Filtration Terms and Definitions

ABSOLUTE PRESSURE - The pressure above an absolute vacuum. One atmosphere (14.70 psi) greater than gauge pressure. Symbolized as psia when the pressure is expressed in psi units.
Absolute - A degree of filtration that guarantees 100% removal of suspended solids over a specified size

ABSORPTION - (gen) The taking in, incorporation or reception of gases, liquids, light or heat. (phys/chem.) Penetration of one substance into the inner structure of another (cf. adsorption, in which one substance is attracted and held on the surface of another). Occurs between a gas or vapor and a liquid. (pharm.) The process of movement of a drug from the site of application into the extracellular compartment of the body.

ACTIVATED CARBON - Charcoal activated by heating to 800 - 900ºC to form a material of high adsorptive capacity for many gases, vapors, organics, etc. Has a large internal surface area (approx 1,000 m2/g). Commonly used in the pharmaceutical industry to remove organic contaminants. Can be used either as an additive in granular form which is then filtered out or as a filter media in a filtration device itself.

ADSORPTION - Retention of gas, liquid or solid on a surface due to positive interaction (attraction) between the surface and the molecules of the adsorbed material.

AEROBIC BACTERIA - Organisms which require oxygen to live.

AEROSOL - A dispersion of small liquid particles in a gas.

AMBIENT - Refers to "common" environmental conditions in which experiment is conducted. For example - 14.7 psia and 20º to 25ºC (room temperature).

ANAEROBIC - Organism capable of growing without the presence of oxygen.

ANISOTROPIC (ASYMMETRIC) MEMBRANE - A membrane in which the pore size and structure are not the same from one side of the membrane to the other. Such membranes are usually considered "directional" because of the difference in flow characteristics depending on which side of the membrane faces the feed stream.

ANODE - Positive pore or electrode of an electrolytic system.

AQUEOUS - Similar to or resembling water. In reference to solution made in water.

ASEPTIC - Refers to an operation performed in a sterile environment designed to prevent contamination through introduction of bacteria.

ASSAY - Analytical procedure to determine purity or concentration of a specific substance in a mixture.

AUTOCLAVE(ING) - A chamber for sterilizing with saturated steam filters or equipment by using constant high temperature and pressure (121ºC, 15 psi). One method of ("terminal") sterilization using saturated steam.

BACKPRESSURE - A backward surge of pressure from downstream to upstream of the filter. Can be the result of closing a valve or air entrapped in a liquid system.

BACKWASH - Reversal of a fluid flow through the filtration media, as an attempt to clean or "regenerate" a filter.

BACTERIA - Free living simple celled, microscopic organisms having a cell wall and characteristic shape (e.g., round, rod - like, spiral or filamentous); lack a defined nucleus.

BACTERIAL CHALLENGE - Term used when testing the bacterial retention of a filter.
BAFFLE - A plate or deflector to provide flow distribution in a filter. Primary functions are to prevent erosion of pre - coat and settling of body feed in the filter tank.

BAR - A unit of pressure. One bar = 14.5 psi.

BETA RATIO - Measurement of filter retention efficiency. Ratio of particles exposed to a filter (as feed stream) to particles present in the filtrate.

BIOBURDEN - The load or level of microorganisms in a substance to be filtered.

BIOHAZARD - Biological refuse, possibly pathogenic in nature.

BIOSAFETY - Biological safety or non - toxicity of a substance to a living organism by passing tests as listed in the United States Pharmacopeia. Analogous to "chemically inert." For filters used in biological and health care application, Plastic Class - VI tests apply, which include Systemic Injection, Intracutaneous and Implantation Tests.
BLOWDOWN - The use of air or an inert gas pressure to displace a liquid out of a filter cake. Continued blowdown is used to dry the cake in situations.
BOD - Biochemical Oxygen Demand - A measure of the amount of oxygen required by bacteria for the biochemical degradation of organic material in a water sample.
BODY FEED - The pre - coat media that is continuously added to the filter while it is on stream. Its purpose is to create a permeable filter cake.

BROWNIAN MOTION - The continuous zigzag motion of suspended minuscule particles. The motion is caused by impact of the molecules of the fluid upon the particles.

BUBBLE POINT PRESSURE - A test to determine the maximum pore size openings of a filter. The differential gas pressure at which a wetting liquid (usually water) is pushed out of the largest pores and a steady stream of gas bubbles is emitted from a wetted filter under specific test conditions. Used as filter integrity test with specific, validated, pressure values for specific pore - size (and type) filters.

