Principle of Operation
The Model 42i operates on the principle that nitric oxide (NO) and ozone (O3) react to produce a characteristic luminescence with an intensity linearly proportional to the NO concentration. Infrared light emission results when electronically excited NO2 molecules decay to lower energy states. Specifically:
Nitrogen dioxide (NO2) must first be transformed into NO before it can be measured using the chemiluminescent reaction. NO2 is converted to NO by a molybdenum NO2-to-NO converter heated to about 325 °C (the optional stainless steel converter is heated to 625 °C). The ambient air sample is drawn into the Model 42i through the sample bulkhead, as shown in Figure 1–1. The sample flows through a capillary, and then to the mode solenoid valve. The solenoid valve routes the sample either straight to the reaction chamber (NO mode) or through the NO2-toNO converter and then to the reaction chamber (NOx mode). A flow sensor to the reaction chamber measures the sample flow. Dry air enters the Model 42i through the dry air bulkhead, passes through a flow switch, and then through a silent discharge ozonator. The ozonator generates the ozone needed for the chemiluminescent reaction. At the reaction chamber, the ozone reacts with the NO in the sample to produce excited NO2 molecules. A photomultiplier tube (PMT) housed in a thermoelectric cooler detects the luminescence generated during this reaction. From the reaction chamber, the exhaust travels through the ozone (O3) converter to the pump, and is released through the vent. The NO and NOx concentrations calculated in the NO and NOx modes are stored in memory. The difference between the concentrations is used to calculate the NO2 concentration. The Model 42i outputs NO, NO2, and NOx concentrations to the front panel display, the analog outputs, and also makes the data available over the serial or Ethernet connection