Aztec 600 aluminium, ammonia, color, iron, manganese, phosphate
Single- and multi-stream colorimetric analyzers
Appendix C
–
Principle of Operation
86
IM/AZT6CR–EN Rev. R
C.2 Temperature Control
The measurement head is temperature controlled. Maintaining
all the optics in a temperature-controlled environment provides
excellent stability and improves analyzer repeatability.
Heating the sample makes it possible to speed up certain
chemical reactions, enabling a greater frequency of
measurement to be achieved. It also improves the acid digestion
performed with certain determinants (for example, aluminium
and iron). If the environment is very cold it takes longer for the
analyzer to reach operational temperature (as the analyzer is
reaching temperature the message 'Stabilising' is displayed on
the LCD).
The operational temperature can be viewed and changed –
Section 6.2.1, page 32. In low temperature environments it is
recommended that the reagents are kept off the floor and are
situated on a reagent shelf and insulated.
C.3 Measurement Cycle
The sample pot fills with sample and overflows at the top to
maintain a constant head from where sample is taken to be
measured. The float inside the pot contains a small magnet that
operates a reed switch. When the float is in the uppermost
position the switch is held closed. If the sample stops flowing
the float drops slowly allowing the reed switch to open, giving a
sample flow failure indication.
The piston pump collects a discrete sample of water from the
side sample pot at intervals of 10 to 60 minutes (determined by
the user). The transmission of light is measured to remove the
actual background color and turbidity effects.
Reagents are added and mixed with the aid of the air. After
allowing time for color development the light transmission is
re-measured, the background value removed and the remaining
light level converted into a concentration value.
The result is logged, shown on the display (as a value, or
graphically) and transmitted as a current signal. If the value is
under, or exceeds pre-programmed limits for the measurement
stream, the appropriate alarm limit relay is activated.
C.4 Calibration Cycle
Automatic calibrations can be performed from every 6 hours to
once per week. The measurement cycle is interrupted for
calibration. Calibration involves repeating the measuring cycle
with de-ionised water and then a known standard. The two
values obtained are used to produce a calibration graph. The
analyzer compares the calibration graph with the ideal working
curve stored within its memory.
If the variation between the ideal working curve and the 2-point
calibration is acceptable, the analyzer returns to the
measurement cycle – if it is not the analyzer fails calibration.
C.5 LED Calibration
Before each calibration the analyzer checks the LED output, and
if required adjusts it accordingly. This is achieved by analyzing
the detector response on de-ionised water.
The detector output is scaled in counts. The full range of the
scaling is 0 to 4095 counts. De-ionised water gives the
maximum transmission, so the electronic scaling is set to
provide a detector response with de-ionised water of 3300
detector counts (80 % of scale).
If the detector response is 3300 ±100 detector counts, no
adjustment is necessary; otherwise the LED current is adjusted
automatically until the detector reads 3300 ±25 counts.
This adjustment allows the use of different LEDs to measure
different determinants, operate the analyzer at different
temperatures and overcome cell fouling with no detrimental
effect on performance.
C.6 Dilution Cycle
The Aztec 600 analyzers are capable of automatic dilution to
increase their range.
If a sample is too concentrated, the measured value is outside
the working range of the analyzer. The next time the sample is
measured, the analyzer dilutes the sample to bring the value
within the working range.
When in dilution mode the sample is diluted with de-ionised
water. The dilution ratio can be programmed from 1:1 to 1:4 (1
part sample to 4 parts dilution water).
The ability to reduce the dilution factor can be an advantage in
allowing more accurate measurement where the sample
concentration is close to, but just above, the undiluted range
limit. A reduced dilution factor also reduces the usage of
deionised water. However, caution must be taken when
reducing the dilution factor, to ensure that the working range of
the analyzer is not exceeded.
The analyzer continues to measure the sample in dilution mode
until the concentration is such that dilution is no longer
necessary.
Analyzer
Enter Dilution
Mode
Exit Dilution
Mode
Aztec 600 Aluminium
0.33 ppm Al
0.27 ppm Al
Aztec 600 Ammonia
0.55 ppm NH
3
0.45 ppm NH
3
Aztec 600 Iron
1.1 ppm Fe
0.9 ppm Fe
Aztec 600 Manganese
2.2 ppm Mn
1.8 ppm Mn
Aztec 600 Phosphate
11.0 ppm PO
4
9.0 ppm PO
4
Table C.1 Analyzer Dilution Points
Note.
The Aztec 600 Color and Aztec 600 Manganese Low
range analyzers do not enter dilution mode.