BUNA - N - A Nitrile rubber seal compound. This is a generic term covering many formulations.

CAKE - Solids deposited on the filter media.

CATHODE - Negative pole or electrode of an electrolytic system.
Cellulose - A fibrous material of vegetable origin used as a filter media.

CENTIPOISE (cP) - (N s/m2; N = Newton) A unit of absolute viscosity. One centipoises equals 0.01 stoke.

CENTISTOKE (cSt) - A unit of kinematic viscosity (m2/s). One centistokes equals 0.01 stoke.

CENTRIFUGATION - Process of separating two substances of differing densities by high speed spinning to create centrifugal force. Typically used to separate suspended particles from liquid.
CFM - Cubic feet per minute.

CHROMATOGRAPHY - The separation of substances in a mixture based on their affinity for certain solvents and solid surfaces.

CLARIFICATION - To clear a liquid by filtration, by the addition of agents to precipitate solids, or by other means.
Clarity - The clearness of a liquid as measured by a variety of methods.

CLASS 100 ENVIRONMENT - A room environment maintained by air conditioning and filtration so that fewer than 100 particles of size 1µm or larger are found in a cubic foot of air.
Cloth - A type of woven filter spectrum made from natural or synthetic yarns.
COALESCING - The separation of mixtures of immiscible fluids, (oil and water) with different specific gravities, can be accomplished due to the fact that small droplets of each pass through the coalescing media to enlarge, thus become larger droplets, then separate out of solution.

COLD STERILIZATION - Removal of all bacteria by filtration through a sterilizing grade 0.2µm absolute filter.

COLUMN - Tube or cylinder containing the chromatographic bed or stationary phase, usually in the form of beads.

COMPATIBILITY - Term used in relation to the non - reactivity of filter materials with the substance to be filtered.

CONCENTRATOR - An apparatus or method for removing some of the water from a sample to concentrate the substances dissolved or suspended in it; usually used to concentrate solutions of biological macromolecules, e.g., proteins and nucleic acids.

CROSSFLOW (TANGENTIAL FLOW) FILTRATION - A filtration system in which the feed stream flows across the filter media and exits as a retentate stream. The retentate stream is recycled to merge into the feed stream, while a portion of it passes through the filter media, resulting in concentration of the feed stream (referred to as concentrate).

DELTA () P - See "Differential Pressure".

DIATOMACEOUS EARTH FILTRATION - A filtration method that employs a medium consisting of microscopic shells of single celled plants known as diatoms.

DI WATER - Deionized water; water processed through an ion exchange process by passing through both cation and anion exchange resin beds, or a mixed resin bed to remove both positive and negative ions. The purity of water is measured by its electric resistance. High quality DI water has a minimum resistance of 18 megohm per cm at 25ºC.

DEAD END (CONVENTIONAL) FILTRATION - Feed stream flows in one direction only, perpendicular to and through the filter medium to emerge as product or filtrate.
Delta P - A commonly used symbol denoting the pressure drop across a filter.

DEPTH FILTER - A matrix of randomly distributed fibers creating a tortuous path with pores of undefined size and shape.

DIFFERENTIAL PRESSURE - The difference in pressure between the upstream and downstream sides of the filter. Also called P, psid or pressure drop. May be modified with applied, available, clean, dirty, initial, or maximum.

DIFFUSIONAL INTERCEPTION - In gas filtration, at low gas flow velocities, very small particles <0.1µm are subject to Brownian motion. Thus they can move out of the gas streamlines and become intercepted by the filter.

DIFFUSIONAL FLOW TEST - A test to determine the integrity of a filter. The test is based on the measurement of diffusive (diffusional) flow of a gas through a wetted filter. Either the gas or the downstream liquid, displaced by the gas, may be measured. In addition, the transition from diffusional flow to bulk flow (i.e. bubble point) can be determined.

DIRECT INTERCEPTION - In gas filtration, particles larger than the pores are removed by direct interception with the filter surface. Some particles smaller than pores can be removed as well depending on the probability, which is proportional to their size, of hitting the surface.

DIRT - HOLDING CAPACITY - Amount of dirt or debris retained by a filter in grams per unit area of the filter medium.
DISSOLVED SOLIDS - Any solid material that will dissolve in the liquid that is being filtered such as sugar in water. Filters cannot remove these "solids". Also the residual material remaining after filtration and evaporation.

