Absolute Rated – Referring to a filter that has a filter efficiency at least greater than 97% and often greater than 99%.
Absorb – A process where particles move into the absorbent material because of material structure.
Activated Carbon – A filtration aid that uses adsorption to remove specific contaminates. These contaminates often are related to an undesired smell or color.
Adsorb – A process that happens on the molecular level. Particles are moved by chemical attraction.
Bag Filter – Can refer to any filter made of a fabric or lofted material sewn or welded together.
Batch Filtration – Running a discrete amount of fluid through a filtration set up at a time. Can be contrasted with continuous filtration. Often involves the use of recirculation to achieve a more uniform end state.
Beta Ratio – A measure of how efficient a filter is with a higher ration corresponding to a higher efficiency.
Biological Filtration – Where the separation process is enacted by microbes and other biological organisms as opposed to by mechanical or chemical separation. A biological filter is a desirable surface for the biological layer to form, often with pores to increase surface area.
Blinding – When a filter can no longer allow the fluid to pass through as intended. The filter is spent when it is blinded.
Cake – Refers to a build up of separated solids on the face of a filter but is still permeable (in contrast to filter blinding). A filter cake is often a desired thing as it can increase throughput and increase filter efficiency simultaneously using the principles of depth filtration.
Continuous Filtration – Where the fluid goes from raw state to a finished product in a continuous pass. This flow does not stop between runs.
Diatomaceous Earth – A filtration aid made from the remains of diatoms. Abbreviated “DE”. Prevents blinding and increases filtration efficiency by spreading out the particulate. Normally, a layer of particulate builds on the surface of the filter eventually blocking flow. Because of the structure of the diatoms, fluid can continue to move through the diatomaceous earth even when it looks completely covered. So, when you mix diatomaceous earth with the fluid and the particulate, the combination will build up on the surface of the filter creating a filter cake.
Diffusion – Movement from a region of higher concentration to a region of lower concentration. Osmosis is the diffusion of water molecules.
Filter – A separation device that removes… The substance that is removed could be rigid, semi-rigid, viscoelastic, or is … as is the case with adsorptive and absorptive filters like molecular sieves, fullers earth filters, silica desiccants, activated carbons, and other filters that take advantage of electron affinity to attract and retain retentates for permeates.
Filter Efficiency – The percentage of particulate of a stated size stopped by the filter.
Filtrate – The liquid passing through a filter.
Filtration Aid – A substance added to a solution to increase the filtration efficacy and/or increase throughput.
Filtride – The particulate suspended in the filtrate. What is caught by the filter.
Fluid – A viscous substance that will deform continuously under infinitesimal sheer stress. Encompasses both liquids and gases.
Fuller’s Earth – Another name for diatomaceous earth.
HEPA – An acronym for High Efficiency Particulate Air [filter]. It is an industry classification for air filters that remove > 99.97% of airborne particles with a size of 0.3-micron diameter or larger.
Lofted – When a material is expanded to have pockets of air. Usually means something fluffy.
Membrane – A surface that separates out contaminates by diffusion. They work on a molecular scale to perform their separation.
MERV – An acronym for Minimum Efficiency Reporting Value. It is an industry standard for specifying the performance of a filter used in HVAC and other air filtration applications. It specifically refers to the filter’s ability to arrest particles between 0.3- and 10-micron. Higher MERV rated filters are more effective/efficient. The US EPA list the performance criteria for MERV ratings here.
Particulate – Particles that are in a fluid.
Permeate – The liquid (and entrained solids and semi-solids below the rating of the filter) that pass through the filter.
Retentate – The substance captured by the filter.
Strainer – A mechanical separation device that blocks rigid and semi-rigid particles that are entrained in a flowing fluid. The fluid can be a liquid or gas, but the particles retained by a strainer a visible with the naked eye, which is generally accepted to be the 40- to 50-micron range.
ULPA – An acronym for Ultra Low Particulate Air [filter]. It is an industry classification for air filters that remove > 99.999% of airborne particles with a size of 0.12-micron diameter or larger.
Aramid – A synthetic material with a particular chemical structure. Nylon and aramid are often used synonymously, although sometimes aramid includes para-aramids such Kevlar and meta-aramids such as Nomex.
Calendaring – A finishing process for fabric where the material is run through two hot rollers that press the fibers further together. This can positively affect filtration efficiency as the fibers are compressed together. This can enhance the filtration efficiency in two ways: pressing of the fibers creates a more uniform structure and calendaring also helps to lock the fibers in place which provides more consistent performance of the textile in use.
Chain Stitch – One of the two methods of machine sewing, along with lock stitch. Less common but is often used as the safety stitch on overlocked edges. First invented method of machine sewing.
Cover Stitch – A type of stitch that includes threads looping over the edge of the fabric to keep the edge from fraying.
Drape – How easily a fabric falls and bends.
