Where can combustion gases be sampled for analysis using an Orsat apparatus?

The correct choice is where combustion gases can be effectively sampled for analysis using an Orsat apparatus, which is typically done in the flue gases after combustion has taken place. Sampling from the last pass or breeching allows for the collection of combustion gases that have been produced in the combustion process, making it suitable for analyzing the composition and efficiency of the combustion.

This location provides a good representation of the gases being emitted from the combustion system and is essential for monitoring performance, emissions, and ensuring compliance with environmental regulations. The gases collected here reflect the end products of the fuel being burned and can yield important information about excess air, carbon monoxide, and overall combustion efficiency.

In contrast, sampling from locations such as the air intake would not yield combustion gases, as it is capturing unburned air. The fuel line is similarly inappropriate for gas sampling, as it carries fuel rather than combustion byproducts. Lastly, sampling from the water supply would not provide any relevant combustion gas data, as water is not a product of combustion gases. Each of these incorrect options illustrates the importance of sampling combustion gases at the correct point in the system to obtain meaningful analysis results.