Aquaprobe
®
AP-7000 Instruction Manual
10401-00894 Rev M
The inside of the sleeve should be wiped over with a soft damp cloth and
non-abrasive
detergent.
Always re-calibrate the zero point after cleaning the sleeve or lenses.
26.2. About Turbidity Measurement
Turbidity is a measurement of the light scattering properties of solids suspended within a
liquid and is therefore an
indirect
measurement of clarity. Turbidity is not a direct
measurement of suspended solids, clarity or colour.
Particle size relative to the wavelength of the transmitted light, particle shape and refractive
index modify the distribution of scattered light. Sample colour, (particularly dark colours)
can also reduce a certain portion of the scattered light by varying degrees. Combined,
these effects result in wide variability in the distribution and intensity of light scattering from
a turbid water sample. As a result, different combinations of particle shape, size, colour and
refractive index can produce similar turbidity effects.
By contrast, changing only the incident light wavelength and detector distance can
dramatically change the measured turbidity of a given sample. As a result, different model
sensors from different manufacturers can measure different turbidity values for the same
sample.
This highlights the qualitative nature of turbidity measurements. Integrated monitoring
programs, where turbidity measurements from different locations are to be compared,
must
use a single model of sensor and maintain a strict QA and calibration program to accurately
characterise, compare, and interpret observed turbidity values.
26.3. Precautions During Use
In common with all other submersion type Turbidity Probes, air bubbles and stray
reflections can be a problem when trying to measure low turbidity values. In order to avoid
air bubbles, keep the Turbidity electrode clean, and agitate the Probe after submersion to
dislodge any air bubbles which may be clinging to the lenses. In order to maintain a
common reflective pattern between calibration and use,
always calibrate and measure
turbidity with the protective Sleeve End Cap fitted
.
26.4. Negative Turbidity Readings
Although the notion of negative turbidity seems strange, it is, in fact, a very useful way to
ensure correct zero point calibration.
When an Aquaprobe
®
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, reflections from the calibration bottle 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.
Similarly, if the turbidity electrode was subject to stray reflections from the inside of the
calibration bottle during calibration, the absence of those reflections during deployment will
result in the sensor seeing less reflected signal and therefore a negative reading will be
displayed.
© 2017 Aquaread
®
Ltd.
www.aquaread.com
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