DMF - Drug Master File. A written document that explains the formulation of an active ingredient, and is referenced in an Investigational New Drug (IND), New Drug Application (NDA), or Amendment to New Drug Application (ANDA) from a company.

DOP - Dioctyl phthalate, a plasticizer that can be aerosolized to particles of extremely uniform size of the order of 0.3µm. Retention of DOP aerosol is used a s standard procedure for pore size rating of air filters.

DOWNSTREAM SIDE (OF FILTER) - The filtrate or product stream side of the filter.

DRY HEAT STERILIZATION - Sterilization at or above 180ºC using a convection or forced air oven without moisture; may concurrently depyrogenate if adequate time and elevated temperature are employed.

E. Coli - Escherichia coli; The most prevalent bacteria in the gastrointestinal tract of humans and animals. It occurs in solids and water as a result of fecal contamination.

ETO (STERILIZATION) - Chemical sterilization using ethylene oxide usually 12 - 88 (12% ETO in Freon). Employs a slightly elevated temperature, 66ºC (150ºF), and high relative humidity to facilitate permeation of the ethylene oxide into the material being sterilized.

EFFECTIVE FILTRATION AREA - The portion of filter that fluid flows through during the filtration (EFA) process.
Efficiency - The ability, expressed as a percent, of a filter to remove a specified artificial contaminant at a given contaminant concentration under specified test conditions.

EFFLUENT - The fluid which has passed through a filter (syn - filtrate or product stream); also, outflow from other types of treatments such as wastewater treatment plants.

EPA - Environment Protection Agency regulates environmental monitoring. Establishes and enforces its guidelines.

EXTRACTABLES - Chemicals which may be leached from a filter during a filtration process; usually tested for by soaking in water under controlled conditions; may be removed by pre - flushing with suitable liquid.

FAB - Fabrication area (e.g., in electronics industry).

FDA - Food and Drug Administration.
FEED - The mixture of solids and liquids that enters the filter. Synonyms - prefiltered influent and incoming slurry.

FERMENTATION - Generally referred to as enzymatically controlled breakdown of an energy rich compound (as a sugar to produce ethyl alcohol, carbon dioxide, and energy) by the action of yeasts which carry the necessary enzymes (bacterial fermentations also occur).

FILTER (noun) - A device for carrying out filtration which consists of the filter medium and suitable holder for constraining and supporting the filter in the fluid path.

FILTER (verb) - To pass a fluid containing particles through a filter medium whereby particles are removed from the fluid.

FILTER EFFICIENCY - A measurement of how well a filter retains particles. Usually expressed as the percentage of retention of particles of a specific size by a filter; see also "Beta Ratio" and "Log Reduction Value."

FILTER MEDIA MIGRATION - Problem caused by a filter medium which is constructed of a non - continuous or fibrous polymeric matrix such that portions of the filter change structure causing undefined pore size/distribution, as a function of fluid flow.

FILTER MEDIUM - The permeable material that removes particles from a fluid being filtered.

FILTRATE - The effluent of a filtration process. The filtered product.

FILTRATION - The process by which particles are removed from a fluid by passing the fluid through a permeable material.

FLASH PASTEURIZATION - The process of briefly heating a beverage to destroy objectionable enzymes and microorganisms. See "Pasteurization".

FLOW DECAY - Decrease in flow rate as a result of filter plugging or clogging.

FLOW DECAY TEST - An experiment to determine flow rate and throughput of a filter type or combination of filters on a specific liquid, usually by using a small area filters, to determine the sizing of a filter system by extrapolation.

FLOW RATE - It is the speed at which a liquid flows and is measured in gallons or liters per minute. Flow rate of a liquid can be affected by the liquids' viscosity, differential pressure, temperature and type of filter used.

FORWARD FLOW TEST - An integrity test measuring air diffusion. See "Diffusional Flow Test."

FRACTIONATION - In densitometry, the division of peaks into fractions in order to quantitate the electrophoretically separated bands. In chemistry, separation of a mixture of components into different portions (fractions).

GC - Gas Chromatography; similar to HPLC except that mobile phase is an inert gas such as helium.

GASKET - Material inserted between contact surfaces of a joint to ensure a fluid - tight seal.

GAUGE PRESSURE - The pressure measured by a pressure gauge. Pressure above ambient pressure. Symbolized as psig when the pressure is expressed in psi units.

GLC - Gas Liquid Chromatography.

GMPs - Good Manufacturing Practices.
GPH - Gallons per hour.

GPM - Gallons per minute.

