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18
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 (1
g / 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 5: ORP, pH compensated ORP and
rH as function of pH buffer solution with
Chinhydron powder.
pH
ORP (mV) Na ORP (mV) pH rH (mV)
buffer compensated compensated
4.00
228
7.00
51
9.00
-67
The accuracy of the standard values is
approximately +/- 10 mV.
IM 12B06J03-05E-E
51
23.6
51
23.6
51
23.6
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.
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
measuring glass elements and
requires that the sensor has to be
soaked in water for at least
12 hours before it will function again.
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