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ISO 29042-2:2009 — Safety of Machinery — Emission of Airborne Hazardous Substances — Part 2: Tracer Gas Method
A comprehensive technical guide to the tracer gas method for measuring emission rates of gaseous pollutants from machinery according to ISO 29042-2
Introduction to ISO 29042-2
ISO 29042-2:2009 specifies a tracer gas method for measuring the emission rate of gaseous pollutants from machinery. This technique is particularly valuable when the pollutant itself is difficult to measure directly, or when a standard reference method is needed for comparative testing across different machine designs. The method uses a tracer gas released at a known rate at the pollutant source location, with subsequent measurement of tracer concentrations in the exhaust or workplace air to determine the emission rate.
The tracer gas method is especially useful for validating computational fluid dynamics (CFD) models of workplace contaminant dispersion. By comparing measured tracer gas concentrations with CFD predictions, engineers can validate their models before using them to evaluate design alternatives for ventilation systems or machine enclosures.
The standard is applicable to machinery that emits gaseous hazardous substances during operation, including welding fume (gas component), solvent evaporation, chemical processing equipment, and combustion engine exhaust. The method provides both controlled and uncontrolled emission rate measurements.
Tracer Gas Methodology
Selection of Tracer Gas
ISO 29042-2 specifies criteria for tracer gas selection. The ideal tracer gas should be non-toxic, non-reactive, not naturally present in the workplace, measurable with high sensitivity, and have density similar to air. Sulphur hexafluoride (SF6) is the preferred tracer gas due to its excellent detectability at very low concentrations, chemical inertness, and well-characterized behaviour. Nitrous oxide (N2O) is an acceptable alternative where SF6 is restricted.
| Parameter | Specification | Engineering Significance |
|---|---|---|
| Tracer gas (preferred) | SF6 (sulphur hexafluoride) | Detection limit less than 1 ppt, inert, non-toxic |
| Tracer gas (alternative) | N2O (nitrous oxide) | Detection limit less than 10 ppb, lower GWP |
| Release rate accuracy | +/-2% of nominal rate | Critical for overall measurement uncertainty |
| Mixing verification | Uniformity within +/-10% at sampling plane | Ensures representative concentration measurement |
| Sampling duration | Minimum 5 steady-state time constants | Ensures stable concentration field established |
| Detection method | ECD or IR spectrometry | Required sensitivity and selectivity |
A common mistake in tracer gas testing is inadequate mixing between the tracer gas and the pollutant source airflow. If the tracer is not uniformly mixed with the source air, the measured concentrations will not be representative. ISO 29042-2 requires verification of mixing uniformity by measuring at multiple points in the sampling plane.
Measurement Strategy
The standard defines two measurement strategies: the direct method (measuring tracer concentration in the exhaust duct) and the indirect method (measuring tracer concentration in workplace air at specified locations). The direct method is preferred when the machine has a ducted exhaust system. The indirect method is used for machines with unducted outlets or when capture efficiency must be evaluated.
Engineering Considerations for Accurate Emission Measurement
The accuracy of the tracer gas method depends critically on the stability of the tracer release rate and the precision of concentration measurements. Mass flow controllers for tracer release must be calibrated against primary standards, and the gas chromatograph or IR spectrometer used for concentration measurement must be calibrated with certified gas mixtures before each test series.
For machines with variable emission rates, ISO 29042-2 specifies time-integrated sampling over complete operating cycles. The standard provides guidance on determining the appropriate sampling duration based on the process time constant. For machines with multiple emission sources, the tracer gas can be released at each source sequentially to determine the contribution of each source to the total emission rate.
When applying the tracer gas method to machines with large volumetric flow rates exceeding 10,000 m3/h, engineers should consider using multiple tracer injection points. A single injection point may not achieve uniform dispersion in large ducts. CFD pre-analysis can help optimize the number and location of injection points.
Frequently Asked Questions
Q1: Can the tracer gas method be used for particulate pollutants?
No. The tracer gas method is applicable only to gaseous pollutants. For particulate pollutants, the test bench methods specified in ISO 29042-3, ISO 29042-5, and ISO 29042-6 should be used. The different aerodynamic behaviour of particles compared to gases makes the tracer approach invalid for particulates.
No. The tracer gas method is applicable only to gaseous pollutants. For particulate pollutants, the test bench methods specified in ISO 29042-3, ISO 29042-5, and ISO 29042-6 should be used. The different aerodynamic behaviour of particles compared to gases makes the tracer approach invalid for particulates.
Q2: What is the typical measurement uncertainty?
With careful execution, the tracer gas method can achieve measurement uncertainties of +/-10-15% (k=2) for the emission rate. Major contributors include tracer release rate accuracy, concentration measurement precision, and mixing uniformity.
With careful execution, the tracer gas method can achieve measurement uncertainties of +/-10-15% (k=2) for the emission rate. Major contributors include tracer release rate accuracy, concentration measurement precision, and mixing uniformity.
Q3: Are there environmental concerns with SF6 tracer gas?
SF6 is a potent greenhouse gas with GWP approximately 23,500 times that of CO2. ISO 29042-2 requires that SF6 release be minimized and recommends capturing exhaust gases when possible. N2O is specified as a lower-GWP alternative.
SF6 is a potent greenhouse gas with GWP approximately 23,500 times that of CO2. ISO 29042-2 requires that SF6 release be minimized and recommends capturing exhaust gases when possible. N2O is specified as a lower-GWP alternative.
Q4: How does the method account for background concentrations?
The standard requires measurement of background tracer concentration before each test. If background exceeds 1% of expected measurement concentration, corrective action is required. Background concentrations are subtracted from measured concentrations during data analysis.
The standard requires measurement of background tracer concentration before each test. If background exceeds 1% of expected measurement concentration, corrective action is required. Background concentrations are subtracted from measured concentrations during data analysis.
The selection between SF6 and N2O as tracer gas involves practical trade-offs beyond detection sensitivity. SF6 offers superior detectability but has a global warming potential approximately 23,500 times that of CO2, leading to increasing regulatory restrictions. N2O is more environmentally acceptable but requires higher release rates and more sensitive detection equipment. Some testing laboratories have begun using alternative tracer compounds such as perfluorocarbon tracers (PFTs) that combine good detectability with lower environmental impact.
The tracer gas method requires careful control of test room ventilation conditions. The standard specifies that the general ventilation in the test area should be maintained at a constant rate during measurements, with air change rates recorded. For accurate capture efficiency measurements, the interaction between the local exhaust ventilation and the general ventilation must be characterized, as cross-drafts from general ventilation can significantly affect capture performance.
When measuring capture efficiency using the tracer gas method, the positioning of the tracer gas release point relative to the emission source is critical for obtaining representative results. ISO 29042-2 specifies that the tracer gas should be released at the same location and with similar dispersion characteristics as the actual pollutant source. For applications where the source is diffuse or moving, multiple release points may be necessary to characterize the overall capture performance. The standard also addresses the stability criteria for the measured concentrations, requiring that readings remain within specified tolerances during the measurement period to ensure valid results.
The choice of tracer gas detection instrument affects the sensitivity and accuracy of the measurement. Photoacoustic spectroscopy instruments offer high sensitivity and selectivity for SF6 detection at parts-per-billion concentrations, while flame ionization detectors are suitable for hydrocarbon tracer gases. The detection limit must be at least an order of magnitude below the expected tracer concentration in the exhaust to achieve acceptable measurement uncertainty. Regular calibration of the detection instrument using certified gas standards is essential for maintaining data quality in accordance with ISO 17025 laboratory accreditation requirements.
