Gas filters play a key role in ensuring accurate and reliable gas detection readings. They remove impurities and contaminants from air and gas samples, preventing inaccurate readings that can compromise safety, compliance, and proper equipment operation. Common contaminants requiring filtration include humidity, condensate, particulates, acid gases, hydrocarbons, volatile organic compounds, and sulfur gases. Different gas filter materials, or media, are designed to target specific contaminants. Understanding when to use gas filters and selecting the appropriate filter media are essential for maintaining the accuracy and reliability of gas detectors. Proper filtration ensures that gas detectors can effectively identify potential hazards and maintain a safe environment.
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Examples of Gas Filters
When you purchase a gas detector, a gas filter usually comes with the gas detector. The gas filter is an extra component located internal or external to the detector. Here are some examples of how gas filters are used in gas detection.
Metal Mesh Filter on a Gas Sensor
Filter Purpose: large particulate protection
Paper Filter Embedded on Gas Sensors
Filter Purpose: moisture and fine particulate protection
Charcoal Filter Embedded in the Gas Sensor
Filter Purpose: humidity and acidic gas protection
Water Trap and Acidic Gas Filter (in-line)
Filter Purpose: condensation, humidity and acidic gas protection
When Do You Need a Gas Filter?
A gas filter is required whenever you:
- have condensation occurring due to an aggressive dew point change (hot and moist combustion gas, condensation, moisture, water trap)
- have a dirty gas sample that needs conditioning. This means removing particulate, dust, soot and humidity.
- need a clean air gas sample to minimize interference gases adversely effecting the gas sensor.
Acidic Gas, NOx, and SO2 Filter
This popular gas filter is called a NOx filter, SO2 filter or acidic gas filter. NOx is an acidic gas generated as a fossil fuel combustion by-product. The acidic gas can react with a gas sensor (such as a carbon monoxide electrochemical gas sensor) and null (cancel) the electronic signal, hence reducing the accuracy and lifespan of CO readings.
SO2 and NOx filters are designed to install inline on the probe and hose assembly of most brands of combustion analyzers that do not have onboard acidic filters. These filters remove, NO2, SO2, NO and other sulfur compounds such as H2S and mercaptans.
Acid gas filters are often found in a Combustion and Exhaust Gas Emission Analyzers, Biogas and Landfill Analyzers, and Toxic Gas Analyzers used in industries such as power generation, chemical processing, and oil refining. Acidic gas filters are made using various blends of activated carbon, alumina zeolite and Potassium permanganate.
Particulate and Dust Filter
All gas detectors and gas sensors have some type of particulate filter installed. Particulate and dust filters are designed to remove solid particles from the gas sample, such as dust, dirt, and other debris.
These filters typically consist of a porous material that allows gas to pass through while trapping solid particles. The filters are categorized by pore size. For example, below is our 1 inch probe with 1 micron polyethersulfone micropore disc filter made for headspace analysis, packaging gas analysis, modified atmosphere analysis and small volume gas sampling.
Gas filters can be made of various materials, including paper, synthetic polymers, ceramic, and metal. In some cases they are also made to be hydrophobic in order to repel moisture and protect the gas sensor element.
VOC and Odor Filter (Gas)
VOCs are a group of chemicals that have low boiling points and can easily evaporate into any air or gas sample. VOCs have that annoying "plastic" smell that gives people headaches. A VOC filter is designed to remove VOCs from the sample air, ensuring minimal cross interference effects within the gas detectors' sensor element. The most popular VOC filter is made using activated carbon which removes organic VOC compounds by adsorption.
Hydrocarbon Gas Filter (HC Gas)
Hydrocarbon gas filter are required to remove contaminants such as methane, propane, butane and other CH (hydrocarbon) compounds. Removal is often required to not create sensor cross interference issues and further ensure accurate detection of a target gas.
Molecular sieve materials are excellent at filtering hydrocarbons.
Water Trap (Filter)
The water trap is designed to remove water vapor and condensate from gas streams before they can reach the detector or analyzer. A water trap filter is a small compartment of volume that holds the condensate water yet allowing the sample gas to flow to the gas detector without clogging or blocking the air line. Water traps and used in combustion analyzers, flue gas analyzers, emission analyzers and any other gas measurement application where hot and moist is required to be sampled.
