wtw Turb 555, Operating Manual

Page: 36 / 82

Operation Turb 555 / Turb 555 IR
36 ba41113e07 04/2009
4.4 Calibration
4.4.1 Basic information on calibration
Why calibrate? As for any measuring instrument, the measuring accuracy of
the Turb 555 /Turb 555 IR has to be checked and adjusted
at regular intervals.
When to calibrate? Under normal conditions, we recommend to calibrate the
measuring instrument at least every three months. You can
set the time between two calibrations in the instrument set-
up. When the time interval has expired, the instrument
shows by a flashing
c
symbol that a calibration is due. Ad-
ditionally, the instrument has to be calibrated each time the
lamp has been replaced.
Referencing
Formazin
In Ratio Mode
Using the Turb 555 /
Turb 555 IR
For the ratio mode, there are no standard procedures.
The provided AMCO Standards 10 and 100 NTU in the cal-
ibration kit are designed for nephelometric measurements
up to 1000 NTU.
If you perform a user defined process and quality control by
measuring turbidity in ratio mode you can use the AMCO
standards, but take into consideration that the measured
values may not read the same as values measured in non-
ratio (nephelometric) mode (90° scattered light).
To perform ratio measurements that reference formazin with
values below 100 NTU it is necessary to first prepare two
formazin calibration standards of 10 NTU and 100 NTU.
Perform the calibration procedure as described in your op-
erator’s manual, but use the two formazin calibration stan-
dards instead of the AMCO standards of the calibration kit.
Please note that formazin in a diluted form is unstable and
must be used immediately after preparation.