You can just google... what is orsat analyser... except water vapour (dry exhaust), it can measure CO2, O2 gases etc.

Introduction:
The Orsat apparatus is a device used to determine the composition of gases produced during the combustion of fuel. It is commonly used in the field of mechanical engineering to analyze the efficiency and emissions of various combustion processes. The apparatus allows for the measurement of both the dry and wet constituents of combustion gases.

Determination of Dry Constituents:
The Orsat apparatus is primarily used to determine the dry constituents of combustion gases. These dry constituents include gases such as carbon dioxide (CO2), carbon monoxide (CO), oxygen (O2), nitrogen (N2), and sometimes methane (CH4). The apparatus consists of a series of glass tubes and chemical reagents that react with specific gases to provide a quantitative analysis.

Working Principle:
The working principle of the Orsat apparatus is based on the principle of gas absorption. The combustion gases are drawn into the apparatus using a gas sampling pump. The gases then pass through a series of absorption tubes filled with specific reagents. Each reagent selectively reacts with a particular gas, allowing for its measurement.

Measurement of Dry Constituents by Volume:
The Orsat apparatus allows for the measurement of the dry constituents of combustion gases by volume. The volume of each gas component is determined by measuring the level of absorption of the specific reagent in each absorption tube. The difference in gas volume before and after passing through the absorption tubes gives the volume percentage of each gas component.

Measurement of Dry Constituents by Mass:
While the primary purpose of the Orsat apparatus is to measure the dry constituents of combustion gases by volume, it is also possible to calculate the mass percentage of each gas component. This can be achieved by converting the volume percentages to mass percentages using the ideal gas law and the molar masses of the gases involved.

Conclusion:
In conclusion, the Orsat apparatus is used to determine the dry constituents of combustion gases by both volume and mass. It provides a valuable tool for analyzing the efficiency and emissions of fuel combustion processes. By accurately measuring the composition of gases, engineers can optimize combustion processes to improve efficiency and reduce environmental impact.