YOKOGAWA FU20-FTS, Instruction Manual, Page: 28 / 32

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IM 12B06J03-05EN-P
6.3 Calibration of ORP and rH measurements
For calibration of ORP and rH, the procedure for MANUAL CALIBRATION can be used as
described in the Instruction Manual of the analyzer. The rH value is a function of the
reference system and the pH value of the buffer solution. The FU20-FTS/MTS sensor has
a reference system of 1molal Silver/Silver Chloride (Ag/AgCl). The commonly used
standards for ORP and rH calibration are made from Chinhydron (Quinhydrone) powder
dissolved in pH buffer solutions (1g / 200 ml). In Table 5 the measurement values are
given as function of the used pH buffer solution with Chinhydron powder. The accuracy of
the standards is approximately ± 10 mV.
Table 8: ORP, pH compensated ORP and rH as function of pH buffer solution with
Chinhydron powder.
6.4 Maintenance of the FU20-FTS/MTS sensor
A pH sensor requires routine maintenance to keep the measuring elements clean and
functioning. Because the FU20-FTS/MTS sensor does not have an open reference
junction and is hermetically sealed from the process, it does not suffer from poisoning,
diffusion and fouling. This means the FU20-FTS/MTS sensor requires very little
maintenance.
Avoid cleaning the complete sensor with solution. Some cleaning solutions will
damage the model code sticker and connector which are placed on top of the
sensor. Only clean the measuring elements at the bottom side of the sensor.
In most cases cleaning with water, iso-propanol or methanol is sufficient. In other cases,
the measuring elements of the sensor have to be cleaned with specific solutions.
Examples:
1. Deposits of limes, hydroxides or carbonates can be removed by immersing the bottom
part of the sensor in a solution containing dilute hydrochloric acid (5% is recommended).
Afterwards rinse the sensor with water.
2. Deposits of oil and fat can be removed with hot water with a detergent. When the results
are unsatisfactory, a mild (carbonate based) abrasive can be used.
3. Protein deposits should be removed with a protein enzymatic solution, for instance a
solution containing 8.5 mL concentrated hydrochloric acid and 10 grams of pepsin in 1
liter of water.
A soft toothbrush may be used to accelerate the cleaning process.
Note: Avoid cleaning with non-polar solvent like tri-chloro ethylene, toluene or hexane.
The non-polar solvents will break up the gel-layer on the pH glass bulb and
requires that the sensor has to be soaked in water for at least 12 hours before it
will function again.
pH buffer ORP (mV) Na
ORP (mV) pH
compensated rH (mV)
4.00 228 51 23.6
7.00 51 51 23.6
9.00 -67 51 23.6