Improvements in contactless turbidimeters with free-falling stream technology

5 Pages·0.503 MB·English

by  Fetisov V., Dmitriyev O.

XX IMEKO World Congress
Metrology for Green Growth
September 914, 2012, Busan, Republic of Korea

IMPROVEMENTS IN CONTACTLESS TURBIDIMETERS
WITH FREE-FALLING STREAM TECHNOLOGY

V. Fetisov, O. Dmitriyev

Ufa State Aviation Technical University, Ufa, Russia, [email protected]

Abstract: The paper describes the idea and the corre- drain. Light penetrates through water to outlet 4 in the center
sponding research concerning some improvements in the of container bottom and make free-falling stream illuminat-
contactless turbidimeters designed with the so called free- ing in accordance with turbidity of water. Brightness of
falling stream technology. The aim of such improvements is falling stream is registered by photodetector 5 separated
to raise the metrological performance and reliability of on- from the stream by air gap.
line industrial turbidimeters of the mentioned type.
1
2
Keywords: contactless turbidimeter, photodetector, free-
falling stream technology.
6 3
1. INTRODUCTION

Turbidity − an expression of the optical properties of a
liquid that causes light rays to be scattered and absorbed 7 4 5
rather than transmitted in straight lines through a sample.
The cause of the light scattering is the presence of small
particles having optical properties different from ones for Fig.1. FFST-type turbidimeter:
1 – light source; 2 - fluid sample container with water over-
the liquid medium. So it is possible to measure turbidity by
flow; 3 - drainage; 4 - outlet; 5 – photodetector;
light attenuation or by intensity of scattered light.
6 – inlet; 7 – trap for air bubbles
The typical field of application for such measuring de-
vices is in the drinking water treatment process to control
and monitor the quality of the different treatment steps. Turbidimeters of such type have susceptibility to acci-
Other fields of application: control of demineralizers at heat dental fluctuations of the falling stream direction and shape.
electric power plants, monitoring of sewage, various proc- Such fluctuations may be produced due to vibrations, insta-
esses in food industries (especially in breweries). bility of input flow and accidental inclinations of the sam-
Every turbidimeter includes a light source and a ple container.
photodetector (one or more) which can be in direct contact
with a liquid medium or not. In the latter case we deal with 2. PROPOSED DESIGN
a contactless turbidimeter.
Contactless turbidimeters possess the obvious advantage Some features of the proposed FFST-type turbidimeter
in comparision with contact ones. That is the opportunity of are illustrated in Fig.2. In addition to traditional solutions
unattended operation over a long period of time. This oppor- (such as sample container and drainage arrangement) au-
tunity takes place due to the absence of fouling on optical thors propose and explain other elements improving meas-
elements (light sources and photodetectors). Consequently urements. For example, ring-shaped multisensor photode-
there is the stable transparency of optical channels. tector 9 allows to minimize influence of fluctuations of
Two general types of contactless turbidimeters exist: free-falling stream 8.
with translucence of plane surface (PST) [1] and with trans- The idea to use many photosensors distributed evenly
lucence of a free-falling stream (FFST) [2]. Each of them around the falling stream instead of a single photodetector
has special constructive features and preferable working is based on the following: if we use a set of photosensors
range of turbidity. Turbidimeters of PST-type are more with summation of their signals then the stream moving
preferable for very turbid liquids. FFST-turbidimeters are away from some photosensor would produce signal decreas-
applicable in rather wide range of turbidity and have good ing in this photosensor. However at the same time this de-
metrological performance. crease would be compensated by signal increasing in a pho-
A layout drawing of FFST-type turbidimeter is shown in tosensor to which the stream comes nearer.
Fig.1. Light is directed from source 1 through open space to We can make speculative assumption concerning a num-
a stabilized flowing water surface that is formed by a special ber of distributed photosensors: the more this number is the
fluid sample container 2 surrounded by catch basin 3 with a better the result is. However without preliminary modelling
N
it is difficult to conclude about a reasonable number of pho- u  u ,
i
tosensors. i1
where I – brightness of the source; k - proportionality coef-
6
ficient; i – index number of a sensor.
1 2 5
Y

s.2
3 7

12

10
2
4 s.3 Δ r s.1 X
11 α θ
1 R

9

8 s.N

Fig.3. Cross-section of the falling stream in the plane
of the distributed sensors

