What type of instrument is used to read CO2 levels?

The choice of NDIR, which stands for Non-Dispersive Infrared sensor, is the correct answer because this type of instrument is specifically designed to measure concentrations of gases, including carbon dioxide (CO2), by detecting the infrared radiation absorbed by the gas molecules. In an NDIR sensor, a light source emits infrared light that passes through a gas sample. CO2 molecules absorb specific wavelengths of this infrared light, and by measuring the amount of light that reaches the detector, the instrument can determine the concentration of CO2 in the air.

Other instruments mentioned in the options have different functions. A gas chromatograph, for instance, is used for separating and analyzing compounds that can be vaporized without decomposition, making it less suited for real-time monitoring of CO2 levels in the air. A thermocouple is a temperature measurement device and does not have the capability to measure gas concentrations. Finally, a barometer is used to measure atmospheric pressure, not gas concentrations. Thus, NDIR is the preferred method for accurately measuring CO2 levels in various environments, making it the most suitable choice.