Yokogawa SC25V - Manual (Page 10)

10
IM12B6J1-40E-E
5. GENERAL CALIBRATION AND MAINTENANCE PROCEDURE
5.1 Calibration for pH measurement
To calibrate a pH sensor, at least two buffer
solutions with known pH values are required. It
is recommended to use one buffer solution with
a value of pH=7. The second buffer solution
should be either acidic (pH<7) or alkaline
(pH>7), depending on the process value to be
measured. Example buffer solutions are the IEC
buffers with pH=4.01, pH=6.87 and pH=9.18.
The following is a general 2-point calibration
procedure:
1 Clean the sensor (deposits may be removed
using a 5% HCl-solution).
2 Rinse sensor thoroughly with clean water.
Carefully slap out residual fluid drops.
3 Immerse the sensor in the first buffer (pH=6.87
buffer is recommended).
4 Refer to appropriate Instrument Instruction
Manual for detailed instrument calibration input
settings (Auto; Manual; Sample, etc.),
5 Rinse sensor thoroughly with clean water.
Carefully slap out residual fluid drops.
6 Immerse the sensor in the second buffer
(pH=4.01 buffer or pH=9.18 buffer
recommended).
7 Repeat Step 3.
Most instruments automatically correct the
measured pH value at process temperature to
a pH-value at a standard temperature of 25°C.
After the calibration is complete, replace or
re-install the sensor into the process.
5.2 Maintenance
Cleaning the electrodes
When the sensitivity of the electrode has
decreased or the response has slowed down,
the electrode should be cleaned. If cleaning
with hot water is not sufficient, more aggressive
water based agents should be used.
Examples:
• Deposits of lime, hydroxides or carbonates
can be removed by immersing the sensor in
or a dilute solution of hydrochloric acid (5% is
recommended). Afterwards rinse with water.
• Deposits of oils and fats can be removed with
hot water in conjunction with a detergent.
When the results are unsatisfactory, a mild
(carbonate based) abrasive can be used.
(Ref.) B
(Temp 1) E
(Temp 2) F
(LE) D
(pH) A
VP-Connector
4.5 Sensor VP connection
Figure 8