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Engineering Calculator · 121

Water Vapor Permeance & Permeability Calculator

Convert measured WVTR into mass-based permeance and a thickness-normalized permeability for a homogeneous planar specimen.

Pressure & environment
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Live calculation

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Results

Water-vapor mass flux1.157407E-7kg/(m²·s)
Mass-based water-vapor permeance1.157407E-10kg/(m²·s·Pa)
Thickness-normalized permeability2.314815E-13kg/(m·s·Pa)

Formula
Jm=WVTRday1000⋅86400J_m=\frac{\mathrm{WVTR}_{\mathrm{day}}}{1000\cdot86400}Π=JmΔpv\Pi=\frac{J_m}{\Delta p_v}Pm=ΠtP_m=\Pi t

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Direct answer

What this result means

Mass-based water-vapor permeance is mass flux divided by the water-vapor partial-pressure difference. For a homogeneous planar specimen, permeability is permeance multiplied by thickness. A total air-pressure difference cannot replace the water-vapor partial-pressure difference in this calculation.

Water Vapor Permeance & Permeability Calculator engineering relationship diagramWVTR in g/(m²·day) is converted to mass flux in kg/(m²·s). Divide by a positive vapor partial-pressure difference in pascals for permeance, then multiply by thickness in metres for permeability. These mass-based units differ from molar gas-permeability units and require an explicit measured species and conditions. This dedicated schematic represents vapor partial pressure film thickness and is not to scale.Vapor p_v,1Vapor p_v,2Thickness tMeasured mass flux J_mPARTIAL-PRESSURE NORMALIZATION · TOOL 121Measured WVTRWater-vapor mass flux
Calculator-specific engineering relationship diagram — schematic, not a product drawing, and not to scale.

Formula & theory

Make the assumptions visible.

WVTR in g/(m²·day) is converted to mass flux in kg/(m²·s). Divide by a positive vapor partial-pressure difference in pascals for permeance, then multiply by thickness in metres for permeability. These mass-based units differ from molar gas-permeability units and require an explicit measured species and conditions.

Calculation relationship
Jm=WVTRday1000⋅86400J_m=\frac{\mathrm{WVTR}_{\mathrm{day}}}{1000\cdot86400}Π=JmΔpv\Pi=\frac{J_m}{\Delta p_v}Pm=ΠtP_m=\Pi t

WVTR in g/(m²·day) is converted to mass flux in kg/(m²·s). Divide by a positive vapor partial-pressure difference in pascals for permeance, then multiply by thickness in metres for permeability. These mass-based units differ from molar gas-permeability units and require an explicit measured species and conditions.

Variables

tHomogeneous specimen thicknessm
PmThickness-normalized mass-based permeabilitykg/(m·s·Pa)

Use sequence

From measurement to review

  1. Obtain measured WVTR, compatible vapor partial pressures and the specimen’s construction and thickness.
  2. Enter WVTR, the positive partial-pressure difference and thickness in the displayed units.
  3. Report the mass basis, temperature, humidity and whether the thickness-normalized value represents one homogeneous material.

Limits

What this calculation does not confirm

  • The flux and partial pressures belong to the same steady test conditions and water-vapor species.
  • Intrinsic permeability requires a justified homogeneous-material model; multilayers and interfaces need separate interpretation.
  • Vapor-pressure dependence, sorption and temperature effects are not modeled; no liquid-water sealing or complete-gland rating follows from these values.

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Calculator FAQ

Use the number without losing the engineering context.

These answers clarify what the tool calculates and where a product-specific review still begins.

01Calculator guidanceHow does permeance differ from permeability?

Permeance describes transmission per vapor-pressure difference for the specimen. Multiplying by thickness gives permeability under a homogeneous-material model. Their units and thickness dependence differ.

02Calculator guidanceCan total enclosure pressure be used in the water-vapor permeance denominator?

No. Use the difference in water-vapor partial pressure across the tested specimen. Total gas pressure may be nearly equal on both sides while water-vapor partial pressure differs substantially.

03Calculator guidanceIs permeability from a multilayer seal an intrinsic property of its polymer?

No. Multiplying an assembly’s permeance by total thickness provides an effective value on that stated model. Layers, interfaces, voids and bypass paths prevent assigning it automatically to one constituent.

Product selection support

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Bepto Cable Glands can help relate a preliminary result to the product family and current model information for your application. You can start with the details you have; a complete drawing package is not needed to ask a question.

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