Children’s cold rating and advanced moisture testing

New SATRA developments in the assessment of children’s footwear.

by Tom Bayes

When it comes to whole shoe comfort and thermoregulation (a balance of breathability and insulation achieved through construction and materials selection), there are two main alternative approaches to testing. Subject-based tests require resilient subjects, especially as the testing is often carried out in extreme environments. With laboratory-based testing, the subject is no longer a factor or a requirement. This has advantages – especially when the subjects required are children. SATRA has now adapted some of its advanced tests so they can be routinely carried out on children’s footwear.

Historically, testing the thermoregulatory properties of footwear was carried out with human subjects confined to a climate chamber. The subjects would perform an appropriate task representative of the intended application of the footwear, in pre-determined ambient conditions. The subjects were instrumented with temperature and humidity sensors, and the results were logged for analysis. For testing thermal insulation, additional data would often be obtained through questionnaires given to the subjects, as well as by careful study of the sensor data. For moisture management, a similar process would also be conducted with measurements taken of mass gain of hose, foot bed and footwear before, during and after the activity.

Choice of subject is important

This approach is, by its very nature, subjective. In order to gain meaningful results, the subjects should be correctly acclimatised and correctly trained. In other words, if meaningful results for desert combat footwear are required, it would make sense to use service personnel who are familiar with desert conditions and activity levels. Results obtained for this type of footwear, from subjects whose primary profession is as an office worker, would not be so relevant, as the perception of ‘comfort’ may be entirely different.

The approach is also expensive and very time-consuming, as there is a limit to how much useful testing can be done in a day. If sufficient care is taken during planning and execution, the process can provide meaningful data, although it will be largely qualitative and, of course, subjective. It is also very difficult to repeat with a high confidence of obtaining the same results.

In order to address this problem, SATRA developed two laboratory-based tests to determine thermoregulatory performance of whole footwear. The SATRA TM376:2009 'Advanced moisture management test' (AMMT) does more than determine the level of breathability of the footwear. It takes into account other mechanisms that are employed by footwear developers to manage the footwear's ability to control moisture – specifically moisture close to the foot. Breathability, wicking and air exchange are all common methods of allowing moisture produced by the foot to escape to the outside environment.

The key measure is mass gain of the hose, as the drier the foot is kept, generally the more comfortable and thermally stable the footwear will be. The other related test is designed to measure whole footwear thermal insulation. SATRA TM436:2010 – 'Determination of whole shoe thermal insulation value and cold rating' is a method that measures the actual physical insulation value of whole footwear and is carried out on the same test machine (SATRA STM 567) as SATRA TM376. This is a whole footwear test and, as such, the results include all the properties that contribute to thermal insulation. SATRA TM436 contains a means of determining the most comfortable ambient temperatures for a predetermined range of activities, whether 'low', 'moderate' or 'vigorous'. The result represents an optimal value, as the test is carried out without the addition of any perspiration due to the presence of moisture affecting the thermal properties of footwear.

Laboratory-based tests removed the need for subjects. This is advantageous because subjects can be difficult to find and, as already explained, the subjects need to be familiar with the type of footwear and its application. For children's footwear, the ability to test for comfort without having to use subjects is essential, as it is not possible to carry out meaningful tests of this nature with very young subjects. There is a real need for methods of measuring properties of children's footwear – particularly thermal properties.

New size foot forms

Until recently, SATRA STM 567 has been available with the options of UK size 4, 7 and 9 foot forms. SATRA has now developed two further heated, perspiring foot forms in UK adult size 1 and child size 10. The feet are constructed slightly differently from the larger sizes. Because of the relatively small volume and surface area, smaller heating units are employed and the number of water feed pipes reduced from eight to six. Due to their small size, SATRA employed rapid tooling technology to manufacture the foot forms using 3-D printed moulds.

The much smaller size of these foot forms compared to the UK size 9 has also led to a revision of SATRA TM436. This test method and the calculations it contains are based around a UK size 9 foot of a certain surface area. The area/volume of these new foot forms are so different that the same calculation cannot be used if there is to be a correlation of results throughout the range. The new version of the method makes no difference to results carried out on the original sizes, but ensures that the smaller foot forms maintain the same cold rating values for equivalent insulation levels across activity levels.

Publishing Data

This article was originally published on page 6 of the May 2016 issue of SATRA Bulletin.

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