Sensor Calibration; Two-Point Calibration - Burkert 8201 Manuel Utilisateur

4.5 Sensor calibration

Calibration adapts the measuring transmitter beyond the variables already entered for the
characteristic of a pH sensor. Calibrate only with correctly conditioned sensors (see section 4.1).
The following procedures are available for calibration.
Automatic calibration mode
For some measuring transmitters, this mode is stored in the control program by means of software.
Pay attention to the indications in the measuring transmitter's operating instructions.
•

Two-point calibration

In order to achieve impeccable functional safety and a defined measuring precision on
first commissioning as well as after a long break from sensor operation.
•
One-point calibration
To check measuring results and for quality assurance purposes.
4.5.1 Two-point calibration
Two-point calibration is applied very frequently in pH value measurements. During two-point calibra-
tion, the reference voltages of two different buffer solutions with a known pH value are measured suc-
cessively under the same measuring conditions (temperature). The slope and zero point of the sensor
characteristic is re-calculated using these measuring results.
Requirements for two-point calibration
•
Have a thermometer ready for measuring the liquid temperatures
•
Provide distilled water and pH 7 and pH 2 buffer solutions (potentially pH 3 or pH 4.01) of
the same temperature.
• Remove the sensor, complete with the adaptation set for calibration, from the connection fit-
ting.
• Position the sensor in a suitable plastic vessel so that it and parts of the adaptation set
can be immersed in the buffer solution.
• Secure all fluidic and electrical connections of the sensor to the electrolyte vessel and
to the measuring transmitter.
•
Ensure an evenly wetted diaphragm on the sensor by loading the electrolyte
with pressure about 1hour before commencement of calibration.
•
In the standard case for sensor Type 8201, use buffer solutions at pH 7 (neutral) and
ph 2 (acid). Commence the measurement with buffer solution pH 7.
•
For a restricted measuring range, buffer solution at pH 3 or pH 4.01 can also be used for the
second measuring point.
Implementation of two-point calibration
Condition the de-installed sensor and rinse it carefully with distilled water.
•
Place the sensor in the plastic container and pour in buffer solution pH 7 until parts of the
adaptation set are wetted. The sensor is grounded by the wetted adaptation set.
•
Ascertain and record the buffer solution temperature.
•
Activate the calibration mode in the measuring transmitter control menu and select the
"two-point calibration" menu item.
•
Enter the pH value of the first buffer solution (pH 7) via the control menu.
•
Wait until the displayed measuring value is stable and launch the calibration in the control
menu.
•
The measured voltage is allocated to the pH value 7 entered for the first buffer solution.
•
After the first calibration step remove the first buffer solution from the plastic container
and rinse sensor and container thoroughly with distilled water.
•
Place the sensor back in the plastic container and carefully pour in the second buffer solution
(pH 2, optionally also pH 3 or pH 4.01) until parts of the adaptation set are wetted.
• Ascertain the temperature of the second buffer solution and ensure that the second measure-
ment has the same temperature as the first measurement (wait, warm up, cool).
• Enter the pH value of the second buffer solution (pH 2, 3 or 4.01) via the control menu.
• Wait until the displayed measuring value is stable and launch the calibration.
8201 - 17