Measuring Principle

The opacimeter is a device to measure the opacity of exhaust gases o diesel engines. The measuring principle is based on the attenuation of a green light beam by the smoke of the exhaust gas in a sampling head.


The smoke tube is heated at constant temperature of 100ºC in order to avoid the condensation.

The smoke sampling head contains a fan to clear the lens and to draw the smoke sample into the sampling tube via a by-pass system. If some soothing of lens does take place, this cause readings above zero even when there is no smoke in the tube. However the device can be automatically recalibrated to zero, until such time as the lenses become badly obscured by soot.

The value of the opacity is deduced by the attenuation of the beam light through the smoke from the sampling head. The opacity can be represented by two physical values:

  1. a) By percentage:

N= 100(1-F/Fo ) [%]

  1. b) By absorption coefficient

K= (1/L)ln(F/Fo ) [m 1 – ]

Where: L = is the effective length of light beam through the exhaust gases;

Öo = is the light flux on the receiver when the sample tube is filled with fresh air;

Ö = is the light flux on the receiver when the sample tube is filled with exhaust gases;

Device Description

The device is build by the following electronic modules:

– the sampling head;

– the central unit;

– the local panel.

The Sampling Head

This module performs all the right conditions in order to measure the opacity of the exhaust gases of diesel engines. The measuring principle was described in the first paragraph (see Figure8.1). The sampling head performs the followings functions:

head temperature regulation (to 100ºC), in order to avoid the condensation in the sampling tube;

measuring the temperature of the exhaust air flow, in order to perform the gas temperature correction of the opacity;

measuring the pressure of the exhaust air flow, in order to avoid the measurements when the pressure in the sampling tube is greater than 0,735 Kpa against the atmospheric pressure;

fans control, measures the current through the ventilators dc motors in order to verify the right function;

green light emission control provide the emission of a stable light beam owing to the thermal compensation of the emission LED;

light beam receiver provide the level compatibility of the photo diode signal with the Analog Digital Converter;

measuring the engine revolution with piezoelectric sensors or stroboscope, in order to verify the engine accelerations;

measuring the engine oil temperature;

The Central Unit

Performs the algorithms of the temperature regulator, the Analog Digital Conversion of the light signal and temperature signals, the calculations needed to perform the final results of all measured parameters.

The central unit is build around an Intel 80C552 processor with 11,059MHz clock. This module has the following configuration:

– 32K bytes EPROM;

– 32 K bytes RAM;

– one port with 8 analog inputs;

– three ports with 8 digital outputs;

– one port with 8 digital inputs;

– I2C bus;

– RS232 interface;

– one LCD interface for displays with 16 x 2 characters;

– local power supply and voltage reference for the ADC (LM336);

– power management circuit (Dallas1833).

The Local Panel

Displays the followings engine measured parameters:

– The Opacity of the Engine Exhaust Gas;

– The Temperature of the Sampling Head;

– The Engine Revolution;

– The Engine Oil Temperature.

The local panel has also some buttons in order to control the measuring process and a thermal printer to writhe the official report.

Technical Parameters

The main parameters of the “Opacimeter for Exhaust Gases of Diesel Engines” are:

– The Absorption Coefficient                                                              K = 0 m …9.99 [m 1 – ] ;

– The Opacity                                                                                      N = 0% … 100%;

– The Engine Revolution                                                                     n = 0 … 9.999 [rot/min];

– The Engine Temperature                                                                   t = 0 … 150 [ºC].