When a hot and moist gas sample exists, it is drawn through the gas line and quickly cools which changes the dew point and creates condensate (water). Before any filtering, the condensate must be trapped so it does not proceed further in the gas sample line to minimize clogging, blocking or water damage.
Below is a image of a water trap filter with a acidic gas filter in line as part of a combustion flue gas analyzer.
Humidity and Moisture Filter
Gas detectors always prefer to received clean and dry gas. Humidity and moisture present in the gas can cause interference and lead to incorrect readings hence why the usual gas detector maximum operating humidity specification is about 90%RH. Moisture and humidity filters are used to dry the air sample prior to gas detection.
These filters work by removing water vapor and other moisture from the gas stream. They typically contain a desiccant material that absorbs the moisture, leaving the gas dry and free from humidity interference. Some filters also have a hydrophobic membrane that repels water and prevents it from entering the gas stream.
Molecular Sieve Filters (Gas)
Molecular sieve filters are a type of gas filter used in gas detectors and analyzers to remove impurities and contaminants from gas samples. They are highly effective at removing moisture, hydrocarbons, and other volatile organic compounds (VOCs) that can interfere with gas analysis.
Molecular sieve filters work by using a crystalline material that has uniform pores. These pores are small enough to prevent large molecules from passing through, while smaller molecules can. Adsorbed species include SO2, CO2, H2S, C2H4, C2H6, and C3H6. These materials are very stable and can be regenerated when exposed to high temperature up to 200C.
Zeroing Filter
Zero calibration is a technical procedure typically undertaking by nearly all gas detectors and analyzers. This procedures feeds zero gas, or clean gas, with zero target gas onto or into the gas sensor, to achieve a true zero baseline.
Typically, zeroing is mostly achieved by taking your gas detector into open ambient fresh air, pressing the "zero cal" button and you are done. In most cases this is suffice. However, in certain situations, zero gas is needed to be fed to the gas detector because the gas detector may not be portable or the environmental ambient gas may be contaminated. In these cases, a zeroing filter is required to remove the target gas (target analyte). This process ensures that the instrument reads zero when there is no gas present.
Typical zero filters most employ activated charcoal.
Combustion, Flue Gas Filters and Exhaust Gas Filters
Combustion and flue gas streams have many contaminants that can damage gas detection equipment. They gas stream have a high content of moisture, high temperature and many acidic gas components that can hamper gas sensing accuracy.
Similar to combustion and flue gas emissions, exhaust gas from automobiles, motorcycles, forklifts and other combustion fossil fuel burning engines emit "dirty" streams of gas that is contaminated with particulate, soot, moisture and acidic gases.
Combustion flue gas filters are usually a combination of the following:
- Water trap
- Acidic Gas Filter (NOx Filter)
- Particulate Filter (combination of external and internal to the gas detector and or gas sensor)
From our experience, a common blunder is when customer purchase basic carbon monoxide detectors and undertake flue gas measurements. This is obviously not recommended, and only a combustion or flue gas analyzer with the appropriate filters combination will suffice.
High Temperature Gas Filters
In some cases, special situations require custom filtering systems to deal with high temperature gas emissions. Such a special case is exhaust gas measurement from a high temperature oven kiln. In this case, a heavy duty thermal radiator is called for along with a heavy duty NOx acidic filter.
Gas Filters, Scrubbers and Absorbents
To ensure accurate readings, gas filters, scrubbers, and absorbents are used to remove impurities from gas samples. While all three are used to clean gases, they work differently and are used in different applications. It should be mentioned however, that folks in this business down usually break down the nomenclature with such fidelity, so wherever the media, they are all often referred to as "filters". For this part of the article, we will separate them out, so you have some insight into the nomenclature.
Filters
Gas filters are used to remove particles, liquids, and gases from gas samples. They work by trapping impurities in a filter medium, typically made of activated carbon, silica gel, or molecular sieve. Filters are effective in removing impurities that are larger than the pores of the filter medium. These filters include particulate filters and water traps (filtering condensate).
Absorbents
Gas absorbents are used to remove gases from gas samples. They work by exposing the gas sample to a absorbent material, which reacts with the gas and removes it from the sample. Absorbents are commonly used to remove gases such as carbon dioxide, ammonia, and nitrogen oxide from gas samples. They are also used in industrial applications to remove pollutants from exhaust gases.
