation, which will change the
concentration of the buffer. The change in concentration causes a change in the pH,
thus resulting in an inaccurate calibration.
• Buffers have limited shelf lives. Do not use a buffer if the expiration date has passed.
Store buffers at controlled room temperatures.
• Used buffer solutions should not be reused. Buffers should also be protected from
excessive exposure to air. Exposure to air can cause changes in the pH of the buffer
solution.
• Always rinse the sensor with DI water and remove excess water by dabbing with a
clean tissue before placing it in a buffer. Only dab the sensor and do not wipe it.
Wiping the sensor builds static charge which alters the reading.
7.1.3 Auto-calibration procedure
Prerequisites
Ensure that auto-calibration is turned ON and the appropriate buffers are used.
Measurements
Instruction Manual 41