Permeability Converter
Permeability Converter
Convert between 4 distinct types of permeability units with scientific precision. Magnetic (H/m), fluid (darcy), gas (barrer), and vapor (perm) permeabilities measure fundamentally different physical properties and cannot be converted between types.
What is Permeability?
Permeability measures how easily something passes through a material, but this simple definition hides a critical fact: there are FOUR completely different types of permeability in physics and engineering, each measuring different physical quantities.
The Four Types of Permeability
Magnetic Permeability (μ)
Measures how easily magnetic flux passes through a material. Relates magnetic flux density (B) to magnetic field strength (H).
Units: H/m, μH/m, nH/m, relative permeability (μᵣ)
Formula: B = μ × H
Applications: Electromagnets, transformers, magnetic shielding, inductors, MRI machines
Examples: Vacuum (μᵣ = 1), Iron (μᵣ = 5,000), Permalloy (μᵣ = 100,000)
Fluid Permeability (k)
Measures how easily fluids (oil, water, gas) flow through porous media like rock or soil. Critical for petroleum engineering.
Units: darcy (D), millidarcy (mD), nanodarcy (nD), m²
Formula: Q = (k × A × ΔP) / (μ × L)
Applications: Oil/gas reservoirs, groundwater flow, soil drainage, rock characterization
Examples: Shale (1-100 nD), Sandstone (10-1000 mD), Gravel (>10 D)
Gas Permeability (P)
Measures how quickly specific gases transmit through polymers, membranes, or packaging materials. Used in packaging and membrane science.
Units: barrer, GPU (gas permeation unit), mol·m/(s·m²·Pa)
Formula: P = (N × L) / (A × Δp × t)
Applications: Food packaging, gas separation membranes, protective coatings, space suits
Examples: HDPE (0.5 barrer for O₂), Silicone rubber (600 barrer for O₂)
Water Vapor Permeability
Measures moisture transmission rate through building materials, fabrics, or packaging. Critical for moisture control and building science.
Units: perm, perm-inch, g/(Pa·s·m²)
Formula: WVTR = permeance × vapor pressure difference
Applications: Building vapor barriers, breathable fabrics, moisture management, packaging
Examples: Polyethylene (0.06 perm), Plywood (0.7 perm), Unpainted drywall (20-50 perm)
Quick Facts
Cannot Convert Between Types
Magnetic permeability (H/m) ≠ Fluid permeability (darcy) ≠ Gas permeability (barrer) ≠ Vapor permeability (perm). These measure different physics!
Extreme Range
Fluid permeability spans 21 orders of magnitude: from tight shale (10⁻⁹ darcy) to gravel (10¹² darcy)
Unit Name Confusion
The word 'permeability' is used for all four types, but they're completely different quantities. Always specify which type!
Material Specific
Gas permeability depends on BOTH material AND gas type. Oxygen permeability ≠ nitrogen permeability for the same material!
Magnetic Permeability (μ)
Magnetic permeability describes how a material responds to a magnetic field. It's the ratio of magnetic flux density (B) to magnetic field strength (H).
Formula: B = μ × H = μ₀ × μᵣ × H
B = magnetic flux density (T), H = magnetic field strength (A/m), μ = permeability (H/m), μ₀ = 4π × 10⁻⁷ H/m (free space), μᵣ = relative permeability (dimensionless)
Material Categories
| Type | Relative Permeability | Examples |
|---|---|---|
| Diamagnetic | μᵣ < 1 | Bismuth (0.999834), Copper (0.999994), Water (0.999991) |
| Paramagnetic | 1 < μᵣ < 1.01 | Aluminum (1.000022), Platinum (1.000265), Air (1.0000004) |
| Ferromagnetic | μᵣ >> 1 | Iron (5,000), Nickel (600), Permalloy (100,000) |
Fluid Permeability (Darcy)
Fluid permeability measures how easily fluids flow through porous rock or soil. The darcy is the standard unit in petroleum engineering.
Formula: Q = (k × A × ΔP) / (μ × L)
Q = flow rate (m³/s), k = permeability (m²), A = cross-sectional area (m²), ΔP = pressure difference (Pa), μ = fluid viscosity (Pa·s), L = length (m)
What is a Darcy?