GROUNDWATER TESTING - Process of collecting and analyzing groundwater in areas where contamination is suspected such as dumpsites and landfills. Look for pesticides, dissolved metals, etc.

HIMA - Health Industry Manufacturer's Association. A trade association, whose membership includes both pharmaceutical manufacturers and filter manufacturers, that defines and sets standards governing the validation of filters for sterilizing liquids.

HOLD - UP VOLUME - Also called Retention Volume. Volume of fluid retained in a filter and/or housing after purging the assemble with air or suitable gas.

HPLC - High Pressure Liquid Chromatography allows separation and analysis of very small quantities of complex mixtures with high resolution and great sensitivity. Purpose - identify nature of a compound or measure amount or concentration of a compound.

HYDROPHILIC - Having an affinity for water; a membrane which will wet with aqueous solutions.

HYDROPHOBIC - Literally, fearing water; a membrane which cannot be wetted by and repels aqueous and other high surface tension fluids; when pre - wetted with low surface tension fluid, such as alcohol, the filter will then wet with water.

IMPACTION - Retention mechanism in gas filtration. Also called Inertial Collection and Inertial Impact. As the gas stream lines bed in the vicinity of the filter, the carried particles continue in a straight line due to their inertia and impact the filter. Effective primarily for particles about 0.3µm and larger, at high gas velocities and low filter porosity.

IN SITU - Latin for "in place." Sterilization or integrity testing of a filter in the system rather than as an ancillary operation such as in autoclave or bubble point stand.

INERT - Chemical inactivity; unable to move; totally unreactive.


INERTIAL IMPACT - See "Impaction."

INLET PRESSURE - The pressure entering the inlet side of the filter. Also called upstream pressure or line pressure.

INTEGRITY TEST - A non - destructive test which is used to predict the functional performance of a filter. The valid use of this test requires that it be correlated to standardized bacterial or particle retention test. Examples - Bubble Point Test, Diffusion Test, Forward Flow Test, Pressure Hold Test.

INTERCEPTION - See "Direct Interception."

ION EXCHANGE COLUMNS - Vessels filled with ion exchange resin (anion, cation, or mixed) for producing conditioned or DI Water. Also, type of column used for Ion Exchange chromatography (IEC).

ISOTROPIC (SYMMETRIC) MEMBRANE - Membrane in which the pore openings are the same diameter throughout the thickness and on both sides of the membrane. Such membranes are non - directional, i.e., their flow characteristics are independent of which side faces the feed stream.

K or k - The symbol for kilo or 1,000. As in kilogram (kg = 1,000g) or kilometer (km = 1,000m). In information systems, and computers, 1K means 1024 bits of information. A 64K memory stores 65,536 bits.

LINE PRESSURE - The pressure in the supply line. Also called inlet pressure, upstream pressure.

LIVE STEAM STERILIZATION - Sterilization by flowing saturated steam through a vented vessel or system, usually at 125ºC and 20 psi (but can be performed up to 140ºC and 35 psi.)

MEAN FLOW PORE MEASUREMENT - The theoretical diameter of the mean pore. It is calculated as the diameter of the pore of a wetted membrane partially voided of liquid such that air flow of the partially wetted membrane is equal to ý the dry air flow.

MEDIA - In filtration, the material through which fluid passes in the process of filtration and which retains particles. Also, the nutrients containing solutions in which cells or microorganisms are grown.

MEDIA MIGRATION - Migration of the materials making up the filter medium. May cause contamination of the filtrate.

MEMBRANE FILTER - A continuous matrix with pores of defined size.

MICROFILTRATION - Separation of particles ranging from 0.1µm to 10µm from a fluid by passing the fluid through a membrane. Used for clarification, sterilization or to detect or analyze bacteria and other organisms and particulate matter.

MICROMETER/MICRON (m) - Also referred to as "micron." It is a 1/1,000,000 of a meter (1µm = 10 - 6µm = .000039 in);
25.4µm = 0.001 inch;
60µm = approximately the diameter of a human hair.

MIL - A unit of measure equal to one thousandth of an inch. 1 mil = 0.001 in = 0.025 mm.

MINIMUM BUBBLE POINT PRESSURE - Also referred to as minimum critical bubble point pressure, it is a filter specification derived from diffusional flow – bubble point curves for a number of filters. It is a diffusional flow pressure just before the onset of bulk flow.

MIXED CELLULOSE ESTERS - Synthetic materials derived from naturally occurring cellulose. First materials used in the manufacture of membrane filters. Mixed cellulose esters membranes are used in a wide variety of applications, e.g., concentration of bacteria in water analysis (GN - 6) and sampling of air.

