Appendix IX L. Determination of Nitrous Oxide in Gases

Gases absorb light at one or more specific wavelengths. This property is widely used to allow highly selective measurement of their concentrations.

Description and principle of measurement

The concentration of nitrous oxide in other gases can be determined using an infrared analyser.

The infrared analyser generally consists of a light source emitting broadband infrared radiation, an optical device, a sample cell and a detector. The optical device may be positioned either before or after the sample cell and it consists of one or several optical filters, through which the broadband radiation is passed. The optical device in this case is selected for nitrous oxide. The measurement light beam passes through the sample cell and may also pass through a reference cell if the analyser integrates such a feature (some use an electronic system instead of a reference cell).

When nitrous oxide is present in the sample cell, absorption of energy in the measurement light beam will occur according to the Beer-Lambert law and this produces a change in the detector signal. This measurement signal is compared to a reference signal to generate an output related to the concentration of nitrous oxide. The generated signal is linearised in order to obtain the nitrous oxide concentration. To prevent the entry of particles into the sensors, which could cause stray-light phenomena, the apparatus is fitted with a suitable filter.