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Permeation Glossary


Degree of agreement of a measurement with an accepted reference level or value.

Ambient Temperature
Surrounding temperature; temperature encompassing on all sides.

Arrhenius (Svante) (1859-1927)
A native of Sweden, he won the Nobel Prize in chemistry in 1903. He is best known for his fundamental investigations on electrolytic dissociation of compounds in water and other solvents, and for his basic equation stating the increase in the rate of a chemical reaction with rise in temperature: in which, k is the specific reaction velocity, T is the absolute temperature, n is a constant usually referred to as the energy of activation of the reaction and R is the gas law constant. Arrhenius Plot A plot of data on a graph with the x-axis marked in the inverse Kelvin temperature and the y-axis marked in a Log10 format. It derives its name from the chemical reaction versus temperature relationship equation postulated by Svante Arrhenius. Loosely taken, any plot of data that is intended to show a Log10 relationship between the temperature of the test and the resultant data from that test.

Arrhenius Theorem
The Arrhenius Theorem states that the permeation rate, transmission rate, diffusivity or solubility vary with the film temperature according to the following equation(s): P=P0 exp (-Ep/RT) TR=TR0 exp (-ETR/RT) D=D0 exp (-ED/RT) S=S0 exp (-ES/RT) where P, TR, D and S are the measured properties, P0, TR0, D0, and S0 are the respective constants, E is the activation energy, R is the gas constant and T is the absolute temperature. The Arrhenius equation has been verified empirically to give the temperature behavior of the mass transfer properties within experimental accuracy over fairly large temperature ranges.

ASTM International
ASTM International is one of the largest voluntary standards developing organizations in the world. We are a not-for-profit organization that provides a forum for the development and publication of international voluntary consensus standards for materials, products, systems, and services. ASTM standards are used by individuals, companies and other institutions around the world.

ASTM Standard
A standard is a document that has been developed and established through ASTM's consensus principles and which meets the requirements of our procedures and regulations. Full consensus standards are developed with the participation of stakeholders with an interest in their development and use.

Abbreviation for standard atmospheric pressure.


A unit of pressure. The bar is equal to 100,000 Pa (pascal), 750.062 mmHg, 29.53 InHg, 14.504 psi and 0.987 ATM (standard atmospheres). See SI units.

Barometric Pressure
Pressure of the atmosphere. This is usually expressed in terms of the height (in millimeters mmHg) of a column of mercury.

Barrier (barrier film)
Object or device, such as a polymer film, that is designed to restrict the free movement and mingling of populations or areas of higher and lower concentrations.

Barrier Plastics
Plastics that impede the passage of oxygen, water vapor, and other vapors and gasses.

Baseline Zero
See zero.

Blister Pack
Package with multiple small compartments for individual products. Common with, but not exclusive to pharmaceutical capsules.

No gas is being routed to the sensor from the AQUATRAN test cells. The sensor is in a protected state.


Procedure to adjust a quantitative measuring instrument to agree with an accepted reference. See Accuracy.

Carrier Gas
Picks up water vapor that passes through a test barrier and carries (transports) it to the AQUATRACE water vapor sensor. The AQUATRAN uses nitrogen as the carrier gas.

See Test Cell.

CFR 21 Part 11 compliant:
Code of Federal Regulations deals with the United States Food and Drug Administration (FDA) guidelines on electronic records and electronic signatures (ERES). (ERES).

Period allowed for a film or package to acclimate to the conditions within the test cell or Package Environmental Chamber.

Conditioning Time
Number of hours, specified by the operator, during which a sample is conditioned.

Convergence Hours
Number of hours, specified by the operator, over which to compare transmission rate readings. The transmission rate readings are compared to determine whether equilibrium has been reached.

Convergence Testing
Method by which the computer determines when a test material has reached equilibrium. Test materials' transmission rates are examined and compared on a timed basis.