NFR - Non - fiber releasing. A filter which will not release fibers into the filtrate.

NIOSH - National Institute of Occupational Safety and Health develops basic methodology for analytical test procedures.

NYLON - A thermoplastic, polymeric material that has high mechanical strength & compatibility with many different kinds of chemicals. When used as a membrane it is hydrophilic.

OEM - Original Equipment Manufacturers.

ORGANIC - Related to or derived from a living organism. Always contains carbon.

OUTLET PRESSURE - The pressure exiting the outlet side of the filter. Also called downstream pressure.

PARALLEL FILTRATION - Branching a filtration setup so that two assemblies of the same pore size are in parallel, to increase flow rate or simplify filter changes.

PARTICLE - Any discrete unit of material structure; a discernible mass having an observable length, width, thickness, size and shape.

PARTICULATE - Relating to or occurring in the form of fine particles.

PASTEURIZATION - Partial sterilization of a substance and especially a liquid (as milk) at a temperature and time of exposure that destroys objectionable organisms without a major chemical alteration of the substance. Maintaining the high temperature for only a short period of time is referred to as 'flash' pasteurization.

PERISTALTIC PUMP - A pump functioning by alternate pinching and release of tubing which drives the fluid forward in a pulsing action. The major advantage's are that the peristaltic pump is noninvasive, i.e., the pump does not contact the fluid being filtered, only the inner wall of the tubing contacts the fluid and the low shear imparted.

PERMEABILITY - The degree to which a fluid will pass through a permeable substance under specified conditions. The space or void volume between molecules allowing fluid flow.

PERMEATE - The fluid which passes through a membrane.

pH - The inverse (negative) logarithm to the base 10 of hydrogen ion concentration. Measure of a substance's acidity or alkalinity with 7 being neutral. Measure of hydrogen ion concentration.

POISE (ABSOLUTE VISCOSITY) - Numerically equal to the force required to move a plane surface of one square centimeter over another plane surface at the rate of one centimeter per second when the surfaces are separated by a layer of fluid one centimeter in thickness (dyne sec/cm2).

POLYPROPYLENE - A thermoplastic polymeric material which is resistant to a broad range of chemicals. When used as a membrane, polypropylene is hydrophobic.

POLYSULFONE - Commonly used membrane material. Has excellent flow rates, high mechanical strength, resistant to a broad range of temperatures (can be sterilized) and is hydrophilic. Is not resistant to exposure to many organic solvents.

PORE SIZE - Diameter of pore in membrane.

PORE SIZE - ABSOLUTE RATING - The rated pore size of a filter at which particles equal or larger than the rated pore size are retained with 100% efficiency.

POROSITY - The percentage of the filter volume which is void space (syn. Void volume). Also, number of pores per square centimeter of filter area.

PROTEIN BINDING - Adsorption of a protein to a surface such as a cellulose nitrate or nylon membrane due to several types of interactions between the protein molecules and the surface.

PSEUDOMONAS DIMINUTA - A type of bacteria used in sterility testing. One of the smallest bacteria (0.3µm in diameter), used to challenge a sterilizing grade filter during validation testing. Under HIMA challenge conditions (107 c.f.u./cm2 EFA), sterilizing grade filters must retain all 100% of P. diminuta.

PTFE - Polytetrafluoroethylene; More commonly known as Teflon. Highly durable and resistant to a broad range of temperatures and chemicals. PTFE is hydrophobic.

RECOVERY - Ability of a filter to recover bacteria (or other defined particles) from a solution. In Membrane Filtration Technique, expressed as percent of bacteria originally present or observed on a comparable pour plate.

RETENTION - Ability of a filter to retain particles (total number or those of a specific size) suspended in a gas or liquid. Expressed as a percent of particles originally present.

RETENTION VOLUME - See "Hold - up Volume."

REVERSE OSMOSIS (RO) - A filtration separation method (usually crossflow or stirred cell type) operating at 200 - 1500 psi to overcome osmotic pressure. Pore sizes are typically in the order of 10 - 10 meters (107mm). Efficiency is usually described in terms of percent salt rejection with 90% being common.

SCFM - Standard cubic feet per minute, i.e. units of gas flow rate. A standard cubic foot is measured as volume of gas at 760 millimeters of mercury pressure (1 bar) and 0ºC temperature.
SCREEN - A term commonly used for septum, Also a wire mesh screen used to screen out large sized particles that would clog a filter cartridge. Usually installed on the suction side of a pump.

