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Aquaprobe
®
AP-5000 Instruction Manual
10401-00906 Rev M
15.4. 2000/5000-TURB Turbidity Electrode
Turbidity can be measured by the AP-5000 using the optional 2000/5000-TURB optical
electrode.
This electrode employs a Nephelometric technique in accordance with ISO 7027, which
uses Formazin as a reference standard. The Aquameter
®
displays turbidity in
Nephelometric Turbidity Units (NTU) which are nominally equivalent to Formazin Turbidity
Units (FTU).
Turbidity can be calibrated with either Formazin Turbidity Standards or Suspended Polymer
Turbidity Standards, depending upon your preferred turbidity reference.
Be aware, these
two standards will give very different results
. Factory calibration is carried out with a
1000 NTU Stabilised Formazin Turbidity Standard in accordance with ISO 7027.
15.4.1.
About Turbidity
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.
15.4.2.
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
.
If you experience any problems calibrating or using the Turbidity Electrode, refer to
Appendix 7. Troubleshooting Turbidity
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®
Ltd.
www.aquaread.com
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