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Emissions Testing – Gravimetric Determination of Particulate Matter

Filter Weighing for Gravimetric Determination of PM in Engine Exhaust and Evaporative Emissions

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Emissions Testing

Emissions Testing

Weight of particulate matter collected on filters for Emissions Testing can be extremely low, therefore the weighing procedure has to be performed very accurately. See why METTLER TOLEDO Excellence Micro-balance, equipped with especially developed accessories for filter weighing, is the best choice for Emissions Testing application.

Jump to one of the following section to explore and learn more:

  1. Application Workflow and Challenges
  2. METTLER TOLEDO Solutions
  3. FAQ's


Emission Testing Workflow

Whether measuring particulate matter in exhaust gases, evaporative emissions, the atmosphere or ambient air, the gravimetric filter weighing emissions test follows the same procedure.

1. A blank filter is first weighed on a balance in the laboratory.
2. Filters are loaded into labelled filter cartridges, preconditioned and transported to the test site.
3. At the test site, the air or gases are drawn through the filter at a known flow rate for a specific duration. Any particulate matter (PM) present is retained on the filter.
4. In the laboratory, the loaded filters are conditioned again and re-weighed.
5. By subtracting the weight of the blank filter, the mass of PM can be determined (differential weighing). Concentration of PM is calculated using the flow rate of the gas/air sample.

Whilst the test can be performed using just one filter, it is common practice to repeat the test several times as well as to retain a blank filter as a reference to eliminate background count. Filters may be used singly or in combination in a multiple-stage filter unit.

Typical Workflow: Filter Weighing Process

  • The blank filters are weighed on a laboratory balance
  • Filters are loaded into labelled filter cartridges
  • Filters are preconditioned prior to the emissions test
  • The cartridges are transported to the test site
  • PM can be collected using either a single-stage or a multiple-stage filter
  • Several tests may be performed
  • In the laboratory, the loaded filters are conditioned again and re-weighed
  • The mass of the particulate matter is determined from the weight differential
  • The concentration of PM is calculated using the volume of the gas/air sample

Emissions Testing Know-How

The World Health Organization (WHO) considers clean air "to be a basic requirement of human health and well-being" and provides guidelines for selected pollutants which are applicable across all WHO regions.

Emission standards set quantitative limits on the permissible amount of specific air pollutants that may be released from specific sources over specific timeframes.

In the U.S., CFR 40 Part 50 is part of the United States Code of Federal Regulations and concerns national air quality standards. The Occupational Safety and Health Administration (OSHA), part of the United States Department of Labor, published regulations (Standards – 29 CFR) concerning air quality, and the National Institute for Occupational Safety and Health (NIOSH) is also concerned with promoting safety and health at work.

The European Union sets legislation in the form of DIRECTIVE 2008/50/EC on ambient air quality and cleaner air for Europe; DIRECTIVE 2008/1/EC concerns integrated pollution prevention and control; and DIRECTIVE 89/654/EEC specifies the minimum health and safety requirements for the workplace.

The automotive sector is a key source of a variety of air pollutants including carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), particulate matter (PM), as well as greenhouse gases (GHG).  In both the European Union and in the United States, legislation exists to control emissions from motor vehicles and motor vehicle manufacturers are obligated to carry out emissions tests.

There are many methods and guidelines for testing emissions and particulate matter concentration. One of them is the gravimetric filter weighing method. Guidelines and regulations define the procedures to ensure accurate, reproducible filtering of emissions, and specify which filters should be used and which weighing system to use for the measurement as shown in the table below.

Examples of guideline requirements for weighing equipment and recommended balances:



Required repeatability
CFR 40
Part 1065
Engine testing procedure0.1 μg2.5 μg /
0.25 μg
DIRECTIVE 2004/26/ECEmission of gaseous and particulate pollutants from internal combustion engines to be installed in non-road mobile machinery1 μg2 μg 
DIRECTIVE 2005/78/ECEmission of gaseous and particulate pollutants from compression-ignition engines for use in vehicles and the emission of gaseous pollutants from positive ignition, engines fueled with natural gas or liquefied petroleum gas for use in vehicles1 μg (70 mm filter) / 0.1 μg (47 mm filter)2 μg

It is foreseen that, as a result of increasingly tighter regulations combined with technological advancements in combustion engines, the amount of PM collected on filters will gradually decline. With guidelines typically specifying a balance with a readability of 1 μg or 0.1 μg, a highly accurate ultra-microbalance is therefore the only solution to meet both today's and future needs.