1 darcy is the permeability that allows 1 cm³/s of fluid (1 centipoise viscosity) to flow through 1 cm² cross-section under 1 atm/cm pressure gradient.
SI Equivalent: 1 darcy = 9.869233 × 10⁻¹³ m²
Permeability Ranges in Petroleum Engineering
| Category | Permeability | Description | Examples: |
|---|---|---|---|
| Ultra-tight (Shale) | 1-100 nanodarcy (nD) | Requires hydraulic fracturing for economic production | Bakken shale, Marcellus shale, Eagle Ford shale |
| Tight Gas/Oil | 0.001-1 millidarcy (mD) | Challenging to produce, needs stimulation | Tight sandstones, some carbonates |
| Conventional Reservoir | 1-1000 millidarcy | Good oil/gas productivity | Most commercial sandstone and carbonate reservoirs |
| Excellent Reservoir | 1-10 darcy | Excellent productivity | High-quality sandstones, fractured carbonates |
| Extremely Permeable | > 10 darcy | Very high flow rates | Gravel, coarse sand, highly fractured rock |
Gas Permeability (Barrer)
Gas permeability measures how quickly specific gases transmit through polymers and membranes. The barrer is the standard unit, named after physicist Richard Barrer.
Formula: P = (N × L) / (A × Δp × t)
P = permeability (barrer), N = amount of gas transmitted (cm³ at STP), L = material thickness (cm), A = area (cm²), Δp = pressure difference (cmHg), t = time (s)
What is a Barrer?
1 barrer = 10⁻¹⁰ cm³(STP)·cm/(s·cm²·cmHg). This measures the volume of gas (at standard temperature and pressure) that permeates through a unit thickness per unit area per unit time per unit pressure difference.
Alternate Units: 1 barrer = 3.348 × 10⁻¹⁶ mol·m/(s·m²·Pa)
Example: Silicone rubber: H₂ (550 barrer), O₂ (600 barrer), N₂ (280 barrer), CO₂ (3200 barrer)
Applications
| Field | Application | Examples |
|---|---|---|
| Food Packaging | Low O₂ permeability preserves freshness | EVOH (0.05 barrer), PET (0.05-0.2 barrer) |
| Gas Separation | High permeability separates gases (O₂/N₂, CO₂/CH₄) | Silicone rubber, polyimides |
| Medical Packaging | Barrier films protect from moisture/oxygen | Blister packs, pharmaceutical bottles |
| Tire Liners | Low air permeability maintains pressure | Halobutyl rubber (30-40 barrer) |
Water Vapor Permeability (Perm)
Water vapor permeability measures moisture transmission through materials. Critical for building science, preventing mold, condensation, and structural damage.
Formula: WVTR = permeance × (p₁ - p₂)
WVTR = water vapor transmission rate, permeance = permeability/thickness, p₁, p₂ = vapor pressures on each side
What is a Perm?
US Perm: 1 perm (US) = 1 grain/(h·ft²·inHg) = 5.72135 × 10⁻¹¹ kg/(Pa·s·m²)
Metric Perm: 1 perm (metric) = 1 g/(Pa·s·m²) = 57.45 perm-inch (US)
Note: Perm-inch includes thickness; perm is permeance (already divided by thickness)
Building Material Classifications
| Category | Description | Examples: |
|---|---|---|
| Vapor Barriers (< 0.1 perm) | Block nearly all moisture transmission | Polyethylene sheeting (0.06 perm), aluminum foil (0.0 perm), vinyl wallpaper (0.05 perm) |
| Vapor Retarders (0.1-1 perm) | Significantly slow moisture, but not complete barrier | Oil-based paint (0.3 perm), kraft paper (0.4 perm), plywood (0.7 perm) |
| Semi-Permeable (1-10 perm) | Allow some moisture transmission | Latex paint (1-5 perm), OSB sheathing (2 perm), building paper (5 perm) |
| Permeable (> 10 perm) | Freely allow moisture transmission | Unpainted drywall (20-50 perm), fiberglass insulation (>100 perm), house wrap (>50 perm) |
Cold Climate: In cold climates, vapor barriers go on the warm (interior) side to prevent indoor moisture from condensing in cold wall cavities.