Coulometric Sensor
Fuel cell that performs in accordance with Faraday's Law. When exposed to water vapor, the coulometric sensor generates an electrical current proportional to the amount of water vapor entering the sensor.

Coulox® coulometric sensor
MOCON’s patented sensor. Every molecule that enters the cell is passed to the sensor. It is an absolute sensor and no calibration is required.

Carbon Dioxide Transmission Rate. Typically measured in cc/m2day. Learn more about CO2TR Analyzers here.


Removing a module from the test rotation. Modules are deactivated by clicking on Control-Pause.

To give up or release from a barrier.

The process by which matter is transported from one part of a film to another as a result of random molecular motions. In a film each molecule behaves independently of the other. When collisions occur, molecules move towards regions of lower concentration and sometimes towards regions of higher concentration. Because the number of molecules on the test gas side of a film is higher than the number of molecules on the carrier side, the overall movement of molecules will be in the direction of the carrier gas. Diffusion is usually expressed in units of cm2/sec. Diffusion varies with temperature and sometimes with permeant concentration (see Fick's first law).

Diffusion Coefficient
A coefficient specific to a given barrier and permeant that describes the relationship between the permeant mass flux and the permeant mass concentration change across the barrier. The diffusion coefficient (D) is defined by Fick's law and is usually expressed in terms of cm2/sec. Other terms are m2/day, cm2/day, m2/in, in2/sec and in2/min.

Driving Force:
Substances naturally tend to move from a higher chemical potential to a lower one. It prompts a molecule to diffuse within a polymer.


Point at which the amount of water vapor passing through a test material has stabilized at a steady rate. The computer reports equilibrium value as the material's final transmission rate.


Fickian Behavior
Fickian behavior is observed when a given permeant diffuses through a given polymer film according to the mathematical equations governed by Fick's First and Second Laws. In the case of an isostatic transmission rate test, the equilibrium values of the permeation rate, transmission rate, solubility and diffusion rate of a polymer-permeant pair exhibiting Fickian curve behavior can be predicted according to Pasternak's solution of Fick's law (see Transmission Rate curve). NOTE: Many organic permeants react with polymeric materials causing non-Fickian behavior and prevent the prediction of equilibrium values.

Fick's First Law
The mathematical equation adopted by Fick in 1855 which correlates diffusion to heat conduction. The mathematical theory of diffusion in a substance is therefore based on the hypothesis that the rate of transfer of a diffusing substance through a unit area of a section is proportional to the concentration gradient measured normal to the section, i.e. F=-D ∂C/∂x Where F is the rate of transfer per unit area of section, C the concentration of diffusing substance, x the space coordinate measured normal to the section, and D is the diffusion coefficient.

Fick's Second Law
Fick's Second Law is the fundamental differential equation of diffusion in a medium. It states that: ∂C/∂t=D∂2C/∂x2 where C is the concentration of diffusing substance, x the space coordinate measured normal to the section and D is the diffusion coefficient. This assumes there is a concentration gradient in only the x-direction.

Flow Rate
The volumetric rate at which gas is traveling through the module.

The rate of flow or transfer of permeant. The term is normally used to denote the quantity of permeant that crosses a unit area of a given surface in a unit of time.

Foil Mask
Reduces the exposed area of a film sample (as with a low barrier), or supports a fragile sample in the test cell.


Gas Flow Rate
See Flow Rate.

Gas Pressure
Pounds per square inch (psi) of the test and carrier gasses. Used to regulate relative humidity via the module-mounted pressure regulator knobs.

Glass Transition Temperature (Tg)
The temperature at which an amorphous material (such as glass or polymer) changes from a brittle glass-like state to a plastic state. Many polymers such as acrylics and their derivatives have this transition point which is related to the number of carbon atoms in the ester group. The Tg of glass depends on its composition and extent of annealing. Example: The Tg of PET is about 70C.

Gravimetric Method
Also known as cup test – measuring loss or gain by weight.


