Improved sweating guarded hotplate

Highlighting the benefits of the revision to this key piece of test equipment.

Comfort is of critical importance to the wearer, especially in performance, occupational or military footwear. SATRA has developed a number of test machines for assessing all aspects of comfort, including the SATRA STM 511 sweated guarded hotplate. The latest generation of this machine has been introduced, following a series of major upgrades to improve ease of operation and to further develop the precision of measurement.

The sweating guarded hotplate measures the thermal resistance or water vapour resistance of a material or composite – both very important for comfort.

The test simulates the transfer of heat and moisture through materials next to the skin and measures the rate of transfer of heat or moisture. It is particularly relevant when assessing the ability of footwear and clothing to transmit moisture or heat, primarily under steady-state ambient conditions.

How it works

The SATRA STM 511 test machine has an environmental cabinet which allows the temperature and humidity to be closely controlled. A ‘measuring unit’ consists of a porous metal plate attached to a conductive metal block fitted with heating elements. A reservoir keeps the water level up to the underside of the porous plate.

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 or can be raised to be flush with the top of the test material. A support table is set outside the guard ring and is flush with the top of the ring. A crossflow fan provides a controlled rate of airflow through a polycarbonate duct over the test specimen during the test.

The unit is ideal for steady-state measurement of thermal resistance and water vapour resistance of fabrics, films, coatings, foams and leather. The new version is fully compatible with EN ISO 11092:2014, which is used when assessing the performance of breathable and insulating materials. Other similar standards include ASTM F 1868:2017 and GB/T 11048:2008.

Measuring resistance

Standards specify two test methods. The first measures 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 called ‘Ret’ and is expressed in square metres pascal per watt. The second method measures thermal resistance (‘Rct’), expressed as square metres Kelvin per watt, and defined as the temperature difference between the two faces of the test specimen divided by the heat flow per unit area.

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Publishing Data

This article was originally published on page 16 of the October 2017 issue of SATRA Bulletin.

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