What is the proper instrument for determining the percentage of carbon dioxide in the flue gas of a gas burning furnace?

The correct instrument for determining the percentage of carbon dioxide in the flue gas of a gas-burning furnace is a combustion analyzer. This device is specifically designed to measure various combustion-related gases, including carbon dioxide (CO2), carbon monoxide (CO), oxygen (O2), and others. The ability to analyze these gases helps technicians assess the efficiency of the combustion process and ensure that the furnace operates safely and effectively.

By using a combustion analyzer, technicians can make necessary adjustments to improve combustion efficiency and reduce emissions, thereby enhancing the furnace's performance. This instrument provides critical information that can assist in troubleshooting and maintaining proper furnace operation.

Other devices mentioned serve different purposes. For example, smoke detectors are designed to detect the presence of smoke as an indicator of fire, rather than measuring gas composition. Ventilation gauges measure airflow and pressure differentials, which is important for assessing the efficiency of ventilation systems but not for direct gas composition analysis. Pressure gauges measure gas pressure but do not provide information about the chemical compositions of the exhaust gases. Hence, the combustion analyzer is the most suitable choice for determining CO2 levels in flue gas.