Aztec ATS430 | Turbidity sensor |
OI/ATS430–EN Rev. H 27
Appendix A – Principle of operation
A.1 Turbidity
Turbidity provides a measurement of water clarity. When there
is material in water that scatters light, its presence manifests
itself as turbidity, this material may be (for example) algae, silt,
air bubbles.
The ABB turbidity sensor determines turbidity by measuring the
amount of light scattered by the sample at 90° from the
direction of illumination, see Fig. A.1. This arrangement is
commonly referred to as Nephelometric detection.
The optical design of the instrument follows the guidelines set
out in the ISO7027 Standard*. The light source is an LED
emitting at a wavelength of 850 nm. Light scattered at 90 ° is
collected by a photodiode.
*Reference: Water Quality –
Determination of turbidity: ISO, 1999. ISO 7027:1999(E).
A.2 Suspended solids
Suspended solids content in water is usually measured
using ASTM method D5907-10**, that involves filtering
the sample through filter paper and measuring the increase
in weight of the filter paper. This method can provide quite
accurate results, but is rather time consuming, requires
trained personnel and precision laboratory equipment,
and cannot provide results in real time.
It is possible to use the long known relation between
the amount of solids in suspension and the turbidity of a
sample to estimate, in real time, the suspended solids
content of the sample.
For a given sample, it is possible to build a calibration
curve to convert the turbidity value to a suspended solids
value, as shown in Fig. A2:
As can be seen in Fig. A2, the relationship between turbidity and
suspended solids is specific to a particular sample, as is the
range of suspended solids values that can be measured.
**Reference:
Standard test methods for filterable matter
(Total Dissolved Solids) and nonfilterable matter (Total
Suspended Solids) in water: ASTM, 2010. ASTM D5907-10.
For a detailed explanation of the measurement of turbidity and
suspended solids, refer to white paper WP/ANAINST/002-EN
Fig. A.1 Light scattering from a turbid sample
LED
Back scattered
light
Forward
scattered light
Light scattered at 90 °
Detector
Fig. A.2 Relationship between suspended solids and turbidity for Fullers
earth and kaolin
20,000
1000
2000
3000
4000
5000
6000
40,000
60,000
80,000
100,000
Kaolin
Fullers earth
SiO
2
Turbidity (NTU)
Suspended solids c
on
ten
t (mg/l
)
