TECHNICAL INSIGHT

Thermophysiological comfort

Definition

Comfort or confort is a purely subjective sensation perceived by the user, in the work environment or in certain service conditions and serves to indicate the perceived "level of well-being". (Wikipedia)

Comfort = Well-being

According to this generally accepted definition, the word comfort is associated with well-being, understood as the set of pleasant sensations deriving from external or internal stimuli to our body. The degree or level of comfort would therefore be considered a completely subjective condition, impossible to measure or compare.
In order to arrive at a complete definition of comfort, it is necessary to introduce an aspect of objectivity by approaching the problem from a physical and/or physiological point of view, therefore analyzing what can objectively happen to our body when it comes into contact with clothing, home textile products or various accessories.
Comfort therefore becomes the degree of well-being offered by a product, determined by the sensation perceived when it is worn.
The transition turns out to be that of a purely sensorial evaluation, strictly conditioned by some identified and measurable variables: type of model, ability to adapt to the morphology of the user and to maintain itself over time and in different conditions, ability to respond in terms of sizes to the needs of the user, intrinsic engineering to respond to different performance needs, aesthetics, ability to avoid physiological inconveniences such as irritation, dermatitis, thermal stress, excessive sweating, proliferation of bacteria and allergies.

Thermal comfort

We talk about thermal comfort to indicate that psychophysical state of satisfaction with the thermal environment.
The body must be kept in thermal equilibrium as much as possible: the metabolic heat generated by the body, added to that generated by external sources, must correspond to an equivalent loss of heat.

If the heat gained and the heat lost are not in balance, the body temperature will decrease or increase, leading to a decrease in the comfort level, therefore all the parameters that influence the thermal exchanges, between the individual and the environment, must compensate for the sensations of heat or cold perceived by the individual himself.

There are two types of parameters to take into consideration:

Environmental or objective :

Ambient air temperature (convective heat exchange)
Mean radiant temperature (radiative heat exchange)
Relative air velocity (convective heat exchange)
Relative humidity of the air (evaporative exchange of the body)

Individual or subjective :

Metabolic expenditure related to the activity performed
Conductive and evaporative thermal resistance of clothing

Thermophysiological Comfort

Thermophysiological comfort represents the indicator par excellence that allows us to move from the concept of sensation to that of objective evaluation : Comfort and Thermophysiological nature associated with comfort.

The transition from the concept of comfort to thermophysiological comfort is achieved by taking into consideration three fundamental elements:

the physical activity of the human body
the external environment
the characteristics of the garment.

The subtle link between these three variables is represented by the ability of any product, be it an item of clothing, an accessory or a padded blanket, or a padding, to best perform the complex of three distinct functions:

thermal insulation;
barrier to vapor transpiration;
behavioral mechanism of thermoregulation.

The measurable quantities to be considered are:

RCT (Thermal Resistance Coefficient) – determinable through instrumental laboratory analysis Skin Model UNI EN ISO 11092:2014 and ASTM F1868)
RET (Evaporative Resistance Coefficient) – determinable through instrumental laboratory analysis Skin Model UNI EN ISO 11092:2014 E ASTM F1868

The Skin Model Test

The laboratory analysis that is best able to evaluate the thermophysiological comfort index and the variables described above is the one defined as the “Sweating guarded hot plate method”, or more commonly Skin Model:

The SkinModel allows to determine the RCT and RET measures expressed in m2K/W and m2Pa/W respectively.
The measurement must take place in an environment where relative humidity and temperature can be controlled.
Measurement conditions:

RCT - Climatic chamber at 20°C±0.1°C and 65%±3% RH
RET - Climatic chamber at 35°C ±0.1°C and 40%±3% RH
Plate: 35°C±0.1°C
Air speed: 1m/s±0.5m/s
Time: 12h sample conditioning

Measurement methods and meaning

RCT The sample is placed on an electrically heated plate. Air is made to flow parallel to its surface. To determine the thermal resistance, the heat flow through the sample is measured when the stationary state (quasi-equilibrium state) has been reached. The RCT value is obtained by subtracting the thermal resistance of the plate on which the sample rests and the relative layer of air from the value obtained.
RET The sample is placed on an electrically heated porous plate covered by a membrane permeable to vapor but impermeable to liquid water. No liquid must come into contact with the sample. The heat flow required to maintain the plate at a constant temperature is in practice a measure of the evaporation flow of water and allows us to trace the evaporative resistance value. The RET value is obtained by subtracting from the obtained value the evaporative resistance of the plate on which the sample rests and the relative layer of air

The UNI EN ISO 11092:2014 standard

Thermal resistance RCT

The temperature difference between the two faces of the material, divided by the heat flux per unit area in the direction of the gradient.

Dry heat flux may consist of one or more of the conductive, convective and radiant components. Thermal resistance is a specific quantity of textile and composite materials that determines the dry heat flux through a given surface when a stable temperature gradient is applied over time.

Water vapour resistance RET

The difference in water vapor pressure between the two faces of the material, divided by the evaporative heat flux per unit area in the direction of the gradient. The evaporative heat flux can consist of both diffusive and convective components.

Water vapor resistance is a specific quantity of textile and composite materials that determines the flow of latent heat of evaporation through a given surface when a time-stable water vapor pressure gradient is applied.

Results and meanings attributable to the different variables

High RCT : material that conducts little
High RET : low breathability material
Wd : water vapor permeability - expressed in g/(m2hPa) = 1/(Ret x ΦTm)
with ΦTm=latent heat of evaporation of water at the measurement temperature.
Imt : water vapor permeability index = (RCT/RET) x S with S=60Pa/K
Imt : is a pure number whose value is between 0 and 1. A material that has a value of 0 has a total impermeability to water vapor. A material with an index equal to 1 has the thermal and evaporative resistance of a layer of air of equal thickness.

Imt is the parameter that more than any other expresses the thermo-physiological comfort coefficient

The 100% T.Silk® silk padding has a very high water vapour permeability index (IMT), close to 1.