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Aquaprobe
®
AP-5000 Instruction Manual
10401-00906 Rev M
15.4.3.
Negative Turbidity Readings
When a Probe is deployed in clean/clear water and negative turbidity readings occur, the
cause is usually an erroneous zero point calibration, caused either by contaminated
calibration solution, aeration or changes in the measurement chamber between zeroing
and deployment.
It follows that if the Probe has been zeroed in a solution that has a turbidity greater than
true zero, subsequent measurements taken in a less turbid sample will be displayed as
negative. 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
aeration, i.e. 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 5NTU as each bubble is seen as a solid particle.
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.
15.4.4.
Calibrating the Turbidity Electrode
The Probe Sleeve and Sleeve End Cap form an integral, working part of the Probe’s
turbidity measurement system, and MUST be fitted during calibration and
measurement for correct operation.
15.4.5.
Calibration Points
Turbidity electrodes have three calibration points. Careful calibration is essential in order to
ensure consistent and reliable results across the full measurement range.
When a turbidity electrode is first installed,
it MUST be calibrated at three points
in order
to establish the individual electrode's slope.
The Zero NTU point must always be
calibrated first, followed by the other two points, all within the same calibration
session (i.e. without turning the Aquameter
®
off).
The Turbidity electrode should subsequently be Zeroed (calibrated at the Zero NTU point)
before each day’s use. A three point calibration should be carried out once a month to
ensure optimum accuracy.
Zeroing of the Turbidity electrode is normally carried out automatically during RapidCal
(see
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®
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