SANITIZATION, SANITIZE - To make clean by removing dirt and other extraneous materials with soap and general disinfectant so as to reduce possibility of growth and spread of pathogenic organisms.
SEPARATOR - A spiral - wound membrane cartridge or element in cross flow membrane systems. Modular and replaceable.

SERIAL FILTRATION - Filtration through two or more filters of decreasing pore size one after the other to increase throughput, filtration efficiency, or to protect the final filter.

SIEVE - A filter with straight - though capillary pores with identical dimension, e.g. a screen filter.
SOP - Standard Operating Procedure. A written document that explains how to complete a specific production - oriented task.

SPARGING - The process by which steam, compressed air, or gas is forced into a liquid through perforations or nozzles in a pipe as part of fermentation.

STANDARD (NORMAL) PRESSURE - A pressure of 1 atmosphere (14.70 psi or 760 mm of mercury) to which measurements of quantities dependent on pressure are often referred.

STERILE, STERILITY, STERILIZATION - To make or be free of any viable microorganisms. Demonstrated by testing to show the absence of microorganisms.

STERILIZING FILTER - A non - fiber releasing filter which produces an effluent in which no microorganisms are demonstrable when tested by the method specified in the current edition of the United Sates Pharmocopeia. Usually accepted as 0.2µm pore - size absolute rating.

SURFACE TENSION - Also "interfacial tension." Tendency of the surface of a liquid to contract to the smallest area possible under the existing circumstances. Defined as a force in dynes acting on a line 1 cm long lying in the surface of the liquid.

SURFACTANT - A soluble compound that reduces the surface tension of a liquid, or reduces interfacial tension between two liquids (causing formation or micelles) or between a liquid and a solid, thereby functioning as a wetting agent.

SVP - Small Volume Parenteral; Typically administered to a patient as a bolus or single syringe injection.

TARE - A deduction of weight, made in allowance for the weight of a container or medium; the initial weight of a filter.

TENSILE STRENGTH - Resistance to breaking as a function of tensile force (tension). The amount of force required to break a membrane by stretching. Usually accompanied by measurement of Elongation - at - Break, the total amount of stretching realized at break, expressed as percent of the original length.

THROUGHPUT - The amount of solution which will pass through a filter prior to clogging.

TOTAL DISSOLVED SOLIDS - Is the portion of the total solids in the sample that passes through the filter and is indicated by the increase in weight in the vessel after the filtrate has been dried at 180ºC.

TOTAL SOLIDS - The material residue left in the vessel after evaporation of a sample and its subsequent drying in an oven at 103 - 105ºC. The increase in weight over that of the empty vessel represents the total solids. Used in analyzing drinking water.

TOTAL SUSPENDED SOLIDS - Is the portion retained on the filter and indicated by an increase in the weight of the filter after drying at 103 - 105ºC. Used in analyzing drinking water.

TORTUOUSITY - An imaginary continuous course or path that can be traced from a point on the upstream side of a filter to a point on the downstream side. Pathway traveled by the liquid or gas during filtration.

U.S.D.A. - United States Department of Agriculture.

U.S.P. - United States Pharmacopeia/National Formulary.

ULTRAFILTRATION (UF) - A separation method operating at 50 - 200 psi in crossflow filtration mode. Efficiency is approximately 90%. Used to separate large molecules according to their molecular weight.

UPSTREAM SIDE (of filter) - The feed side of the filter.

VACUUM - The depression of pressure below atmospheric pressure.

VALIDATION - Demonstration that a process or product does what it is supposed to do by challenging the system and providing complete documentation.

VISCOSITY - A resistance to flow as a function of force, or gradual yielding of force. Viscosity is in units of centipoises or centistokes. For a given filter and differential pressure, flow rate will decrease as viscosity increases; e.g. oil will have a flow rate much slower than water. The viscosity of water is 1 centipoise.

VOLATILE - Evaporates easily, converts easily from liquid form to gas.

WATER BREAKTHROUGH TEST - An integrity test for hydrophobic filters in which the resistance to water flow is overcome by a specific pressure such that water will flow through a correspondingly specific pore size of the filter. Also called a water intrusion test. Useful test to determine gross loss of integrity (e.g., installation integrity) and filter hydrophobicity.

WETTING AGENT - A surfactant added to a membrane to assure complete intrusion (wetting) by a high surface tension fluid such as water.