Body thermal sensation

Conditions that are warmer than the applicable still-air comfort zone of Fig. 5.7b can often be made comfortable by increasing the air speed. If the conditions are 1 to 6 °C warmer than the still-air comfort zone of Fig. 5.7b, the necessary air speed v) to restore thermal balance and comfort can be estimated from Fig. 5.8, where Tis the temperature difference between the environment and the still-air comfort temperature. Though the increased air speed will bring the whole-body thermal sensation to the comfort level, air motions above 0.8 m/s or so may cause other kinds of discomfort frojn... 

Body thermal sensation The response of the body to changes in the thermal environment, relating to moisture, air movement, or temperature. 

The temperatures monitored in Fig. 5.2 are used by the brain to regulate shivering, blood flow to the skin, and sweating. The sensed temperatures also contribute to our overall feelings of warmth and other thermal sensations. 7 hermal sensation (TS) can be predicted over a wide range of activities (0.8 to 4 met) from simple deviations in the mean body temperature (T j,) from the mean bodv temperature when the person feels neither warm or cool but neutral (Fig. 5.2). 

During transients the rate of change of mean body temperature can have a strong effect on thermal sensation. 

In general, when a person is thermally comfortable, the person s thermal sensation for the whole body is at or near neutral as depicted in Fig. 5.7a. As we have seen, the thermal conditions necessary for comfort are affected by clothing insulation. Figure 5.7b shows the range of temperatures and humidities... 

For < 0.05 m s, use t, - 0.05 m s b For DR > 100%, use DR = 100%, The model applies to people at light, mainly sedentary activity with a thermal sensation for the whole body close to neutral. The sensation of draft is lower at activities higher than sedentary and for people feeling warmer than neutral. 

As it was mentioned earlier, if liquid water (sweat (sweat) cannot be dissipated quickly, the humidity of the air in the space in between the skin and the fabric that contacts with the skin rises. This increased humidity prevents rapid evaporation of liquid water on the skin and gives the body the sensation of "heat" that triggered the sweating in the first place. Consequently, the body responds with increased sweating to dissipate excess thermal energy. Thus a fabric s inability to remove liquid water seems to be the major factor causing uncomfortable feeling for the wearer. 

This necessity in creating inner spaces which satisfies most of their occupants is considered a great challenge to design engineers, architects and building operators as the human-being thermal sensation is very subjective since each human s body react differently and because of that it differs from person to person (Leal Neves, 2013). 

Thermal sensation is the human thermal perception which according to thermal environment surround, triggers a human body reaction (Parsons, 2003). The skin temperature is considered by some authors (Kataoka et al., 1998 Zingano, 2001) as a reference temperature to evaluate human thermal sensation. A thermal comfort sensation is defined as a neutral sensation or the state of satisfaction of an individual when exposed to a thermal environment (ASHRAE, 2001 Chow, Fong, Givoni, Lin, Chan, 2010). On the other hand a stress thermal sensation is the state of dissatisfaction of an individual when exposed to a thermal environment (Meles, 2012 Talaia, Meles, Teixeira, 2013) When an individual are under extreme thermal environments, either hot or cold, the risk of failures and work accidents increase (Riniolo Schmidt, 2006). Furthermore, many studies demonstrate a strong relation between comfort and productivity (Bluyssen, Aries, van Dommelen, 2011). 

Thermal conductivity Everyone knows that touching a metallic surface at room temperature produces a colder sensation than touching a piece of wood or plastic at the same temperature. The very high thermal conductivity of metals allows them to draw heat out of our bodies very efficiently if they are below body temperature. In the same way, a metallic surface that is above body temperature will feel much warmer than one made of some other material. The high thermal conductivity of metals is attributed to vibra-... 

We owe our knowledge of temperature to the direct action of the senses. By touching a body we can tell whether it is hot or cold, and it is neither possible nor necessary to define the conceptions hot and cold, nor to trace their origin back to other conceptions. We attribute our sensation of temperature to what we may call the thermal condition of the body if a body appears to be hot or warm, we say that there is much heat in it if cold, that it contains little heat, much, for example, as we say that a body appears bright when it is emitting much light, and dark when it is emitting little or none. 

If we wish to study the laws of the flow of heat, we must introduce exact measures for the merely psychological conceptions of heat and temperature, which we have been considering hitherto. Our sensation of temperature cannot help us in this, as sensations cannot be compared with one another quantitatively. We cannot perform a measurement of heat or of temperature until we have found some physical property of a body which varies uniquely and continuously with its thermal condition, and which can be measured in time and space by one of the ordinary scientific methods. Fortunately experiment has shown us that there are a great many physical properties of bodies which vary uniquely in this way with their thermal condition, and which may all, therefore, be used for the measurement of heat or of temperature. Nearly all the physical properties of a body are altered when it is heated. One of the easiest properties to measure is the volume. A body occupies more space when it is hot than when it is cold. We say, therefore, that heat causes expansion, and cold causes contraction. The property of expansion on heating is common to all bodies with very few exceptions (one exception is water between 0° and 4°C.), and is generally used as the basis of temperature measurements. 

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