# Thermal Effusivity Tester (TET)--A New Device to Determine Thermal Effusivity of Textiles
## 1 Introduction  
The perception of coolness and warmth by handfeel of textiles is an important aspect of evaluation and, in some cases, one of the selection factors at the point of sale. A textile surface that quickly dissipates heat will be perceived as cool, which is considered pleasant under normal conditions. Subjective fabric cool/warm perception can be assessed by consulting a group of human assessors in the form of a consumer inquiry or an expert panel. As a higher variation in subjective assessment is expected, an objective measurement as reference has significant advantages in terms of reproducibility and time and personnel resources [1]. In the Kawabata KES-f system for objective handfeel assessment, this transient thermal feeling is measured in the thermal module (Thermo Labo), which determines the maximal amount of heat transferred through the fabric (heat flow) in a short measurement time (100 s), expressed as Qmax (J\(\cdot\)s/cm\({}^{2}\)) [2]. Later, the Alambeta device was developed to assess the cool/warm feeling of fabrics, defined by the term \"Thermal Absortivity\" [3; 4; 5], later called thermal effusivity [6], i.e., the ability of the material to exchange heat with its surroundings. It is defined by Equation (1), where \(\lambda\) is the thermal conductivity in W/mK, \(\rho\) the density in kg/m\({}^{3}\) and \(c_{p}\) the specific heat capacity in J/kgK. \[e=\sqrt{\lambda*\rho*c_{p}} \tag{1}\]  
The unit for thermal effusivity hence is W s\({}^{1/2}\)/m\({}^{2}\) K. The higher this value, the cooler the surface feels. Textiles show effusivity values between 0 and 400, while this value goes up to 1300-1500 for moist textiles. Water, for example, has a thermal effusivity of 1600. The works of [PERSON] et al. showed that thermal effusivity is theoretically independent from the temperature difference between fabric and skin, as well as from the measurement time. However, it does depend on the contact pressure, as the compression of the fabric reduces the air voids and enhances the material density [7]. As textile fabrics show a degree of compressibility depending on the fabric construction, the applied contact pressure could influence the result. Other developments took place to assess the thermal effusivity on solid surfaces like plastics and concrete. [PERSON] et al. developed a measurement technique to assess effusivity and conductivity on solid materials. The approach was based on the measurement of the electrical and thermal behavior of an internal metallic probe (hot wire) within the material over longer periods of time [8]. Other works took a thermographic approach to assess the effusivity of solid surfaces [9; 10]. A recent development is the TCi device by C-Therm, which has the same theoretical approach as Alambeta to measuring the thermal effusivity. Unlike Alambeta, TCi offers a more flexible approach, as the measurement unit can be positioned in any direction on solid surfaces. The TCi version \"Modified Transient Plane Source\" (MTPS) considers the compressibility of textiles and is equipped with an accessory unit, which offers a manual control of the applied pressure on the textile surface [11]. To measure thermal effusivity on one fabric side, heat penetration beyond the sample must be avoided. Apparently oriented to TCi measurement setting, the test standard ASTM D7984-16 defines a minimal thickness of 1 mm [6]. As textile fabrics are usually thinner than 1 mm, a multi-layered measurement is suggested, where the number of layers is subject to pre-experimentation. Alambeta is able to measure a single layer, where the fabric backside is positioned on a metal plate with the same temperature [4]. This work presents the thermal effusivity tester (TET)--a new device developed by the Instrument Development team at Lenzing's R&D department to perform profound research in the field of thermal perception.