Emissions Testing Challanges
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Emissions Testing Best Solutions

Emissions Testing

Weight of particulate matter collected on filters for Emissions Testing can be extremely low, therefore the weighing procedure has to be performed very accurately. See why METTLER TOLEDO Excellence Micro-balance, equipped with especially developed accessories for filter weighing, is the best choice for Emissions Testing application.


NVR Testing - Micro weighing Guide

Software solution for Emissions Testing
Automated Filter Weighing

What to Do If You Need to Weigh Large Filters?

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What's the best way to get fast and reliable results when weighing filters?

Firstly, the environment is important. Ideally, your balance should be located on a dedicated weighing table in a room with a stable temperature and controlled humidity. The filter weighing kits for METTLER TOLEDO microbalances and ultra-microbalances contain a special weighing pan which ensures filters are correctly placed on the balance. Correct filter handling with asymmetric tweezers will help ensure your filter weighing processes are fast and efficient.

I'm using PTFE filters but my balance doesn't settle easily and the figures are drifting. Can you explain?

PTFE filters are particularly prone to electrostatic charging. The unstable figures on your balance suggest that the weighing cell is being affected by electrostatic charges. The influence from electrostatic charges can significantly affect weighing results. Use an AntiStatic Kit before weighing each filter to remove any charges which may be present.

I have to weigh different types and sizes of filters.

METTLER TOLEDO offers filter weighing solutions for both microbalances and for analytical balances. There are two different filter weighing kits for microbalances, enabling you to weigh filters with diameter from 20 mm to 70 mm; it's easy to switch between the kits. For weighing larger filters up to 150 mm in diameter on an analytical balance, there are two filter holder solutions available.

The regulations we are following require that filters must be positioned centrally when weighed. How can I ensure this?

The METTLER TOLEDO micro filter weighing kit includes two filter pans. Each has a unique contour which ensures the filter is automatically centered. Centering the filter eliminates eccentricity errors to ensure reproducible and reliable results.

How can I be sure my filter weighing SOP is followed?

A clear SOP should be in place for weighing filters in a regulated environment. However, in a manual process, human errors can occur, and in a process which requires many steps, it can be easy to make a mistake. LabX software enables you to create your own dedicated filter weighing SOP workflow. The step-by-step instructions for the SOP are provided on the balance display. Each step requires confirmation from the user so you can be sure the SOP has been followed.

Which balance do I need to use to get accurate filter weighing results?

Every measurement on ANY balance is subject to uncertainty. Understanding this uncertainty is the key to ensuring accurate weighing results. It is not the readability that determines the accuracy of a weighing instrument, but rather its repeatability. With the very small sample weights involved in filter weighing, the balance's minimum net sample weight capability is also of critical importance.

To find the appropriate balance for your needs, you need to know the smallest amount you want to weigh and how accurately you need to weigh it (i.e. at what tolerance).

METTLER TOLEDO's global weighing standard, GWP® helps you choose the right balance to meet your application requirements. Ask your local representative for a free balance recommendation. Determine if your existing balance meets your quality requirements.

GWP Recommendation

How can I improve productivity when weighing filters?

METTLER TOLEDO filter weighing kits have glass lids so it's easy to see what you are doing when placing and removing filters. The microbalance draft shield doors can be set to open to the left or the right according to the needs of the operator so that filter handling is more ergonomic. To speed up the process further, the SmartSens infrared sensor can be used to open the doors with the wave of a hand. For high throughput filter weighing, we recommend an automated solution with the PFS-ONE filter weighing robot.

My results have 5 or more decimal places. How can I avoid mistakes when writing the results down?

Manual transcription of data and results carries an inherent risk of error. By connecting your balance to LabX software you can eliminate manual transcription and the associated risk of errors entirely. LabX also performs all your calculations, including air buoyancy corrections, removing a further source of potential errors. All data is saved securely in a centralized database so you benefit further from complete data integrity and traceability.

How can avoid potential mix-ups of my filters? I need to ensure traceability too.

It's vitally important to label the filter cartridges and ensure that results are logged correctly against each filter. When handling many filters, this can be a challenge. The process can be greatly simplified by connecting your balance to LabX, a barcode reader and a printer. LabX prints barcoded labels automatically. By using barcodes, the samples are uniquely identified. LabX uses the barcodes to automatically log the weight results against the correct sample. All data is saved securely in a centralized database providing full traceability. The PFS-ONE filter weighing robot can process up to 1,000 filters per day in a fully automated process with all data handled automatically.

FAQs – Frequently Asked Questions on Emissions Testing