39
W3T331759 / WT.050.585.003.UE.IM.1014
MICRO/2000® AND DEOX/2000® MODULE
e. The pump is now drawing sample from the beaker. Allow the
sample to purge the pump and cell for five minutes. Wait for the
measured concentration to stabilize. Watch the water level in the
flask during this period and refill as required. Do not allow the
pump to suck in air, as this disrupts the calibration. If this occurs,
get out of the calibration procedure and wait for the analyser to
stablize (10-minute delay) before reattempting calibration.
f.
The screen will display the “BIAS” and “Temp Calibration” on the
left and the current bias value and microamp (μA) cell signal on the
right. Press the down arrow to access the “Temp Calibration” menu.
The cell signal value will be replaced by the current temperature
value. If necessary, enter the “Temp Calibration” menu and calibrate
the temperature as required.
g. Return to the “BIAS” calibration menu. Monitor the microamp cell
signal until it stablilizes. The signal will vary by ± 5%, but will
stabilize around a middle point.
NOTE:
The microamp cell signal is dependant on the operating range
of the unit, with higher range units having a higher signal strength.Once
the signal is stable, enter the desired bias value (1½ x range). Return to
main display and allow unit to operate on demand free water. Displayed
residual should be zero (± 5%). Adjust bias as required for zero residual.
h. If the calibration is completed before the 10-minute delayed return
to operation, the mA is still frozen. The main display continues to
function, with a message indicating the delay is in effect.
4.4.2.5 Calibration Tips
The bias calibration method requires that the sample be taken from the
cell discharge, where the BIAS produced by the reagents can be directly
measured. This sample flow is approximately 16 ml/min. The amperometric
titration measurement method requires a 200 ml sample, which requires
a sample time of from 12 to 30 minutes. Because of the unstable nature
of the iodine residual, particularly in a reactive process, it is not good
practice to wait this long to take the sample. If direct measurement of
the sample cell residual is desired, it can be done as follows:
1
Take a sample of exactly 200 ml (e.g., use a volumetric pipette).
2
Dilute it to 200 ml with distilled water in the titration cup. Titrate
with 10:1 PAO solution (or, if standard strength PAO is used,
multiply the titration value by 10).
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