BS ISO 20852:2020 pdf download – Textiles — Determination of the total heat transfer through textiles in simulated environments
This test evaluates two forms of heat transfer which are dry heat and evaporative heat emission. The total heat transfer results from combining both by calculation. Dry heat emission represents the heat loss resulting from the external environment due to the temperature gradient 10 °C and it is drawn from the standardized total thermal resistance of the test specimen and air layer. Evaporative heat emission represents the heat loss resulting from the external environment due to the vapour pressure gradient 3,57 kPa and it is drawn from the standardized total evaporative resistance of the test specimen and air layer
6.1 Sweating guarded hot plate test machine, as described in ISO 11092.
7.1 Water For the evaporative resistance measurements, water for analytical laboratory use over grade 3, according to ISO 3696, shall be used to wet the test plate surface.
7.2 Liquid barrier
A smooth, water-vapour permeable but liquid-water impermeable cellophane membrane of thickness 10 μm to 50 μm shall be fitted over the porous plate.
8 Test specimens Three test specimens are used. Use test specimens large enough to cover the surface of the hot plate test section and the guard section completely. Remove any undesirable wrinkles from the test specimens. Possible techniques for removing wrinkles include smoothing, free-hanging, pressing, steaming, ironing, and so forth. Allow the test specimens to come into equilibrium with the atmosphere 25 °C, 65 % R.H. of the testing chamber after conditioning them at the same environment for at least 12 h.
9 Test procedure
9.1 Test conditions Maintain the temperature of the test plate, guard section and bottom plate at (35 ± 0,5) °C without fluctuating more than ± 0,1 °C during a test. Air temperature should be (25 ± 0,5) °C and it should maintain the air flowing over the test plate at the same condition without fluctuating more than ± 0,1 °C during a test. Maintain the relative humidity of the air flowing over the plate at (65 ± 4) % R.H. during the test. Set the air velocity 1 m/s. Maintain the same air velocity for all calibrations and tests, and without fluctuating more than ± 0,1 m/s over the duration of the test measurement.
9.2.1 Determination of R ct0 Measure the bare plate thermal resistance R ct0 according to ISO 11092:2014, 7.1.1 at the test condition (25 ± 0,5) °C, (65 ± 4) % R.H. of 9.1. The bare plate thermal resistance shall be average of at least three measurements with nothing mounted on the test plate.
9.2.2 Measurement of total thermal resistance R ct for a test specimen Maintain the same condition of 9.1. Place the test specimen to be tested on the measuring unit surface and measure the total thermal resistance R ct . Measure and calculate the total resistance to dry heat transfer R ct for a test specimen including the air layer resistance when equilibrium is reached, using Formula (1). The total thermal resistance shall be the average of the measurements for the 3 specimens at least.
R ct is the total resistance to dry heat transfer provided by the test specimen and air layer, K·m 2 /W;
A is the area of the measuring unit, m 2 ;
H is the heating power supplied to the measuring unit, W;
T m is the temperature of the measuring unit, °C;
T a is the temperature in the air flowing over the test specimen, °C.
9.2.3 Measurement of intrinsic thermal resistance of the test specimen, R cf
Determine the intrinsic thermal resistance provided by the test specimen alone, R cf , by subtracting the average thermal resistance value measured for the bare plate including the air layer, R ct0 in 9.2.1 from the average total thermal resistance value measured for the test specimen and air layer, R ct in 9.2.2. See Formula (2):where R cf is the intrinsic thermal resistance of the test specimen only.BS ISO 20852 pdf download.