Comparing turbidity sensors performance for TSS evaluation in field measurements Boogert F.J.a, Martins M.A.S. a, Bilro L. b, Prats S.A. a, Nogueira R.N. b, Keizer J.J.a*
a Centro de Estudos do Ambiente e do Mar (CESAM), University of Aveiro. 3810 Aveiro, Portugal (
[email protected]) b Instituto de Telecomunicacões (IT-Aveiro), 3810 Aveiro, Portugal.
Introduction The studies about forest fires effects have been focused on identifying erosion risks and on assessing the effectiveness of post-fire land management procedures. The FIRECNUTS project (PTDC/AGR-CFL/104559/2008) has been providing insight in sediment and nutrient transport. The collection and analysis of runoff samples are widely established practices and frequently used in these studies. However, they are not only costly but also time-consuming. Regarding to the determination of sediments concentration, several methods can be applied, namely turbidity sensors. Commerciallyavailable turbidity sensors like e.g. the “OBS-3+ Suspended Solids and Turbidity Monitor” have existed for over two decades now.
Furthermore, the use of fiber optic sensors is an uncommon method for runoff samples analysis but in the literature several studies can be found reporting strong correlations between their outputs and the turbidity values obtained with the OBS-3+ 1,2,3 .Envisaging the development of a cost-effective, a small-sized and easier operation mode, a plastic optic fiber (POF) based turbidity sensor was developed 4. The performance of this sensor was compared against the OBS-3+ and results showed a positive correlation showing that they both can be used for the determination of runoff samples TSS.
Samples and Laboratory analysis
•
What is the Plastic Optic Fibre sensor's performance with field sediment samples
OBS and POF performance 200 to 50 um fraction 1200 1000
0.8
0.6 600
0.5 0.4
400
POF (TDP)
0.7
800
NTU TDP
0.3 0.2
200
0.1 0 0
2
4
6
8
10
12
14
16
18
0 20
Sediment concentration in Grammes per Litre
Preliminary Results Preliminary results show that both the OBS and the POF sensors have a reasonable correlation with sediment concentration in water. The OBS and POF performance graph shows the obtained values for both NTU (Nephelometric Turbidity Units) and TDP (Transformed Direct Power) with a sample fraction of 200µm to 50µm. It clearly shows a more consistent value for the POF output. Regression analysis for the different fractions confirm the correlation between sediment and turbidity for both sensors.
1 0.9
OBS (NTU)
In this study sediment samples where taken from the bottom of a small flume structure and from an area where sedimentation takes place in the FIRECNUTS study area located in the municipality of Sever do Vouga, north-central Portugal. The samples were dried and sieved in three different fractions; from 1mm to 200µm, 200µm to 50µm and smaller than 50µm. Both sensors (OBS and POF) were placed in the same bucket containing 3 l of demineralized water. A rotor and a magnetic stirrer where used to create a homogeneous mixture. Sample fractions were added continuously and sensors output registered for each addition.
Preliminary Conclusions and future work In conclusion, the presented preliminary results show that the POF sensor is suitable for the determination of sediment concentrations and can even be more capable than the OBS. Further work will be focused on determining calibration curves that can be used outside laboratory conditions and for un-sieved samples.
References 1 Omar et al., 2009 2 Postolache et al., 2007 3 Ruhl et al., 2001 4 Bilro et al., 2010
OBS and POF correlation with sediment concentration
R²
Key Question
1mm to 200µm
200µm to 50µm
< 50µm
OBS
POF
OBS
POF
OBS
POF
0.623
0.917
0.751
0.982
0.855
0.982