Fig.2. The proposed layout of FFST-type turbidimeter: The model allowed to obtain 3D-plots (Fig.5) for the
1 – sample container with water overflow; 2 – bottom outlet; 3 – relative estimation of u as a function of Δ and α .The results
inle t; 4 – drainage system; 5 - light source; 6 – dual track fiber were calculated as ratio u/u , where u is the sum of sensors’
0 0
bundle; 7 - photodetector; 8 – free-falling stream; 9 - ring-shaped
signals with the not devoted stream. So we can observe the
multisensor photodetector; 10,11 – amplifiers; 12 - controller
behaviour of the summary photosensors signal shift induced
by stream deviation. The plots in Fig.5 show the ratio u/u
0
for different values of the photosensors number N.
3. MODELLING A relative shift of u may be expressed percentagewise as
well. Thus, the value u/u = 1,6 corresponds to 60%, u/u =
0 0
Suppose the multisensor photodetector consists of N 0,8 corresponds to -20% and so on.
sensors (s.1..s.N) distributed evenly along a circumference Note that if N>3 the shift of u is always positive
of radius R around the falling stream. A Cross-section of the (u/u >1). Increasing N we can significantly improve the
0
stream in the plane of the distributed sensors is shown in ratio u/u . It means that application of the proposed mul-
0
Fig.3. The centre of the stream cross-section 1 is located in tisensor photodetector proves its efficiency. But if N is
the centre of the rectangular coordinate system with axes X greater than 8, the maximal value of the ratio (under the
and Y. Deviated location of stream cross-section 2 may be in determined value of Δ = 2 mm) is stable and equals 1,03
any point inside the circumference. (3% of u ). Obviously, it is not reasonable to increment this
0
The mathematical model for estimation of behavior of number further.
the sum u of all sensors’ signals u as a function of Δ and α Therefore, the described solution doesn't exclude influ-
i
was developed, where Δ – deviation of the stream, α – an ence of the stream deviation completely. The cause of the
angle between the deviation direction and the axis X. The boundedness is in nonlinear dependence of u on r and θ.
i
model was developed under certain assumptions, such as: Hypothetically, a certain conversion f must exist which,
1) cross-section of the stream is considered as an illumi- after application to each of sensor signals u before their
i
nating point and only this point was examined; summation, would help to produce a result u not depending
2) output signal of each sensor is in a direct proportion to on the stream deviation (Fig.4).
cosine of incidence angle (θ);
3) illuminance at the input of a sensor is in an inverse 1 u1 f
proportion to square of distance r between the source (the u
2 2 f Σ u
centre of the stream cross-section) and a sensor.
Finally the model was obtained as it follows: ... u ... ...
N N f

 2(i1) 
kIRcos  Fig.4. The proposed conversion scheme
  N   ;
u 
i 3 Really, we found out a few types of simple conversions
 2(i1)  2
2R22Rcos  that provide a satisfactory results.
  N 
N=1 N=2

u/u
0 u/u
0

1.4 1.1

1.2

1.05

1.0

1.0
0.8
0
2.0Δ ,1 .5m 1m.0 0.5 0.0 6 5 4 α3 ,2 r1 ad 0.95 2.0 1.5 1.0 0.5 0.0 6 5 4 α3 2, 1 r0 ad
Δ, mm

N=3
N=4

u/u u/u
0 0

1.05

1.04 1.03

1.03
1.02

1.02

0
1.01 1.01 0
1.0 2
2
2.0 1.5 4 α, rad 1.0 α, rad
1.0 2.0 4
Δ, mm 0.5 0.0 6 1.5 1.0 6
0.5
Δ, mm 0.0

N=8
N= 36

u/u
0 u/u
0
1.03 1.03

1.02
0 1.02

1.01 2 1.01 0
α, rad

1.0 4 2
2.0 1.0 α, rad
Δ1.5, m1.0m 0.5 0.0 6 2.0 1.5 1.0 4
0.5 6
Δ, mm 0.0

Fig.5. Results of calculating u/u for N=1, 2, 3, 4, 8, 36
0
N=1 N= 2

u/u0 u/u0

Δ, mm α, rad Δ, mm
α, rad

N=3
N=4

u/u
0
u/u
0

α, rad

Δ, mm α, rad Δ, m m

N=8

u/u

0

α, rad
Δ, mm

Fig.6. Results of calculating u/u for N=1, 2, 3, 4, 8 (after additional conversion of u)
0 i
The elementary way is to extract the root from each pho- 5. CONCLUSIONS
tosensor signal value with following accumulation of the
derived values. The positive influence of such a conversion 1. Application of the proposed multisensor photodetector
is a considerable reduction of the sum u value shift (im- proves its efficiency.
provement of the ratio u/u ). 2. Preferred number of sensors is from 4 to 8. To incre-
0
As we can see (Fig.6), the ratio u/u for the second ment this number further is not reasonable. The reached
0
method in comparision with the corresponding cases for the level of additional error produced by deviation of the stream
first method (Fig.5) under the same conditions is considera- (for example, this error is about 3% for Δ =2 mm) is almost
bly better. not reducible when increasing N.
Thus, when Δ=2mm and α=0, relative change of the sum 3.The simple mathematical operation (such as taking the
u equals to: 60% for the first method and 25% for the sec- root) under sensor signals u before their summation could
i
ond method for one sensor; 15% and 4% correspondingly - improve the result. The additional error produced by devia-
for two; 6% and 1,2% - for three; 3,5% and 0,65% - for tion of the stream may be reduced by this method signifi-
four; 3% and 0,5% – for eight sensors. As in the first cantly (3-6 times).
method, with the further increase of the photosensors quan-
tity N, the shift of value u will not decrease. 6. REFERENCES
Therefore, the second method of signal processing
makes possible to neutralize the stream deviation influence [1] US Patent № 5400137, Int. Cl. G01J 003/30, Publ. March 21,
quite efficiently. 1995.
[2] "On-line turbidimeter WTM-500", Sigrist-Photometer AG
Datasheets, accessed at:
http://www.photometer.com/en/products/details/downloads