Aquameter
™
Series Instruction Manual
10401-00188 Rev M
15. Turbidity Electrodes Calibration and Maintenance
Turbidity is measured by the Aquaprobe using a dual electrode, optical, 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).
A Turbidity Transmitter electrode emits a high energy, narrow beam of pulsed infrared light
across the centre of the Probe. Light scattered at 90° to the beam axis by suspended solids
within the water is collected by a Turbidity Receiver electrode on the opposite side of the
Probe.
Turbidity can be calibrated with either Formazin Turbidity Standards or Suspended Polymer
Turbidity Standards, depending upon your preferred turbidity reference. Factory calibration
is carried out with a 1000 NTU Stabilised Formazin Turbidity Standard in accordance with
ISO 7027.
15.1. 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.
15.2. Precautions During Use
In common with all other submersion type Turbidity Probes, air bubbles can be a problem
when trying to measure turbidity values below 5NTU. In order to avoid air bubbles, keep the
Turbidity electrodes clean, and agitate the Probe after submersion in order to dislodge any
air bubbles which may be clinging to the lenses.
When taking Turbidity readings in a flask or beaker, ensure the Aquaprobe is at least
25mm (1”) away from the bottom of the vessel in order to avoid reflections which may affect
accuracy. This
does not apply
to the purpose-made 300mL Aquaread calibration bottles,
which have non-reflective pads fitted, the Aquaread Flow-Through Cell or when a protective
Sleeve End Cap is fitted to the Probe.
© 2011 Aquaread Ltd.
www.aquaread.co.uk
Page 39 of 64
