The PLC 860 photolytic converter is capable of selectively converting NO₂ in concentrations of a few ppt. This instrument is the right choice for areas with excellent air quality.

In so called “background areas on mountain tops, sea coasts or polar regions, the nitric oxides to be measured in the ambient air are typically in the low ppt range. For these applications the PLC 860 selectively converts the NO₂ into NO. In the photolytic process the sample gas passes through a cell where it is exposed to light at a specific wavelength from a metal halide lamp. This causes the NO₂ to be selectively converted to NO with negligible interference to other gases.

Step by Step

The principle for measuring NO and NO₂ precisely in low ppt concentrations is as follows:

In  alternating cycles the sample gas flows either through the photolytic cell or by passes it. In the later case the sample gas enters directly the CLD, where only the NO content reacts with the ozone (PHASE 1).

In the other case the sample gas is directed through the chamber of the PLC 860 and then guided to the CLD where the original content of NO plus the converted NO₂ are reacting with the ozone to give the value of NO.c (phase 3) referring to the value NO converted.

The difference to the previously obtained NO content is the value which multiplied with the converter efficiency results in the true NO₂ concentration.

In the two other intermediate steps (PHASES 2 and 4) the ozone is added in the pre chamber. Thus, the selectivity of the CLD reaction gets significantly increased due to the slower reactivity of interfering compounds.

The PLC 860 is operated in connection with the ECO PHYSICS NO analyser model CLD 88 p. The valve control and the calculations are performed in the master CLD.