Aquaprobe
®
Lite Instruction Manual
10401-00976 Rev F
11. Electrode Calibration and Maintenance
All Aquaread
®
Optical Electrodes are incredibly sensitive. For example, the Turbidity
electrode is capable of measuring between 0 and 3000NTU with an internal resolution of
greater than 0.1NTU. This means that the electrode is able to detect changes in turbidity
that are less than 0.003% of the full range! The other optical electrodes have a similar level
of sensitivity.
It follows, therefore, that in order to provide stable, repeatable readings, the environment in
which the measurements are made must be completely stable and repeatable.
The AP-Lite is constructed with a
matt black aluminium sleeve and
end cap that enclose the sensing
electrode and provide a closed,
constant condition, non reflective
measurement chamber.
This is essential for the correct
calibration and operation of all types
of optical electrodes.
In order to obtain consistent
results, the measurement
chamber created within the AP-
Lite must remain physically
constant during both calibration
and measurement.
Both the Sleeve and Sleeve End
Cap must be fitted during
calibration and measurement
including when using the AP-Lite
with the optional Flow Through
Cell (Flowcell).
If the optical electrode is calibrated under one set of conditions then used to measure under
another set of conditions, the readings will naturally be erroneous, especially at low
concentrations.
A perfect example of this is calibrating with the end cap removed then measuring with the
end cap fitted (or vice-versa). By changing the physical characteristics of the measurement
chamber, you also change the calibration and response of the electrode.
Another particular problem when trying to measure very low
concentrations is air in the form of both visible and microscopic
bubbles. These act like tiny prisms and can refract and reflect both
the excitation light and the return signal being measured. The
photograph to the right was taken in a calibration tube after fresh
water was poured in. The bubbles are clearly visible in the light
beam.
© 2015 Aquaread
®
Ltd.
www.aquaread.com
Page 22 of 72
