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η is the air dynamic viscosity, Pa·s Q is the flowrate, m 3 ·s −1. It can be seen that the specific surface is proportional to the square root of the ratio of pressure to flow. Various standard methods have been proposed: Maintain a constant flowrate, and measure the pressure drop; Maintain a constant pressure drop, and measure the flowrate
Heated building volume V. At 50·Pa, it is called the air change rate at 50 Pa and usually noted n 50 (units: h −1). [13] [14] Envelope area A E. At 50 Pa, it is called the air permeability at 50 Pa and noted usually q 50 or q a50 (units: m 3 /(h·m 2)) [13] [14] Floor area A F.
A standardised testing procedure is crucial for various fabrics as it uncovers fundamental qualities such as warmth in blankets and air resistance in parachute cloth. [30] The air permeability test method is for measuring the ability of air to pass through textile materials. [31] ASTM D737-96 alternative test method is; ISO 9237:1995
Air permeability is a measure of the ability of air to pass through a fabric. [27] Air permeability is defined as "the volume of air in cubic centimetres (cm 3 ) which is passed through in one second through 100 cm 2 of the fabric at a pressure difference of 10 cm head of water", [ 28 ] also known as the Gurley unit .
The SI unit for permeability is the square metre (m 2).A practical unit for permeability is the darcy (d), or more commonly the millidarcy (md) (1 d ≈ 10 −12 m 2). The name honors the French Engineer Henry Darcy who first described the flow of water through sand filters for potable water supply.
The absolute permeability number, which has no units, is determined by the rate of flow of air, under standard pressure, through a rammed cylindrical specimen. DIN standards define the specimen dimensions to be 50 mm in diameter and 50 mm tall, [ 2 ] while the American Foundry Society defines it to be two inches in diameter and two inches tall.
Air permeability is the ability of a fabric to allow air to pass through it. While air permeable fabrics tend to have relatively high moisture vapor transmission, it is not necessary to be air permeable to be breathable. Moisture Vapor Transfer (MVT) in waterproof fabrics occurs by two processes:
ISO 2393:2014 Rubber test mixes — Preparation, mixing and vulcanization — Equipment and procedures; ISO 2394:2015 General principles on reliability for structures; ISO 2395:1990 Test sieves and test sieving – Vocabulary; ISO 2398:2016 Rubber hoses, textile-reinforced, for compressed air — Specification