TEST METHODS MEASURING BREATHABILITY
Confused about all the different test methods measuring breathability? Look no further as we explain in
greater detail the different standards used in industry to test breathable fabrics.
A variety of test methods have been developed for measuring the breathability of fabrics. Since the environmental conditions, setup, and protocol of each method differ, results from one standard are NOT comparable to
that of another.
In particular, the vapor transport properties of hydrophilic monolithic polymers increase as the concentration
of water vapor on the membrane side of the fabric increases. The rate of water vapor transport is also affected
by the air temperature and direction, and amount of wind flow.
The test results supplied by fabric manufacturers are especially unreliable. Although standard protocol is usually followed to some degree, specific conditions under which the tests are conducted and methods for obtaining the final results are often not specified. Manufacturers tend to use test methods and conditions that give
the best results for their specific product.
Moreover, it is important to realize that even the most carefully selected test performed in the most rigorous
manner only approximates (at the very best) performance in the field. Tests are performed on small pieces of
fabric in a laboratory, under a limited set of controlled conditions. Even if tests were adopted for field conditions, it would be nearly impossible to take into account every variable including individual perspiration level,
metabolic activity, venting characteristics, available surface area for vapor transport, ground temperature, wind
speed and direction, ambient temperature, precipitation level, etc.
In the absence of a single test that accurately characterizes WP/B fabrics, a number of test methods are used to
provide a relative basis for comparison. These test methods are described in further detail below.
1. ASTM E 96, Procedure B (upright cup method)
Tests are conducted in a wind tunnel which is housed in an environmental chamber. The
air temperature in the chamber is 23±0.5˚C, and the dew point temperature is 12±1˚C (50%
relative humidity). The air velocity in the wind tunnel is 2.8±0.25 m/s. Six circular specimens
of 7.4 cm diameter are cut from the fabric. Each specimen is placed on a 155 mL aluminum
cup that is filled with 100 mL of distilled water, covered with a gasket, and then clamped into
position (Figure 4a). Coated or laminated fabrics are placed with the coated or laminated
side facing the water in the cup. Each cup is first weighed to the nearest 0.001g and then
placed inside the wind tunnel. Subsequent weighings are made at 3, 6, 9, 13, 23, 26, and 30
hours after placement in the chamber.
The water vapor transmission rate (WVTR) is calculated using the following formula, where
G = weight change (g), t = time during which G occurred, G/t = slope of the straight line for
weight loss per unit time (g/h), and A = test area (m²).
WVTR =
G/t
A
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2. ASTM E 96, Procedure BW (inverted cup method)
The test conditions and cup preparation are identical to that of ASTM E 96, Procedure B
except that cups are placed in an inverted position in the wind tunnel (see Figure 1). This
procedure may only be used for waterproof samples because fabrics that allow liquid water
to pass will leak. A sealant may be used to secure the fabric to the cup and prevent leakage.
The weighing protocol and calculation of the WVTR is identical to the upright cup method.
Fabric
Water
Air Layer
Gasket
Water
Gasket
Fabric
Figure 1. (L) Upright cup assembly and (R) Inverted cup assembly (fabric is in contact with water)
3. JIS L 1099 (dessicant inverted cup method)
A solution of potassium acetate is used to fill two-thirds of a cup. The solution acts as a dessicant and generates 23% humidity on one side of the fabric. Once the potassium acetate
solution is added, PTFE film is placed over the cup and fixed into place.
Three 20 cm x 20 cm square specimens are then cut from the fabric. Each specimen is placed
on the test piece supporting frame. Coated or laminated shell fabric are placed such that the
coated or laminated side faces away from the dessicant. Another piece of the PTFE film is
placed on top of the fabric specifem and secured ot the frame. The test piece frame assembly is placed in a floating position in a water tank of temperature 23˚C.