Hot Humid Climate: In hot humid climates, vapor barriers should be on the exterior OR use permeable walls to allow drying in both directions.
Quick Conversion Tables
Magnetic Permeability
| From | To |
|---|---|
| 1 H/m | 1,000,000 μH/m |
| 1 H/m | 795,774.7 μᵣ |
| μ₀ (vacuum) | 1.257 × 10⁻⁶ H/m |
| μ₀ (vacuum) | 1.257 μH/m |
| μᵣ = 1000 (iron) | 0.001257 H/m |
Fluid Permeability (Darcy)
| From | To |
|---|---|
| 1 darcy | 1,000 millidarcy (mD) |
| 1 darcy | 9.869 × 10⁻¹³ m² |
| 1 millidarcy | 10⁻⁶ darcy |
| 1 nanodarcy | 10⁻⁹ darcy |
| 1 m² | 1.013 × 10¹² darcy |
Gas Permeability
| From | To |
|---|---|
| 1 barrer | 10,000 GPU |
| 1 barrer | 3.348 × 10⁻¹⁶ mol·m/(s·m²·Pa) |
| 1 GPU | 10⁻⁴ barrer |
| 100 barrer | Good barrier |
| > 1000 barrer | Poor barrier (high permeability) |
Water Vapor Permeability
| From | To |
|---|---|
| 1 perm (US) | 5.72 × 10⁻¹¹ kg/(Pa·s·m²) |
| 1 perm-inch | 1.459 × 10⁻¹² kg·m/(Pa·s·m²) |
| 1 perm (metric) | 57.45 perm-inch (US) |
| < 0.1 perm | Vapor barrier |
| > 10 perm | Vapor permeable |
Frequently Asked Questions
Can I convert darcy to barrer or perm?
No! These measure completely different physical properties. Fluid permeability (darcy), gas permeability (barrer), vapor permeability (perm), and magnetic permeability (H/m) are four distinct quantities that cannot be converted between each other. Use the category filter in the converter.
Why does gas permeability depend on which gas?
Different gases have different molecular sizes and interactions with materials. H₂ and He permeate faster than O₂ or N₂. Always specify the gas: 'O₂ permeability = 0.5 barrer' not just 'permeability = 0.5 barrer'.
What's the difference between perm and perm-inch?
Perm-inch is permeability (material property independent of thickness). Perm is permeance (depends on thickness). Relationship: permeance = permeability/thickness. Use perm-inch to compare materials.
How do petroleum engineers use darcy?
Reservoir permeability determines oil/gas flow rates. A 100 mD reservoir might produce 500 barrels/day; a 1 mD tight gas reservoir requires hydraulic fracturing. Shale formations (1-100 nD) are extremely tight.
Why is relative permeability (μᵣ) dimensionless?
It's a ratio comparing a material's permeability to vacuum permeability (μ₀). To get absolute permeability in H/m: μ = μ₀ × μᵣ = 1.257×10⁻⁶ × μᵣ H/m. For iron (μᵣ = 5000), μ = 0.00628 H/m.
Is high permeability always good?
Depends on the application! High darcy is good for oil wells but bad for containment. High barrer is good for breathable fabrics but bad for food packaging. Consider your engineering goal: barrier (low) or flow (high).
What determines building vapor barrier placement?
Climate! Cold climates need vapor barriers on the warm (interior) side to prevent indoor moisture from condensing in cold walls. Hot humid climates need barriers on the exterior OR permeable walls to allow drying both ways. Wrong placement causes mold and rot.
What materials have the highest/lowest permeability?
Magnetic: Supermalloy (μᵣ~1M) vs vacuum (μᵣ=1). Fluid: Gravel (>10 D) vs shale (1 nD). Gas: Silicone (3000+ barrer for CO₂) vs metallized films (0.001 barrer). Vapor: Fiberglass (>100 perm) vs aluminum foil (0 perm).
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