Testing heat and moisture transmission

The next generation of SATRA’s STM 511 Sweating Guarded Hotplate test machine.

by Peter Allen

In many situations, comfort of footwear and clothing is of critical importance to the wearer. It is a major contributory factor in determining the success or failure of products brought to market or presented against a tender specification. Two important factors contributing to product comfort are the overall heat and moisture transmission properties of the product’s construction materials. One method of assessing these material properties is with a Sweating Guarded Hotplate test machine. This article introduces SATRA’s redesigned STM 511 test machine and outlines the tests that can be conducted with it.

The Sweating Guarded Hotplate (often referred to as the ‘skin model’) is a device for measuring the thermal resistance or water vapour resistance of a material or a composite. The tests conducted using the Sweating Guarded Hotplate are designed to simulate the transfer processes of heat and moisture through materials next to the human skin, and to measure the rate of transfer of heat or moisture in such processes.

The test methods detailed in EN 31092:1993/ISO 11092:1993 are steady state tests, suitable for the measurement of thermal resistance or water vapour resistance of fabrics, films, coatings, foams and leather. These tests are referenced in a number of protective clothing standards for assessing the vapour permeable and insulating properties of materials, as well as in a number of footwear specifications, particularly those used by the military. The test method also has applications in clothing, sleeping bags, upholstery and similar textile-type products.

EN 31092:1993/ISO 11092:1993 specifies two test methods. One of these is used for measuring water vapour resistance – the water vapour pressure difference between the two faces of the test sample divided by the resultant evaporative heat flux per unit area in the direction of the water vapour pressure gradient. This is called ‘Ret’ and is expressed in square metres pascal per watt.

The other method measures thermal resistance, and is referred to as ‘Rct’. It is expressed as square metres Kelvin per watt – defined as the temperature difference between the two faces of the test sample divided by the heat flow per unit area.

The SATRA STM 511 test equipment

The test machine comprises an environmental cabinet, which allows the temperature and humidity to be closely controlled, and contains the specific hardware required to conduct the tests. At the core of the machine is a ‘measuring unit’, which consists of a porous metal plate attached to a conductive metal block fitted with heating elements. Also contained within the block is a water reservoir fed by a water supply system, which closely controls the water level relative to the underside of the porous plate (typically set 1mm below the plate surface).

During a test, the test material is placed on top of the porous plate. Surrounding the measuring unit is a guard ring which is set level with the top of the porous plate. An adjustable support table is set to the outside of the guard ring. This table can be set flush with the top of the porous plate and guard ring, or can be raised to be flush with the top of the test material – for example, in the case of a thick foam or composite material. During the test, a crossflow fan provides a controlled rate of airflow through a duct placed over the test specimen. An airspeed sensor fitted to the machine is used to confirm the correct rate of airflow is set.

In a very simple way, the equipment attempts to mimic the heating and sweating mechanisms at the surface of human skin. As a result, the performance of breathable fabric/membrane systems can be studied under theoretical conditions. The temperatures of the measuring unit and thermal guard ring are controlled at 35°C ± 0.1°C in order to represent the temperature of the skin. The test specimen is laid over the measuring unit and thermal guard ring, so that it overlaps the thermal guard.

When measuring water vapour resistance, the temperature of the environmental chamber is set to 35°C and the humidity is set to 40 per cent RH. However, when measuring thermal resistance, the temperature of the chamber is set to 20°C and the humidity is set to 65 per cent RH.

When used to determine thermal resistance, no water is used and the heat flux is measured through the test specimen after steady state conditions have been achieved.

If measuring water vapour resistance, however, a thin water vapour permeable membrane (a consumable item for the STM 511) is placed over the surface of the measuring unit, and the sample under test is placed on top. Water fed to the heated plate evaporates and passes through the membrane as vapour, so the test sample is not in direct contact with the liquid water. The heat flux required to maintain the plate at steady state temperature is a measure of the rate of water evaporation from which the water vapour resistance can be determined.

The operation of the device is, in principle, straightforward. The hotplate and guard ring are temperature controlled, and the environmental chamber is set to the appropriate temperature and relative humidity – depending on whether the water vapour permeability or thermal resistance is being assessed. Initially, the bare plate constants of the equipment for water vapour resistance or thermal resistance are determined. This is accomplished by conducting tests without a test specimen in place. The STM 511 software, supplied with the machine, determines the R base value (R_et0 or R_ct0, depending on the type of test conducted).

With the test specimen in place, the apparatus is allowed to achieve a steady state condition (in which the rate of heat loss from the upper surface of the test specimen is equal to the heat input from the measuring unit). Once steady state conditions occur, new R values are calculated with the sample in place. The result for a particular material is the value obtained with the test specimen in place minus the bare plate constant determined as above. These values (R_et or R_ct) are output to the test machine display screen. An option is available for transferring data and the calculated R values to an external PC.

New SATRA STM 511 features

Development of the new SATRA Sweating Guarded Hotplate test machine has benefited from the experience of routinely using our earlier design for contract testing within SATRA’s test laboratory. The new STM 511 test machine has two porous plate options: a large plate (250 x 250mm) or a small plate (204 x 204mm). A large test sample of up to 500 x 500mm can be accommodated, and the machine now allows test samples up to 90mm thick to be tested. Improvements in the user interface with the machine using touch-screen technology and improved export of data to spreadsheets have also been introduced. New improved airflow and airflow measuring systems have been added, along with improvements in the supply and control of the water feed system.

SATRA produces a range of other test equipment which can be used for the assessment of water resistance, water permeability or water absorption of materials. The SATRA equipment sales team can offer advice on the selection of the most appropriate test equipment, best suited to your specific requirements.

Click here for comprehensive information on SATRA's range of test equipment.

Publishing Data

This article was originally published on page 32 of the April 2012 issue of SATRA Bulletin.

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