MSR147WD in use at VAUDE to measure the thermophysiological wearing comfort of backpacks
Authors: Markus Weder, MSR Electronics GmbH, Seuzach, Switzerland, Robert Klauer and Frank I. Michel, VAUDE Sport GmbH & Co. KG, i-team, Tettnang, Germany, Franziska Breuner and Stephan Odenwald, TU Chemnitz, Sports Equipment & Technology, Germany
Even if a rucksack only covers part of the body, wearing a rucksack has a significant influence on thermoregulation during sporting and sweat-inducing activities.
The overall feeling of comfort is often negatively influenced by extreme local temperature or moisture sensations. For example, a wet back due to a backpack can lead to a thermophysiological discomfort sensation, although the clothing itself would be optimal and there is no increase in core temperature.
Measuring method
In an elaborate series of test subjects at VAUDE, three different backpack types (U, A and T, see illustration above) with different designs were tested on 4 test subjects on a bicycle ergometer in a climate chamber. Wireless data loggers of the type MSR147WD were used to measure the temperature and relative humidity both in the microclimate on the body and on the backpack (on the contact surface to the back). In order to better quantify the measurement data obtained, the same measurements were also carried out without the rucksack.
Data logger used
The MSR147WD wireless data logger used in the study was specially designed for clothing physiology measurements. Equipped with Bluetooth Low Energy (BLE) short-range wireless technology and a memory capacity of 1 million measured values, the thumb-sized logger uses five plug-in humidity or temperature sensors to precisely record physiological parameters such as skin temperature and moisture. Figure 3 shows the positions of the temperature and humidity sensors on the subject's back.
With the help of an automatic configuration function, the data logger recognizes when a sensor is plugged in. The calibration data is stored in the corresponding sensor. The measured values can be retrieved online at any time using BLE or via the USB port.
When testing functional clothing by measuring comfort on the skin and in the microclimate between the individual layers of clothing, Bluetooth Low Energy wireless technology makes the user's work much easier. This is because the wireless transmission of measurement data allows the current measured values to be read out at any time using the MSR smartphone app. It is also possible to check whether the recordings are working properly without the wearer having to spend time putting on and taking off their clothing. Thanks to the built-in OLED screen in the data logger, the current measurement data can also be checked during the measurement without the aid of a cell phone. Using the "MSR DataLogger" app, data recordings can be started, stopped, read out and sent to the MSR SmartCloud from a cell phone. The MSR SmartCloud enables measurement data to be stored on a server via the internet. The user can thus view the measured values of his data logger from any location and, if required, grant other participants access to his measurement data.
Measurement results
The backpack types tested all have special design features that are designed for their desired area of application. Depending on the application, a close-fitting backpack or a backpack with a spacer composite and corresponding ventilation is optimal. Thanks to the measurement data, it was possible to observe that the three types of backpack showed different measurement results. The closer the temperature and relative humidity are to zero (measurement series without backpack), the less the backpack is perceived as such by the wearer
In terms of temperature, backpack A is the one with the lowest temperature increase both in the microclimate and on the backpack; followed by backpacks T and U.
In terms of relative humidity, the two backpacks U and T are slightly more advantageous compared to backpack A (approx. 5 to 10 % lower). This was measured both in the microclimate and in the rucksack. In contrast to the other two samples, sample U has a significantly higher backpack moisture content. A higher humidity on the backpack may mean that the moisture has migrated away from the microclimate into the backpack, or that the moisture evaporates too slowly from the backpack back into the ambient air. In principle, a high backpack humidity and a lower microclimate humidity are advantageous. Human skin sensors do not register humidity because humans do not have humidity sensors per se, only temperature sensors (in addition to pressure and pain sensors). However, moisture changes the sensory perception of the skin, such as the stickiness of underwear on the skin.
Damp skin under a rucksack can lead to a negative perception of comfort, especially during intermittent activities (stop and go). On the other hand, a damp back can have a positive effect during prolonged, relatively active physical exertion, provided the moisture can evaporate quickly enough and thus help to cool the body. In this case, performance is again more important than dry skin.
The differences in temperature at the end of the exertion phase are relatively large at 3 to 4 °C, both in the microclimate and on the rucksack. Humans can already perceive temperature differences of approx. 1 °C - and the thermoregulation of the rucksack is correspondingly more advantageous with a lower temperature rise on the back (compared to the empty value). This results in increased evaporative cooling, improved heat dissipation and therefore better performance.
Backpack T comes closest to the optimum zero value (measurement without backpack) and has the most favorable humidity values of the three backpack samples, both in the microclimate and in the backpack. Although backpack U still shows a slightly lower humidity in the microclimate, the relative humidity in the backpack is significantly higher, even slightly higher than in backpack A, where the highest microclimate humidity was measured.
Summary
Depending on the application, better performance or a less humid back is more important. Where the rucksack is used plays a major role here, but also in which load levels and also in which temperature and weather conditions the rucksack is used.
Thanks to these measurements, the different design features and materials of the individual VAUDE layers can be optimized to meet the desired requirements. The wearer thus receives the best possible backpack for the intended use.