Objectives_template

Smokemeters

In the filtration type smokemeters like Bosch smokemeter a fixed volume of the exhaust gas is drawn through a white filter paper of specified quality. The density of smoke stain obtained on the filter paper is evaluated using a reflectance meter which gives the measure of smoke density of diesel exhaust gas. Now, mostly light extinction/absorption smokemeters based on Beer-Lambert Law are used. The light extinction type smokemeters are more commonly called as ‘opacimeters' as these provide a more realistic measurement of the visible smoke emissions from diesel engines.

. Both the sampling type and full flow type opacimeters are in use. The construction requirements, installation and operational details of opacimeters are described in the relevant international standards. A sampling type smokemeter is shown schematically in Fig. 4.13.

Figure 4.13

Schematic of a sampling type light extinction smokemeter.

An incandescent lamp with a colour temperature in the range of 2 800 K to 3 250 K or a green light emitting diode (LED) with a spectral peak between 550 nm and 570 nm is used as light source. The transmitted light is received on a photocell or a photo diode (with filter if necessary). When the light source is an incandescent lamp, the receiver should have maximum response in the range 550 nm to 570 nm wavelength as is for the human eye.

When light from a source is transmitted through a certain path length of the exhaust gas, smoke opacity is the fraction of light that is absorbed in the exhaust gas column and does not reach the light detector of smoke meter. The absolute smoke density is given by the absorption coefficient, k_s which has units of m -1 and is given by:

(4.4)

where L is length of smoke column in meter through which light from the source is made to pass, I₀ is the intensity of incident light, I is the transmitted light falling on the smokemeter receiver.

In the full flow type smokemetersm, the light source and detector are placed directly across the exhaust gas stream usually at the end of exhaust pipe. In this case, path length of smoke measurement varies with the cross sectional size of the exhaust gas stream or tail pipe. Hence, conversion charts of the measured value to the absolute smoke density, k_s for different exhaust pipe diameter or path lengths are made available for the full flow smoke meters.

;