High Vacuum Grease
Highly refined grease that is used to seal a test material in the test cell and to lubricate parts of the module.

Homogeneous Material
Barrier material that has the same basic characteristics through the thickness of the barrier.

Hygrophobic Material
Material that does not readily absorb water.

Hygroscopic Material
Material that readily absorbs water.

HPLC-grade Water
High purity distilled water (sometimes called triple-distilled water). HPLC-grade water prevents the formation of scale, which can obstruct the plumbing. It is used in the humidifier where its high purity is needed for accurate RH results. It is the only recommended water source for the reservoirs within any MOCON permeation instrument. HPLC is an acronym for high-pressure liquid chromatography.


Individual Zero
Method by which more accurate results are achieved for high barriers. The computer sends nitrogen through both halves of the test cell, then routes it to the sensor to determine the amount of water vapor it is carrying. Because the carrier gas obtained this water vapor due to edge leaks or other factors, the computer subtracts that amount from the transmission rate obtained during testing. This provides a very accurate actual transmission rate.


Isostatic Test Method
A test procedure during which a test film is clamped in a permeation cell. The test divides the cell into two chambers. A test gas flows continually through the high concentration cell chamber, and an inert carrier gas flows through the low concentration cell chamber. The partial pressure gradient of the penetrant provides a driving force for the penetrant permeating through to the low concentration cell chamber, where the penetrant can be conveyed to a detector for quantification. Steady state is reached when the transmission rate, monitored continually, remains constant under constant conditions of temperature and permeant vapor pressure.






Leak Rate
Amount of water vapor entering the carrier gas through edge leaks or other factors. This value is determined during individual zeroing operations and is later subtracted from the apparent transmission rate to determine the actual transmission rate.


Unit of thickness measurement One micron is equal to 3.93 x10-5 inches (1x10-6 meter).

Unit of thickness measurement. One mil is equal to 1 x 10-3 inches.

Millimeters of mercury. A unit of atmospheric pressure. Standard atmospheric pressure at sea level is 760 mmHg. 1 mmHg = .0193371 psi.

Modules contain the environment in which films and packages are tested.

Abbreviation for mole, the International System of Units (Système international d' unités, abbreviated “SI”) base unit for quantity of matter. A mol is numerically equal to molecular weight; 1 mol = 6.023 x 1023 atoms. See Mole.


Chemical abbreviation for the nitrogen molecule. Two nitrogen atoms make a nitrogen molecule. See Nitrogen.

Main constituent in the Permeation instrument carrier gas. It carries the permeant gas that passes through a film or package into the instrument sensor for quantification.

Nitrogen Purge
Process by which nitrogen (carrier gas) is used to expel residual water vapor from a cell, package, or sensor.


Oxygen Transmission Rate. Typically measured in cc/ m2day. Learn more about OTR Analyzers here.


The Si unit for pressure. One pascal is equal to .9869233 x 10-5 atmospheres. See SI units.

Gas, vapor, or other physical entity that passes through a barrier; also called permeant.

A unit of measurement for permeance. One metric perm is equal to 1 gm/m/day•mmHg; one perm in the foot-pound system is equal to 1 grain/in/day•in Hg.

Permeability Coefficient
Product of the permeance and the thickness of a barrier. For example, water vapor permeability is measured in SI units as mol/m2/s•Pa•mm, and in metric units as gm/(m2-day)•mmHg•cm. Permeability is a property of a homogeneous material.

Ratio of a barrier's transmission rate to the partial vapor pressure differential across the barrier. Water vapor permeance is measured in SI units as mol/m2•s•Pa, and in metric units as gm/m2•mmHg (also called a metric perm).

See Penetrant.

To pass through the pores or interstices of; to penetrate and pass through without causing rupture or displacement; applied especially to fluids or gases which pass through porous substances or materials.

