Measuring thermal and water vapour resistance

Describing the use of SATRA’s sweating guarded hotplate.

by David McKeown

An acknowledged contributing factor to overall footwear comfort is the heat and moisture transmission properties of component materials. Measuring these properties reliably requires equipment designed to maintain and control the test environment to very tight specifications, as well as accurately measure the various parameters involved. The SATRA STM 511-2 ‘sweating guarded hotplate’ has been designed to meet the requirements of ISO 11092:2014.

It is important to note that the sweating guarded hotplate can only be used for testing sheet materials. It is difficult to use it with specimens cut from footwear, as they must have a minimum area of 400 x 400mm. Therefore, thermal and moisture vapour resistance tests on finished uppers or whole footwear cannot easily be carried out with this equipment. However, the SATRA STM 567 Endofoot machine can be used to test whole footwear, as described in the article ‘Testing thermal performance and moisture management’.

The relationship to comfort

The sweating guarded hotplate (often referred to as the ‘skin model’) is a device for measuring the thermal resistance and water vapour resistance of a material or composite. The test used – EN ISO 11092:2014 – is designed to simulate and measure the transfer of heat and moisture through materials next to the human skin.

For example, a material with a low level of ‘breathability’ has a high resistance to the transmission of water vapour. Such a material will lose water vapour only gradually through the material. Moisture will build up inside the footwear and often accumulate in the sock and lining materials. By contrast, a material with a high level of breathability will have low water vapour resistance.

By contrast, some types of materials having high levels of thermal resistance are often desirable – particularly for footwear worn in cold conditions. Convective heat loss from the surface of the upper material is one of the major heat loss mechanisms. If the upper material and lining combination has a high thermal resistance, the loss of heat may be slowed down sufficiently so that the wearer’s feet do not get cold.

About the test method

EN ISO 11092:2014 is a ‘steady state’ test in which the temperature and relative humidity of the test environment are stable and the heat lost from the upper surface of the test specimen is equal to the heat input from the measuring unit. It is suitable for the measurement of thermal resistance and water vapour resistance of woven and nonwoven textiles, films, coatings, foams and leather.

As previously indicated, ISO 11092 specifies two test methods. One is used for measuring water vapour resistance (the water vapour pressure difference between the two faces of the test specimen divided by the resultant evaporative heat flux per unit area in the direction of the water vapour pressure gradient). This is referred to as ‘R_et’ and is expressed in square metres pascal per watt (m² Pa/W).

The second method measures thermal resistance, or ‘R_ct’ (expressed as square metres Kelvin per watt – m² K/W). This is the temperature difference between the two faces of the test specimen divided by the heat flow per unit area.

The test equipment

SATRA’s STM 511-2 sweating guarded hotplate consists of a measuring unit fitted with heaters and temperature sensors, and equipped with a water supply. The measuring unit incorporates a porous metal plate, approximately 3mm thick with a surface area of 254mm x 254mm (0.0645 square metres). This is attached to a conductive metal block containing heating elements and incorporating a small shallow water reservoir underneath the porous plate. A thermal guard surrounds the metal plate, which is located within a measuring table.

The top surfaces of the measuring unit and thermal guard ring are flat and level. The upper surface is porous to allow water (at the same temperature as the measuring unit) to evaporate. The water level is closely monitored and adjusted to be at the optimum level throughout a test – that is, in contact with the lower surface of the porous plate. The equipment is mounted in and interfaced to an environmental chamber where the temperature and humidity are tightly controlled, as well as being monitored and recorded by the hotplate’s own control and measurement software.

When used to determine thermal resistance, no water is used. The heating power applied to the hotplate is measured after steady state conditions have been achieved. However, if measuring water vapour resistance, a thin membrane, permeable to water vapour, is placed over the surface of the measuring unit, and the test specimen is placed on top. Water fed to the heated plate evaporates and passes through the membrane. Hence, the specimen is not in direct contact with the water. The heating power 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.

How the equipment works

In a very simple way, the equipment attempts to mimic the heating and sweating mechanisms at the surface of human skin. The performance of breathable fabrics or membranes can therefore be studied under theoretically more realistic conditions than conventional water vapour permeability tests. Irrespective of the type of assessment being conducted, the temperatures of the measuring unit and thermal guard ring are controlled at 35°C ±0.1°C in order to simulate skin temperature.

When measuring water vapour resistance, the temperature and the relative humidity of the environmental chamber are set to 35°C and 40 per cent relative humidity (rh) respectively. As a result, when performing a water vapour resistance test, the set-up is ‘isothermal’ (no thermal gradient exists between the hotplate and the environment in the chamber). Therefore, any evaporation of water from the specimen is because of the water vapour pressure difference across the specimen. However, when measuring thermal resistance, the chamber is set to 20°C and 65 per cent rh, which provides a temperature gradient across the specimen.

Operation of the equipment

In principle, the operation of the device is very simple. The hotplate and guard ring are controlled to the temperature outlined above, and the environmental chamber is set to the appropriate temperature and relative humidity for the test being conducted. Initially, the bare plate constants of the equipment for water vapour resistance (R_et0) or thermal resistance (R_ct0) are calculated. These measurements are carried out without a test specimen in place.

The specimen is then mounted on the hotplate so that it covers and is in close contact with the measurement part of the hotplate and the guard plate immediately around the measuring plate to a distance of at least 50mm on all sides. The test equipment is allowed to settle and achieve steady state conditions. The power at the measuring unit required to maintain this balance between heat loss and heat applied is then measured (in watts) over a period of time.

The result (R_et for water vapour resistance measurements or R_ct for thermal resistance measurements) for a particular material is therefore the value obtained with the test specimen in place, minus the relevant bare plate constant (R_et0 or R_ct0) determined, as described above.

Publishing Data

This article was originally published on page 46 of the June 2019 issue of SATRA Bulletin.

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