In addition to nitric oxide NO, combustion gases contain also higher oxides o f nitrogen (mainly NO2). Not all portable flue gas analyzers have the nitrogen dioxide sensor in the basic version. But it is possible to calculate the NO2 contents on the basis of the measured NO. It is generally assumed that nitric oxide NO contained in combustion gases makes up about 95% of the total amount of nitrogen oxides NOx. Some combustion analyzers calculate the total concentration of nitrogen oxides NOx according to the following formula:
If the gas analyzer is fitted with NO and NO2 sensors, there is no need to calculate the amount of NO2. The NOx concentration is calculated by the analyser as a simple sum of measured NO and NO2 concentrations. This may be a requirement in some areas. Calculation of NOx from NO is only possible if there is a reliable and known ratio between NO and NO2 in the gas measured. This may not always be the case.
In general there is a common ratio fixed in the standards of most countries. This ratio is not always the same in all countries and NOx measurements must t herefore state clearly what ratio of nitric oxide to nitrogen oxides or nitric oxide in NOx was applied for the results. The value will lie around 3 - 5 % NO in NOx in most cases. Cases where the NOx proportions are thought to be very much different will require measurement of both nitric oxide and nitrogen dioxide. The higher oxides of nitrogen can generally be ignored for most stack applications, although the measurement of N2O (nitrous oxide or "laughing gas") is becoming more common in certain countries.
The tendency is now growing to insist on a separate measurement of NO and NO2 in many countries. The low levels of NO2 that are generally encountered make this slightly difficult, but sensors for low ranges are becoming available and reliable. The move is probably to be greeted favourably. The older methods and the rules of thumb that have been used for years are not always accurate and measurement is generally to be chosen over guesswork!