Emissions testing is an important procedure to measure and control air pollution from sources such as vehicles with combustion engines, power-plants, and other motorized equipment that produce exhaust and evaporative emissions.

Particulate matter (PM), also known as particle pollution, is the general term used to describe solid particles and liquid droplets found in the air. Once inhaled, these particles can affect the heart and lungs, and cause serious health problems. Particle classes PM10 and PM2.5 (i.e. particles with diameter less than 10 µm and 2.5 µm respectively) are carefully monitored, both at emissions sources and in the air in general, due to potential health risks.

Gravimetric analysis of PM collected via filtration is the most widely used technique for determining ambient concentrations of PM as well as the amount of PM in emissions.

Why Accurate Weighing is Important

Weighing particulate matter on filters is a differential weighing application. As the amount of particulate matter collected is very small in terms of weight, the filters must be weighed very accurately, both before and after loading of the particulate matter.
As a general rule, guidelines specify a balance with a resolution of 1 μg or 0.1 μg, with corresponding repeatability.

Weight of particulate matter collected on filters for emissions testing can be extremely low, therefore the weighing procedure has to be performed very accurately. Excellence Level microbalances with a dedicated filter weighing kit are perfect for emissions testing. The outstanding repeatability and lowest possible minimum weight provide the weighing performance you need.

The following illustration describes the process for determining the amount of Particulate Matter on filters gravimetrically:

Ideally, a cleanroom is preferable for filter weighing to avoid potential contamination from the surrounding air as well as to have more control over temperature and humidity. Alternatively, a glove box can be used but this makes the process rather cumbersome, particularly when weighing large numbers of filters. In addition, there are several other challenges associated with weighing filters:

Filter Handling

• Material: Some filter materials are soft and difficult to handle. Users must take care not to touch the surface or damage the filter (which can happen all too easily).
• Humidity: One of the biggest challenges when weighing filters is that they are highly sensitive to humidity in the environment. Their hygroscopic behavior makes them appear heavier, affecting the accuracy of results. In addition, PM deposits can also retain water affecting the overall result.
• Electrostatic Charging: Filters are also prone to electrostatic charging which,
  if not addressed, can attract additional particles and lead to skewed results.
  These electrostatic charges can also set up fields of attraction/repulsion in the surroundings, influencing the weighing result.
• Buoyancy: Filters have a low density which makes them susceptible to air buoyancy and they appear to be lighter than they actually are. Air buoyancy influence differs with changing temperature, air pressure and humidity.
  By weighing the filters in the same environment (e.g. cleanroom) before and after PM loading, the buoyancy effect could be considered to be equal in both cases. However, to reduce uncertainty the true weight must be obtained by compensating for the air buoyancy influence.

Sample Tracking

To add further process complexity, the filters must be tracked precisely. Each filter is pre-weighed, placed in a labelled holder, acclimatized, and then transported to the test site. After PM loading, filters are transported back to the lab and reweighed. Accuracy is put at risk if there is not a failsafe system in place to ensure filters are not mixed up during processing.

Weighing Accuracy

With several guidelines specifying the weighing system to be used, it is crucial that the balance used meets the needs of the filter weighing application. For such small samples, the minimum net sample weight of the balance is of critical importance – weighing a sample below this weight cannot be trusted to be of the required level of accuracy.

Time

Filter weighing for emissions testing can take a long time to complete. Acclimatization processes, transport to and from the test site, laborious handling procedures and a balance which is slow to settle contribute to a typical car exhaust emissions test taking up to 2 days or longer to finalize.

METTLER TOLEDO XPR microbalances offer unique precision and exceptionally low minimum weights. Outfitted with a special filter weighing kit and with 0.2 µg readability, the XPR2U is ideal for filter weighing and has the additional benefit of built-in active quality assurance functions to ensure weighing results meet defined tolerances and standards. Built-in user guidance ensures the differential weighing process is followed precisely.
 

The Right Tools

The addition of a barcode reader offers quick sample identification and the convenience to automatically match loaded filters with pre-weighed results for faster processing and fewer handling errors. Plus, passing the filters over the electrode of an AntiStatic Kit neutralizes any electrostatic charges present and ensures the true PM result is measured.
 

Thanks to multiple connectivity options, weighing results can be exported directly to a spreadsheet on a PC or to a company database system for inspection and further analysis.

METTLER TOLEDO tweezers are specially designed to ensure correct handling of the delicate filters as they are transferred to and from the storage cartridge and the filter weighing kit on the balance.

Watch this video to learn the main product highlights of METTLER TOLEDO's filter weighing solutions for emissions testing.Choose from innovative manual solutions using an analytical or microbalance to a fully automated solution with a robotic system, developed together with HORIBA.

Maximize the quality of your weighing results within ten steps and make the delicate process of micro-weighing more efficient.

METTLER TOLEDO XPR microbalances are fully compatible with LabX™ laboratory software. LabX provides full SOP user guidance on the balance. It recognizes the loading and unloading of filters, simplifying and speeding-up the process.

LabX automatically records the weight values against the correct sample IDs and performs all calculations automatically, including correction for air buoyancy.

All data and results are saved securely in a centralized database providing full traceability. Customized reports can be printed out or transferred to an internal information system.

When more than 40 filters are weighed per day, the PFS-ONE Filter Robot, in combination with the XPR2U, is the perfect automated solution.

The innovative filter holder allows identification, acclimatization, storage, transport and weighing all in one cartridge, without interruption or removal of the filter. It also acts as a Faraday Cage thereby reducing electrostatic influences.

Automated sample changing allows effortless filter pre- and back-weighing and results are immediately transferred to a PC for analysis.

If you're using large filters, for example for analyzing industrial filtrations, METTLER TOLEDO has handy and cost-effective solutions for weighing larger filters on your analytical balance. With the addition of a filter weighing kit, filters up to 110 mm can be conveniently weighed on a METTLER TOLEDO Excellence analytical balance. To weigh filters up to 150 mm, the use of a dedicated ErgoClip is recommended. Both solutions can be quickly and easily installed on your existing balance.