Development of a Nephelometer for in-situ continuous measurement of atmospheric dust levels*


Dust is a major pollutant of the atmospheric air. Dust levels in atmospheric air are measured routinely by a high volume sampler. This device provides an integrated value over the period of sampling. It is semi-continuous in operation, rather complicated and expensive. Besides, it is necessary to carry out filteration of air for particulates before measurements are made. It has increasingly become necessary to carry out continuous measurements of dust levels in atmospheric air for a variety of reasons. Firstly, the levels of dust in air are measured for ambient monitoring for purposes of air quality surveillance. Next, it is essential to carry out measurements of particulate levels in a number of industries, such as mines, cement factories, coal or oil fired thermal power plants, etc. Besides, it is also important to know the quantity of particulate matter released from chimneys or stacks of factories, etc.

In order to overcome the aforesaid problems, a nephelometer has been developed at Central Scientific Instruments Organisation, Chandigarh. It carried out measurements in a continuous manner and without the need for filtering the particulate matter from the ambient air. Besides, the instrument is relatively simple and inexpensive and is not manufactured in the country. In this instrument, a beam of light from a halogen lamp is focussed on a spot of dimensions 2x2x1 mm, through which atmospheric air to be sampled is made to flow. The cloud of particles in the spot scatters light in all directions. In the nephelometer, the light scattered at right angles is measured. To have higher sensitivity and resolution, forward scattering principle has been adopted. The new system is easier to fabricate with least background noise. It has optimum sensing response for dust load. The instrument is highly accurate and stable. It can be further improved by replacing the halogen lamp light source with a Helium-Neon laser diode source. The scattered light is detected and measured using a sensitute photodiode or a photo-multiplier tube. The current generated is measured using suitable electronic circuitry. The source of light usually is a quartz halogen lamp or a helium neon laser. In the later case, the optics of the instrument is relatively simpler. A strip chart recorder is used to measure the dust level continuously. The specifications of the Nephelometer developed are as follows.

*Principal Investigator:

Director, Central Scientific Instruments Organisation,

Sector 30-C, Chandigarh-160020,

E-mail: root@cscsio.ren.nic.in

Specifications of Nephelometer

Full Range

:

0-100 mg/m3

Sub-ranges 4

:

(i) 0-100 g/m3

(ii) 100 g 1 mg/m3

(iii) 1 mg 10 mg/m3

(iv) 10 100 mg/m3

Sensitivity

:

0.1 g/m3

Accuracy

:

0 5% FSD

Resolution

:

2 g/m3 long term

0.15 g/m3 short term

Flow Rate

:

0.3 m3/mt

Display

:

(i) Digital

(ii) Continuous Recording

(iii) Audio & Visual Alarm when concentration exceeds a present level.

Type

:

Table Mounting

Availability

:

Centre for Economic Research and Development, Department of Environment & Pollution Control Technologies, XIII/4550, Deputyganj, Sadar Bazar, Delhi 110006

Period of Study

:

20.05.85 to 29.02.88