Permeation is the penetration of a permeate (such as a liquid, gas, or vapor) through a solid. It is directly related to the concentration gradient of the permeate, a material's intrinsic permeability, and the materials' mass diffusivity. Permeation is modeled by equations such as Fick's laws of diffusion

Permeation Rate
The transmission rate normalized to thickness. Permeation Rate is usually expressed as a (gm or cc) of (gas or vapor) at 1 (cm or mil) per unit area of material (m2 or 100in2) in a discreet unit of time (day or sec) and usually is normalized to one atmosphere (760 mmHg or atm). If the material is homogeneous, Permeation Rate can be converted to Transmission Rate of any thickness. Transmission Rate is inversely proportional to thickness (TR = P/x).

Degree of mutual agreement among individual measurements. Relative to a method of test, precision is the degree of mutual agreement among individual measurements made under prescribed like conditions.

See gas pressure.

P = SD Equation
Permeation through a polymer film or sheet is a measure of the steady-state transfer rate of the permeant, which is normally expressed as the permeability constant P. The permeability constant is the product of two fundamental mass-transfer parameters: the diffusion and solubility coefficients. The diffusion coefficient D is a measure of how rapidly penetrant molecules are going through the barrier, in the direction of lower concentration or partial pressure. The solubility coefficient S describes the amount of the transferring molecules retained or dissolved in the film at equilibrium conditions. This equation is applicable only for situation where D is independent of permeant concentration and S follows Henry's law of solubility.

Abbreviation for pounds per square inch (pressure).

Abbreviation for pounds per square inch (gauge).




See Relative Humidity.

Relative Humidity (RH)
Ratio of the quantity of water vapor in air to the maximum capacity of the air at a specific temperature and pressure. The transmission rate of many barrier plastics is dependent on the RH at which the transmission rate is tested.

Measure of the accuracy of a system and the testing method used. A highly repeatable system will provide very consistent results. That is, if the same test is run twice on a test material, the results should be very nearly the same.

Minimum discernible difference in the change of output signal of an instrument.

Method for minimizing the effects of environmental or other factors that should not be included in test data. The operator-entered value in, ReZero, tells the computer when to perform ReZero to "reset" the test parameter baseline.


Abbreviation for standard cubic centimeters per minute.

Degree to which an instrument responds to a defined incoming quantity of the phenomenon being detected. Avery sensitive instrument will detect very small changes in the area being measured.

Shelf Life
Period of time a product can be stored and remain suitable for use. Information obtained when testing for water vapor transmission rate can help determine a product's shelf life using a particular barrier film or package.

Single Cell Testing
Testing conducted in one cell of a module.

The equilibrium amount of permeant absorbed into a material per unit volume of material. Solubility is measured assuming a constant vapor pressure and temperature. Units are usually (gm or cc) of permeant per unit of volume (cc) of material at a set vapor pressure i.e. (Gm/cc). Units of solubility can also be normalized to vapor pressure i.e. (Gm/cc-mmHg). Solubility varies with temperature and permeant condition.

Solubility Coefficient
Amount of a gas or vapor that will dissolve into a given amount of a barrier.


Test Cell
Environment within a module in which film testing takes place. Test cells can be modified to test a package, or to accommodate an Environmental Chamber.

Test Gas
Gas that passes through a test film. For example, the AQUATRAN uses water vapor as the test gas.

Test Gradient
The partial vapor pressure difference on each side of the test film.

Passing of a gas or vapor through a barrier, normally by absorption of the gas into the barrier, the diffusion of the gas through the barrier, and the desorption of the gas on the opposite side of the barrier.

Transmission Rate
Quantity of a gas or vapor that will cross a unit of a barrier in a unit of time by diffusion.




Vapor Pressure
The pressure (usually expressed in mmHg) characteristic of a liquid or solid at any given temperature, in equilibrium with its vapor.


Water Vapor Transmission Rate. Typically measured in g/ m2day. Learn more about WVTR Analyzers here.






Operation that provides a new "zero point” (baseline) for the permeation module. Zeroing gives the computer a reference point to use when generating transmission data.