Aquaprobe
®
AP-7000 Instruction Manual
10401-00894 Rev M
If you experience negative turbidity readings, thoroughly clean the Probe then re-zero in
completely clean water. Still, bottled mineral water is recommended for zeroing the
electrode as it is cheap and readily available.
Never use sparkling or carbonated water.
If you still experience negative turbidity readings and you are certain that your zero
calibration solution is completely clear water, the problem is almost certainly either
aeration, reflection or sensor saturation.
26.5. Aeration
Aeration 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
bottle after fresh water was poured in. The bubbles
are clearly visible in the light beam. This level of
aeration will register the equivalent of around 5 – 10
NTU as each bubble is seen as a solid particle.
If a zero point calibration is conducted under these
conditions, when the Probe is subsequently deployed
in clear water it will register a negative reading between -5 and -10 NTU.
If your zero calibration water is aerated, allow it to stand for a while until the air has all
dispersed, then re-insert the Probe and re-calibrate.
Do not leave the Probe sitting in aerated water, the bubbles will simply cling to the
inside surface of the Probe and make the problem worse.
26.6. Reflection
Although all Aquaprobes are provided with a sleeve and End Cap specifically designed to
maintain a constant measurement chamber, water must be allowed to flow freely over the
sensing electrodes to ensure correct operation. In order for this to occur, a set of holes
have been included around the periphery of the sleeve.
Because both the turbidity electrode and the sleeve are threaded parts, the final position of
the lenses on the turbidity electrode with respect to the holes in the sleeve is random.
As a result, under certain circumstances, the excitation light emitted by the turbidity
electrode can exit the measurement chamber through one of the holes in the sleeve and
can then be reflected back in to the measurement chamber causing an artificially high
turbidity reading of up to 20NTU.
If a zero point calibration is conducted under these conditions, when the Probe is
subsequently deployed in clear water with no reflections, it will register a negative reading
of up to -20 NTU.
In order to avoid erroneous zero point calibration due to reflection it is important to zero the
turbidity electrode in a non-reflective calibration bottle or in a vessel who's sides come no
nearer than 10mm to the probe sleeve.
© 2017 Aquaread
®
Ltd.
www.aquaread.com
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