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7 CALIBRATION
7.1 Method of Calibration
Calibration of the monitor is carried out by replacing the
sample solution sequentially with two solutions of known
phosphate concentration – see Section 4.1. Initially a zero
phosphate solution, and then, if required, a secondary
standard solution are passed through the monitor – see
Section 8.1.2. This calibration sequence can be initiated
automatically at preset times, or manually on demand. Since
most of any drift which takes place affects the zero more than
the sensitivity, the monitor can be set up to carry out regular
AUTO
ZERO
calibrations
SECONDARY calibrations. This reduces the instrument 'down
time' to an absolute minimum. Manual one or two point
calibration sequences can also be initiated. Calibration
programming is covered in Section 6 (single stream) and
Appendix A (multi-stream).
On initiation, either manual or timed automatic, the 'Cal' l.e.d.
is illuminated and the remote Calibration Mode relay is
energized. Two solenoid valves, SV1 and SV2, are energized
sequentially to admit the zero solution and then (if selected) to
admit the secondary standard solution. At each stage of the
sequence, sufficient time is allowed to displace the previous
solution and allow the reading to stabilize. The calibration
sequence is shown in Table 7.1.
After calibration, the outputs from the optical system
corresponding to the two solutions, are used to calculate new
zero and calibration factor values, thus compensating for any
drift or sensitivity in the reagents or liquid handling
performance characteristics.
The new zero and calibration factor can be displayed in
Operating Page 1 (a calibration factor of 1.00 is the nominal
value). This parameter is intended to indicate the performance
of the monitor and in particular the chemical solutions. If the
value is outside factory pre-set limits, a calibration fail alarm is
initiated and the 'Out of Service' l.e.d. is lit.
The calibration factor can be defaulted to 1.00 (see
Programming Page 2.2) following maintenance.
displayed reading can be brought onto scale with the
switches to allow the reading to be observed to assess the
stability of the monitor prior to carrying out a calibration.
Initially, a BASELINE AUTO ZERO calibration is manually
initiated to establish the new baseline zero when a new reagent
solution is installed. This sets the zero offset value, displayed on
Programming Page 1, to 00.0. Following a BASELINE AUTO
ZERO a SECONDARY calibration is initiated. Subsequent timed
ROUTINE AUTO ZEROs generate a new zero offset value which
can then be assessed to check for zero drift within the life of the
I
t n
A
U
T
O
S
E
C
A
c
i t
i v
y t
Z
E
R
O
C
A
L
S
l o
t u
o i
V
a
v l
e
S
V
1
E
n
e
g r
s i
e
d
i T
m
n i
g
(
D
e
a f
u
) t l
2
0
n i
m
n i
u
e t
s
Table 7.1 Calibration Sequence
24
and
less
frequent
o r
d
u
c
e
O
N
D
A
R
Y
I
t n
o r
d
u
c
e
N
B I
R
A
I T
O
N
S
a
m
p
e l
O
n
f i (
s
e
e l
c
e t
) d
S
V
2
N
o
n
e
2
0
2
0
reagent (normally five weeks). If the zero offset is outside factory
pre-set limits, a calibration fail alarm is initiated and the 'Out of
Service' l.e.d. is lit.
7.2 Operation of Color Compensation
The monitor has been designed to compensate for background
color in the sample which could produce higher readings than
expected. However, the compensation sequence takes
20 minutes during which the instrument is off line.
AUTO
It is recommended therefore, that color compensation is only
used when necessary, and then set to a compensation
frequency which is the minimum for the particular application.
For many applications, such as in the power industry and for
potable water, color compensation will not be required. For
certain
environmental
examination of the sample shows it to have a yellow-orange
coloration, there may be a significant absorbance in the region
where
normal
Phosphate readings will be in error until a color correction
(color compensation) has taken place.
Note. If the sample color changes, the monitor
readings will be in error until another compensation is
carried out.
This can easily be achieved by making a measurement on
sample without addition of reagent. If this reading is then
subtracted
from
measurement of true phosphate concentration is obtained.
To decide whether color compensation should be applied,
carry out the following test:
a) Ensure that color compensation is OFF (see Programming
Page 3.3) and that default calibration parameters on
Programming Page 2.1 are set to YES.
b) Carry out a normal BASELINE calibration and make a note
of the sample reading.
c) To compensate for color in the sample, select MANUAL on
The
Programming
and
compensation (Programming Page 2.3). Make a note of
the new sample reading at the end of the sequence.
d) If it is felt that the difference between the readings is
insignificant with respect to the monitor application, then
color compensation is unnecessary.
Note. This test should be repeated at intervals to
check for periodic variations in sample color.
7.3 Calibration/Color Compensation Conflicts
If color compensation is set to MANUAL on Programming
Page 3.3, a color compensation sequence can only be
initiated manually. Alternatively, if a frequency is entered, then
r o
m
l a
the compensation can be initiated manually or automatically.
p
r e
t a
o i
n
If color compensation is taking place and a timed calibration is
N
o
n
e
due, the timed calibration proceeds immediately after
completion of color compensation. Conversely, if a timed
calibration is taking place and a color compensation is due,
the
color
compensation
completion of timed calibration.
applications
where
phosphate
development
the
developed
sample
Page 3.3
and
manually
proceeds
immediately
a
visible
takes
place.
reading,
a
start
a
after
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