FCA
‐
22
Instruction
Manual
Page 5
Influences
on
the
Measurement
pH
Value
The
FCS
only
measures
the
HOCl
component
of
the
total
Free
Chlorine.
The
HOCl
proportion
varies
from
100%
at
pH
5.5
to
0%
at
pH
10,
see
Figure
2.1.
The
PHS10
pH
sensor
provides
automatic
compensation
for
the
pH
dependent
ratio
of
HOCl
and
OCl
‐
present
in
the
water.
The
PTFE
membrane
rejects
charged
ions
allowing
only
neutral
molecules
to
pass
through.
Salts
and
other
ionic
substances
are
blocked
by
the
membrane
eliminating
any
influence
on
the
measurement
by
changes
in
the
conductivity
of
the
sample.
The
HOCl
portion
of
the
free
chlorine
passes
through
the
membrane
to
the
cathode
and
is
measured
while
the
negatively
charged
hypochlorite
ion,
OCl
‐
,
portion
is
rejected
by
the
membrane.
The
C
‐
22
analyzer
uses
the
pH
from
the
PHS10
sensor
to
calculate
the
OCl
‐
value.
The
measured
value
and
the
calculated
value
are
combined
and
displayed
as
Free
Chlorine.
The
DPD
calibration
method
measures
the
Total
Chlorine
by
buffering
the
sample
to
pH
6.3,
thereby
converting
the
entire
amount
of
chlorine
to
HOCl
and
then
measuring
this
HOCl
component.
The
pH
compensation
algorithm
in
the
C
‐
22
is
designed
to
match
this
method.
The
highest
accuracy
is
attained
with
calibrations
performed
at
neutral
pH
values
and
higher
chlorine
concentrations.
The
accuracy
of
the
compensation
decreases
when
the
pH
is
above
pH
8
since
there
is
little
chlorine
to
measure
and
a
large
compensation
to
perform.
Figure
2.1
Flow
The
FCS
consumes
chlorine
to
produce
the
signal.
The
area
near
the
sensing
tip
will
become
depleted
of
chlorine
without
adequate
flow
to
replenish
the
sample.
The
sensor
requires
a
minimum
velocity
of
0.5
ft/sec
past
the
membrane.
Below
this
value
the
sensor
will
indicate
a
lower
concentration
than
the
actual
value.
Higher
flow
rates
have
little
to
no
effect
on
the
measurement.
See
Figure
2.2.
0
20
40
60
80
100
4
6
8
10
%
pH
pH
Effects
%
HOCl
%
OCl
