NOx emissions testing measures nitrogen oxide compounds in exhaust gas under defined operating and sampling conditions. A reported NOx result is meaningful only when the instrument configuration, gas basis, oxygen correction, sampling location and applicable reporting method are stated. It is not the same as a routine residential boiler efficiency check.
NOx measurements require an explicit reporting basis
An instrument may measure NO directly and optionally NO₂; a reported NOx sum is not equivalent to a laboratory reference method unless sensors, conversion and reporting basis align. Specify dry or wet gas, oxygen normalization, operating load and units such as ppm or mg/m³. Do not compare differently normalized readings as though they were the same emission rate. For a reporting checklist covering measured conditions and instrument details, see boiler combustion test report contents.
What NO, NO2 and NOx refer to
NO is nitric oxide and NO2 is nitrogen dioxide. Emissions are often reported as NOx, with NO and NO2 combined according to the required method, sometimes expressed as NO2 equivalent. This convention and any dry-basis or reference-oxygen correction must be stated in the report. A meter that measures only one constituent cannot be assumed to report a legally equivalent total.
Choose the sampling method before the analyzer
Industrial burners, turbines, engines and process stacks have different temperatures, moisture, particulate loads and gas compositions. The sampling train may require a suitable probe, particulate filtration, heated transfer line or controlled sample conditioning. Gas losses in unsuitable tubing or condensate can alter the reported concentration. A hand-held screen value does not automatically satisfy a compliance test method. For a practical example of analyzer setup and diagnostics, see testo 330 combustion analyzer setup.
Field measurement workflow
- Identify the process, fuel, operating load, sampling port and required testing method.
- Confirm that the analyzer has the exact NO and/or NO2 sensor configuration required and that its calibration evidence is current.
- Inspect the probe, filtration and sampling train for leaks, blockages and the correct gas-conditioning arrangement.
- Perform zero and span checks using the procedure and standards required for the application.
- Allow the plant to stabilize at the specified condition and record O2, NO, NO2, exhaust temperature and relevant process data.
- Apply only the reporting corrections required by the governing method, document assumptions and retain raw readings.
How oxygen correction changes the number
Hypothetical dry-basis calculation: if measured NOx is 80 ppm at 8% O2 and a reporting method calls for correction to 3% O2, one common dry-basis expression gives 80 × (20.9 − 3) ÷ (20.9 − 8) = about 111 ppm. This is an illustration of the arithmetic, not a statement that a particular plant is required to report at 3% O2. Do not apply it without checking the specified method, gas basis and applicability.
Testo emission instrument
The testo 350 portable emission analyzer supports configurable gas sensors, including options for NO and NO2. The instrument must be equipped with the sensors, sampling hardware and calibration appropriate to the test. A testo 300 configuration intended mainly for O2/CO combustion service should not be assumed to perform equivalent NOx compliance measurements.
What may invalidate the result?
- Unstable process operation, changing load or inconsistent sampling location.
- Wrong gas basis, incorrect oxygen reference or missing moisture correction.
- Gas losses due to sample conditioning, wet filters or unsuitable tubing.
- Expired checks, sensor cross-sensitivity or extrapolation outside validated measuring ranges.
Emissions compliance work requires trained personnel and the specific approved procedure. A convenient calculated result does not replace that method.