Scrubbers
Gas scrubbers are used to remove gases from gas samples. They work by passing the gas sample through a liquid scrubbing medium, which absorbs the gas. Scrubbers are commonly used to remove acid gases, such as sulfur dioxide, carbon dioxide, heavy metals, hydrogen chloride, from gas samples. They are also used in industrial applications to remove pollutants from exhaust gases or process gases. "Scubbers" are not commonly employed in portable gas detection analysis but "scrubbing" techniques are mostly employed in large industrial facilities to remove contaminants in gas emissions to ensure compliance with EPA requirements.
Charcoal Gas Filter Media
Charcoal gas filters, also known as activated carbon filters, are one of the most common types of gas filters. They are made from activated charcoal, which has a large surface area and high gas adsorption capacity. Charcoal filters are effective at removing organic compounds, such as hydrocarbons and VOCs, from the gas stream. They are also useful for removing odors from the gas.
Silica (SiO2) Gel Gas Filter Media
Silica gel is a commonly used desiccant, which means it absorbs moisture from the surrounding air and often incorporated into gas filter media to remove moisture. The material has a high specific surface area at around 750–800 m2/g. The material can be regenerated by placing the silica beads in a microwave for a few minutes.
Activated Alumina Gas Filter (Aluminum Oxide) Media
Activated alumina gas filters, also known as aluminum oxide filters, are another popular choice for gas filtration. They are made from a porous form of aluminum oxide, which has a high surface area and adsorption capacity. Activated alumina filters are effective at removing moisture and other polar compounds from the gas stream. They are often used in conjunction with other filters, such as charcoal filters, to provide comprehensive gas filtration.
Molecular Sieve Zeolite Gas Filter Media
Molecular sieve zeolite gas filters are a type of synthetic zeolite that is designed to selectively adsorb certain molecules. Sometimes also referred to as Zeolitic imidazolate frameworks (ZIF). They are often used for gas separation and purification, as they can remove specific impurities from the gas stream. For example, molecular sieve zeolite filters can be used to remove water, carbon dioxide, and other polar compounds from the gas. They are commonly used in natural gas processing, petrochemical production, and other industrial applications.
Potassium Permanganate (KMnO4) Gas Filter Media
Potassium permanganate gas filters are a type of chemical filter that uses potassium permanganate to oxidize and remove impurities from the gas stream. They are effective at removing sulfur compounds, such as hydrogen sulfide, from the gas. Potassium permanganate filters are often used in wastewater treatment and other applications where odor control is important.
Metal Organic Framework (MOF) Filter Media
Metal-organic framework (MOF) filters are a relatively new type of gas filter media that are gaining popularity due to their high selectivity and tunability. MOFs are a type of porous material that consists of metal ions or clusters connected by organic ligands.
They can be designed to selectively adsorb specific molecules or classes of molecules, making them ideal for gas separation and purification. MOFs have potential applications in natural gas processing, carbon capture, and other industrial processes.
In fact, they are so selective and tunable, Dr. Koz invented MOF sensors with this filter media.
What Gas Can You Smell In A Gas Filter?
Gas filters can trap various odorous gases including natural gas (methane) with its added mercaptan odorant, carbon monoxide, sulfur compounds, and volatile organic compounds (VOCs). However, if you smell gas around a filter, it likely indicates a leak that requires immediate professional inspection.
Final Words
Gas filters are essential components of gas detectors that remove impurities and contaminants from gas samples, ensuring accurate readings. These filters can be located in various parts of the detector, such as the sample gas line or within the sensor element. Gas filters are necessary when dealing with condensation, dirty gas samples, or when a clean sample is required to minimize interference gases. Different types of filters, such as gas filters, scrubbers, and absorbents, are used to remove impurities but have distinct applications. Gas filter media, including charcoal, activated alumina, molecular sieve zeolite, potassium permanganate, and MOF filters, have unique pros and cons and target specific impurities.
About The AuthorDr. Kos Galatsis ("Dr.Koz") is the President of FORENSICS DETECTORS where the company operates from the scenic Palos Verdes Peninsula in Los Angeles, California. He is a subject matter expert on gas sensor technology, gas detectors, gas meters, and gas analyzers. He has been designing, building, manufacturing, and testing toxic gas detection systems for over 20 years. 
Every day is a blessing for Dr. Koz. He loves to help customers solve their unique problems. Dr. Koz also loves spending time with his wife and his three children going to the beach, grilling burgers and having a cold beer. Read more about Forensics Detectors. Email: drkoz@forensicsdetectors.com |