Felt – Felting is a process where loose fibers (not those spun into yarns) are repeatedly agitated, or crushed to entangle and mat them into a cloth. You may be familiar with craft felt, the felt floor savers to put under furniture, or felt hats. Filtration felts have been rigorously tested so while you cannot observe the exact pore size in the material, we still know the filtration characteristics. Felts use depth filtration, which is a way to describe a filter media that captures particles within the matrix of the material itself. This can make for benefits such as a higher dust/particulate holding capacity and is more effective for trapping certain particulates.
Fiber – Material that can be spun into yarns. Alternatively, a way of referring to the material content of a fabric.
Filament – A long single fiber suitable to be woven into a fabric or twisted into a thread. In the production of synthetic fibers, the polymer is extruded which creates filaments longer than can be found in most natural fibers.
Finished Edge – Edges of woven and knit fabrics will fray overtime. To prevent this there are a variety of ways to finish edges. This usually involves covering them either with thread or a binding, but on some occasions, it could refer to using heat to seal the edges in place.
Hand – A subjective term that refers to the feeling of the fabric. Can encompass softness, weight, and drape, with drape being the most pertinent.
Kevlar – The DuPont corporation brand name for something-er-other aramid fiber that has extremely nifty mechanical properties related to abrasion resistance and high- and low-temperature performance.
Lock Stitch – One of the two methods of machine sewing, along with chain stitch. The most common stitch type. Home sewing machines are lock stitch machines.
Mesh – A British and US customary unit that is defined by the number of orthogonal wires per inch. Any conversion between mesh and micron should be considered approximate in nature because of the variables available with making mesh-denominated screens and cloths. Unlike the micron rating, with mesh-sizes, smaller numbers represent larger pore sizes (eg: a smaller number of orthogonal wires per square inch results in larger openings).
Micron – A metric denomination representing one thousandth of a millimeter. In fraction form: 1/1000mm and in decimal form 0.001mm. The smaller the number, the smaller the particle or pore size.
Monofilament – Refers to the usage of single filaments. Nylon and polypropylene are most often extruded into monofilaments. Monofilaments are usually stronger than multifilaments, but are more expensive. The density of a monofilament in addition to the round cross-section and can lead to fiber migration and slippage of the sewn seam. While not related to filtration, fishing line is an example of a synthetic monofilament and silk is an example of a naturally occurring monofilament.
Multifilament – A fabric or thread that is made of filaments twisted together to form the yarn. Multifilament are more susceptible to tensile forces, but can be made at a lower cost as the fibers are not as difficult to produce.
Nomex – The DuPont corporation brand name for a variant of meta-aramids that can withstand high temperatures. Filtration Nomex is stable until 400 F.
Nylon – A polymer in the aramid family. It can be extruded into long monofilaments that either provide a very stiff and boardy fabric, or an extremely light and silky one, depending on the micron rating. Nylon is a good choice if you need excellent tensile strength as it can be extruded into long monofilaments. Nylon also has the advantage of being very temperature stable. It is safe to use from below 0F to 350F.
Plain Weave – The simplest weave pattern where one yarn always crosses a single other yarn. The same weave used in collared shirts. The filtration capability can be determined by measuring the square gap created between the yarns.
Polyester – An inexpensive, widely available polymer, usually PET. It is typically made into multifilament fabrics and has a soft hand.
Polypropylene – A polymer often used in filtration. Despite its relatively low continuous use temperature (165 F), it is still useful because it is chemically compatible with ethanol and other solvents.
PTFE – Stands for polytetrafluoroethylene. Teflon is the DuPont brand name of this material. It is known for its low coefficient of friction.
Raw Edge – An edge that has not been finished. Depending on the fabric may fray a lot or a little. It may take time for it to fray or will fray due to gravity. This is dependent on the weave and the fiber.
Rayon/Viscose – A “semi-synthetic” fabric made from chemically restructured cellulose. The result is a breathable and drapey material which was extremely popular in the mid 20th century. While less common nowadays, this material is experiencing a resurgence because it has the potential to restructure waste fibers into biodegradable garments. Although the technology isn’t yet to make large declarations about the “greenness” of this material due to it sharing the same dangerous or environmentally unfriendly processes that cotton uses as well.
Screen – Usually similar to an open plain weave, screens create square pores and are sealed at the points where the filaments touch. This gives the material more integrity and prevents the filaments from slipping and changing the filtration characteristics.
Stitch – Referring to thread.
Straight Stitch – The stitch most often used when sewing. A lock stitch that is straight.
Twill – A common heavy-duty weave, this weave creates diagonal ridges across the fabric. The most common is created by weaving two yarns over two other yarns, then on the next row, offsetting the start and continuing the pattern. The offset is what creates the diagonal pattern. Two yarns over and two under would be a 2/2 weave, although others exist as well. This weave can be seen in more rugged garments such as jeans and most trousers. Its filtration efficiency is harder to measure as there are not usually pores you can observe visually. Instead, we measure air flow rate, and take cues from that. While you may not know the efficiency precisely, this can still be a useful material if you’re able to test the results.