Air pollution control
The striving for mobility continuous relentlessly in all areas of daily life. All over the world the volume of traffic has increased steadily and continues to grow.
Passenger transport in germany
Hence it is necessaire to continue the roadmap and to reduce the emission limits with regard to the air pollution control. In this process the emission of particles moves more and more in the public focus. This is due to the fact that the emission limits are often exceeded in metropolitan areas. Particularly the ultra-fine particles are a health risk.
Against this background the legislators worldwide have tighten the emissions limits for combustion engines (gasoline and diesel engines). This trend is continues. The regulations will be extended for all types of vehicles (light and heavy duty vehicle, motorcycles, mopeds etc.).
The verification of this tight emission limits is a challenge in the exhaust air analysis. With the decreasing emission limits the risk of measuring errors is increasing. Hence there are weighing processes required that can provide precise and repeatable results. With the extension of the provisions the weighing and filter volume of those system is arising.
Particle analysis per gravimetry
Gravimetry is a process that is prescribed for the type approval of combustion engines and that is applied for the determination of particulate residues. In this process the weight of the unloaded filter samples and subsequently with soot particles loaded samples, is determined under tight environmental conditions. The soot particles are deposit on the filter samples during defined test cycles. Thereby the filter samples have to be recorded and stored traceable to each test cycle.
For the residue measurements highest accurate balances –so-called ultra micro-balances - with a resolution of 0.1 µg are used. According to the current legislation a repeatability of 0.5 µg is required. For reference filter measurements a long-term stability of 5µg/30d (maximum drift) has to be met.
To be able to converse the weighing value into the real mass, it is required to have the air density. The air density can variate by an average of ± 0,05 mg/cm3 within a day related to the standard density of 1.2 mg/m³. For that reason the air density must calculated for mass determination of soot particles and a buoyancy correction (principle of Archimedes) performed.
In the gravimetric process disturbing factors have to be kept as low as possible. Mayor factors are:
- Stability of temperature and humidity in the environment
- Stability of environmental pressure
(dynamic pressure changes by air flow to be minimized)
- Cleanliness of ambient air
(no contamination of the weighing samples by particles)
- Grounding of components
(no electrostatic charging of the weighing samples)
- Precise determination of the air density
(no unprecise measurement values or drift of the sensors)
- Weighing process
(defined weighing procedures, filter handling with tweezers, observance of conditioning time, correct loading of the balance etc.)
- Compliance to construction physical requirements
(low vibration environment)