SU1492242A1 - Method for determining concentration of particles in a stream of transparent medium - Google Patents

Abstract

The invention relates to instrumentation technology, in particular, to optical methods for monitoring parameters of dispersion media. It allows the determination of the concentration of particles in a stream of a transparent medium and can be used in the electronics industry, biology, medicine and in the control of environmental pollution. The purpose of the invention is to lower the lower limit of detectable particle concentrations. The essence of the invention is that the medium with the test particles is probed by intersecting mutually coherent laser beams with different frequencies. The beat signal of the photodetector is selected at the difference frequency and the mean values of squares of the photocurrents are determined. Signals of scattering from the intersection of the beams and the area outside their intersection are recorded alternately. From the ratio of the mean squares of the photocurrents related to these two scattering zones, the concentration of particles is determined. The method is based on a physical effect consisting in different effects on fluctuations of the photo-mixing signal, fluctuations in the number of particles in the correlation region of the scatter of two beams (beam intersection zone) and in the region of non-correlation scattering of two beams (outside the beam intersection zone) . The method is most sensitive in the range of low concentrations, where fluctuations in the number of particles in a countable volume prevent the effective use of other integral methods due to an increase in statistical errors. 1 il.

Description

The invention relates to instrumentation engineering, in particular, to optical methods for monitoring parameters of dispersed media, which makes it possible to determine the concentration of particles in

flow of a transparent medium and can be used in the electronics industry, biology, medicine and in the control of contamination of the environment.

The purpose of the invention is to lower the lower limit of detectable particle concentrations.

The drawing shows a block diagram of a device that implements a method for determining the concentration of particles in a stream of a transparent medium.

The device contains a continuous radiation laser 1, an acousto-optic modulator 2, a mirror 3, a collecting lens 4, a field diaphragm 5, a photodetector 6, a selective frequency filter 7 and a processing and recording unit 8,

The method for determining the concentration of particles in a stream of a transparent medium is carried out as follows.

A continuous beam of laser 1 with a frequency of CO is directed to the entrance aperture of an acousto-optic modulator 2, the output of which, besides the transmitted beam with the frequency of SOC, also forms a second beam with a frequency tOj extending inclined to the direction of the hepsoro beam. The difference between the CO, and COg frequencies is determined by the frequency of the signal that excites the acousto-optic modulator 2. The difference between B3 and the frequency of the first and second beams is chosen from the condition COi (

WITH

-rg-, as with a larger difference

Frequency selection, selection and signal processing at the beat frequency cannot be performed efficiently due to significant technical difficulties. Mirror 3 intersects the first and second beams inside region 9 occupied by the medium with the particles under study. The light scattered by the particles is collected by the objective 4 and recorded by the photoreceiver 6, which provides for the photo mixing of the signals of the scattering of the first and second laser beams. The field diaphragm 5, provided with two openings, makes it possible to alternately record the light scattered by particles from the intersection zone of the first and second beams and from the region lying inside region 9, but outside the intersection zone of the beams. The beat signal at the difference frequency | c “), from the output of the photoreceiver 6, is extracted using a selective frequency filter and is fed to the input of the processing and recording unit 8.

Imagine that the photocurrent is} 1nka 6 i (due to the photo-mixing of waves scattered by n particles irradiated by the first laser beam in volume V, and n particles irradiated by the second laser beam in volume V, and the volumes V and Vi are intersecting. 1 equals

2

i, in, Z. Z1 lrsr,

,

where the scattering cross section for a pair of particles p and q belongs to 5

0 5

five

AT

lying respectively volumes V and

0

 - the proportionality coefficient, taking into account the intensity of irradiation, the sensitivity of the photodetector and the solid angle of collection of light.

Since the particles are randomly distributed in the flow, the phases of different sections of Lo ro do not correlate and do not depend on fluctuations in the number of particles falling into B volumes of V and Vj. Under these conditions, for the average square of the current of the photodetector 6, you can write

 one

(

C (

P

P-2

five

0

five

0

 G p

z: uG-p,

- R} where A, | ЛО 1 - for particles in volumes V, and V.

Taking into account that n and n are independent random variables, since V (and Vi do not intersect, get, njA,.

Suppose that the first and second laser beams intersect in the volume Vp. Then the current i of the photodetector, caused by the mixing of waves from different beams scattered through particles n in the volume V, is equal to

According to

 2: hp; 1,

For the average square of the current of the photodetector 6 in this case, get

where B is the proportionality coefficient, taking into account the intensity of irradiation, the sensitivity of the photodetector and the solid angle of collection of scattered light.

for particles in volume V.

Bearing in mind that with stat .-; - a uniform homogeneous distribution of particles;

the number of particles in a stream is equine distribution, Po (n,) + n „

Then

((n;) + n) A2.

-rZ.r l / l2

The ratio i, / average squares of the currents in this case can be obtained from the expression

T

72 1.

P.P./

v

A, B,

-g (By) -t-n

By appropriately choosing the solid angles of collecting the scattered light from the volumes V, V and VQ, it is easy to ensure condition A,

Whereas

(to")

get an expression that allows you to determine the concentration of particles T2 / g

n (Y ° v .v.) v

Claims (1)

The positive effect is provided precisely in the concentration range where fluctuations in the number of particles in a countable volume become quite large and other integral methods for measuring concentrations cannot be effectively used due to significant statistical errors. Invention Formula A method for determining the concentration of particles in a stream of a transparent medium, including probing the flow of a medium with the test particles by a continuous laser beam at a frequency of CO, recording the light scattered by particles by a photoreceiver, measuring the current signal of the photoreceiver 92242 ka and opredepsiiio iid it is highly concentrated, about tl and hl - y y and 11 so that, with the aim of reducing the limit of the enlarger en, lx of particle concentrations, the second beam of HcnpepbiBHoj o laser radiation with a photototog satisfying ycJiOBino ten SL | w, -w, / toosh and having mutual coherence with the laser radiation of the first beam, 15 intersects the first and second laser beams in the flow of the medium with the particles under study, and the scattered light is recorded by photographing the signals 2Q scattering the first and second laser beams, extracting the signals from the photodetector currents} ia to the beat frequency and measuring the root-mean-square values of these currents, while the concentration 25 n particles are found from respectively) 0 five 0 five Where P T-1. i V V, HV 0 () V, V, v, (tvili vr - the rms value of the current of the photodetector, caused by the scattering of light by the particles, located at 1c in the intersection zone of the first and second laser beams; - the rms value of the photodetector current caused by light scattering by particles outside the intersection zone of the first and second laser beams; a countable volume of the intersection zone of the first and second laser beams; countable volume r, containing particles, scattering, respectively, radiation 1P1 of the first and second laser P11x beams outside the zone of their intersection.