Inverted cup
Dessicant
(potassium acetate)
PTFE film
Fabric
PTFE film
Tank
Water
Figure 2. Dessicant inverted cup assembly (fabric is between water and dessicant)
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The mass of the test body with the film side upwards is measured. Then, the test body is immediately overturned and placed in the supporting frame. The entire assembly is placed in a
constant temperature apparatus that circulates air at 30±2°C. After 15 minutes, the test body
is taken out of the constant temperature apparatus and weighed.
The WVTR is calculated using the following formula:
P=
4(a₁ - a₀)
S
where a₁ = mass of the test body after the test (g), a₀ = mass of the test body before the test
(g), P = rate of water vapor transmission (g/h/m2), and S = water vapor permeable area (m2).
The results are averaged from three specimens and converted to g/24h/m2.
4. Evaporative resistance (ISO 11092, ISO 1999, and ASTM F 1868)
This test measures the amount of power it takes to keep the plate heated to skin temperature when water vapor is evaporating from the surface of the plate and diffusing through
the fabric to the environment.
Three 50.8 cm x 50.8 cm square specimens are cut from fabric. A PTFE liquid barrier is placed
on the plate to prevent water from contacting the fabric, ensuring that only water vapor
contacts the fabric sample. Each test specimen is placed on the horizontal and flat plate orientated with the side of fabric normally encountering more water vapor facing the hot plate.
The plate temperature and the air temperature are controlled at 35 ± 0.5°C by a main heater
and a set of guard and bucking heaters that eliminate both lateral and axial flow from the
main heater. A dew point temperature of 19°C is used to achieve 40% relative humidity. A
vertical flow of air from a hood is maintained at 1.0 m/s.
Air Flow Hood
Water Reservoir
Fabric
Guard Heater
Bucking Heater
Guard Heater
Figure 7. Sweating Hot Plate
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When the system reaches steady state, the test setup stays at equilibrium for 1 hour. The
basic equation for caculating the total resistance to evaporative heat transfer provided by
the liquid barrier, fabric, and air film is:
R e,t =
(Ps - Pa ) · A
H
where Re,t = total resistance to evaporative heat transfer provided by the fabric system and
air (m²Pa/W), A = area of test speciment (m²), Ps = water vapor pressure at the plate surface
(Pa), Pa = water vapor pressure in the air (Pa), and H = power input.
5. Dynamic moisture permeation cell (ASTM F 2298)
This standard measures WVTR by passing a mixture of dry and water-saturated nitrogen over
the top and bottom surfaces of the test sample located in the test cell. The relative humidity
of the top and bottom cell segments is determined by controlling the ratio of the dry and
saturated nitrogen gas. A maximum humidity gradient of 90% is used in the standard with
the top cell segment at 95% RH and the bottom segment at 5% RH.
95% RH
Test Cell
Fabric
Nitrogen Gas/Water
Bubblers
Nitrogen Gas/Water
Bubblers
5% RH
Mounting Plate
Figure 8. Internal view of dynamic moisture permeation cell
Three specimens of 2.5 cm x 2 cm are tested under air temperature = 20 ± 1°C and nitrogen gas flow rate 2000 cm³/s. The fabric specimen is clamped between the top and
bottom segments of the flow cell with the side normally facing greater water vapor
concentration towards the segment with higher humidity. A computer, flow controllers,
differential pressure transducer, automated valves, and relative humidity measurement
devices are used to produce the desired relative humidity in the upper and lower cells.
The WVTR is calculated using the following formula:
WVTR =
δ ф Psat · Mw · Qs
A R Ts
where A = area of test speciment (m²), δφ = relative humidity difference between the incoming stream and outgoing stream in the bottom portion of the moisture permeation cell,
expressed in decimal format, Psat = water vapor saturation vapor pressure at the test temperature (N/m²), Mw = molecular weight of water vapor (18.105 g/mole), Qs = indicated volumetric flow rate from the mass flow meter (m³/s), R = universal gas constant (8314.5 Nm/(kg · K),
Ts = reference temperature used by the mass flow meter (K).
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