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Working principles of various manikins to evaluate thermal and evaporative resistance

Based on the preceding discussion, ASTM F 1291 and ASTM F 2370 standards are available to evaluate the thermal and evaporative resistance, respectively, of clothing ensembles using a full-scale manikin. By using these standards, different laboratories evaluated the thermal and evaporative resistances of various clothing ensembles (warm weather clothing, cold weather clothing, chemical protective clothing, surgical clothing, flame-resistant protective clothing, and firefighters’ protective clothing) on different sweating thermal manikins [436,442]. The interlaboratory thermal and evaporative resistance test results were compared according to ASTM E 691 in order to evaluate the repeatability (the variability between independent test results obtained within a single laboratory by the same operator using the same equipment) and reproducibility (the variability between independent test results obtained from different laboratories) of ASTM F 1291 and ASTM F 2370 standards. It has been confirmed that there is a variability of ±10% in the test results obtained from various laboratories. It was inferred that the variability from laboratory to laboratory is probably due to the complex nature of the apparatus and the fact that most manikins are one-of-a-kind instruments. It was recommended that the thermal and evaporative resistances of the clothing ensembles need to be measured on the same manikin for comparison unless prior agreement has been established regarding the use of different manikins at different laboratories. It seems that it is essential to understand the working principles of various sweating thermal manikins available in different laboratories in order to accurately evaluate evaporative and thermal resistance.

In the last century, many researchers put their efforts into their individual laboratories to develop the thermal manikin that can evaluate the thermal and evaporative resistances of clothing. The earliest thermal manikin was developed in 1945 by the US Army Research Institute on Environmental Medicine. This was a manually controlled one-segment copper manikin for the measurement of thermal resistance of a complete garment. From 1945 to date, the development of all manikins can be categorized into three generations [306]. In the first generation, the developed thermal manikins were standing (not walkable) and nonperspiring [428]; the second generation thermal manikins were movable (walkable) but nonperspiring, such as the manikins developed by University of Farnborough in England, Technical University of Denmark in Denmark, Hohenstein Institute in Germany (the copper manikin “Charlie”), and Kansas State University in the United States [428]. Among these second generation manikins, “Charlie” has been widely used. The Charlie manikin is made of copper and is divided into 15 segments that can be independently heated and temperature controlled; the manikin has movable joints at shoulders, elbows, hips, and knees and is driven by an external driving mechanism to simulate walking. At this stage, many workers tried to simulate sweating on nonperspiring manikins—in order do so, they put underwear of highly absorbent fabrics on the manikin, and supplied water to the underwear by sprinkling or water pipes. However, this wet underwear technique had the disadvantage that it was not possible to continuously generate sweat as occurs in human beings. As a consequence, the third generation of thermal manikins were developed, which can simulate true perspiration and/or body motions [306,431].

Since 1980, many sweating thermal manikins (standing or movable) have been developed in different laboratories all over the world. Some of these manikins are (1) the Finnish sweating thermal manikin, Coppelius, (2) the Japanese sweating thermal manikin, Taro, (3) the Swiss sweating thermal manikin, SAM, (4) the Hong Kong sweating thermal manikin, Walter, (5) the United States sweating thermal manikin, ADAM, (6) the United States sweating thermal manikin, Newton, and (7) the Japanese sweating thermal manikin, KEM [306]. In this context, it is notable that ISO 9920, ASTM F 1291, and ASTM F 2370 standards designate a test protocol for determining the thermal and evaporative resistance of the clothing; however, these standards do not specify anything about the movability of the manikin. Additionally, ISO 9920 and ASTM F 2370 standards do not stipulate a specific design for sweating simulation and leave the sweating mechanism open to interpretation [436,442]. These unspecified freedoms encouraged the development of manikins in different laboratories with different sweating systems, sweat rate evaluation techniques, moving techniques, and so on. [306,431]. A brief description on the working principles of these manikins, mainly related to sweat generation, sweat rate evaluation technique, and/ or movement is presented here.

Finnish sweating thermal manikin “Coppelius": The sweating thermal manikin “Coppelius” was developed based on the nonperspiring thermal manikin “Tore,” to which an additional sweating mechanism has been added [431]. The basic concept for this manikin (Coppelius) was to produce heat and moisture in a similar way to the human body. Coppelius was the first sweating thermal manikin developed (in 1980), which could deliver a controlled amount of water (200 g/m2/h) onto the surface of the manikin. This manikin has 18 individually controlled body sections. Its skin is made up of two layers: an inner nonwoven material that transfers water via 187 artificial sweat glands to the second, outer layer, which is microporous.

The manikin is housed in a chamber that can control the room temperature as per the requirements of the experiment. This manikin is hung from an electronic balance to constantly measure its weight. In order to evaluate evaporative resistance, the clothed manikin body is saturated with water. This water is supplied from a reservoir, and the amount of water supplied to the manikin is measured by an electronic balance attached to the reservoir. The amount of moisture vapor evaporation from the manikin’s body can be evaluated by calculating the difference between the weight of water supplied to the manikin, and the weight increase of the clothed manikin. The amount of moisture condensation in the clothing is measured by weighing the testing garments before and after the test. The amount of moisture condensation in the skin material of the manikin is measured by deducting the moisture condensed in the clothing from the increased weight of the clothed manikin.

Japanese sweating thermal manikin “Taro”: Dozen et al. [443] reported on the development of a sweating thermal manikin called “Taro”. This bronze manikin is equipped with pores that enable moisture vapors to flow from inside the manikin to the skin’s surface to simulate the body’s gaseous perspiration heat loss. The manikin’s body is divided into several segments, and the water supply in each segment can be individually controlled. The sweating quantity in each segment of the manikin is calculated by Eq. (5.37), where, Qi = sweat rate of a segment (g/m2/h); qi = the amount of air supply in the segment (L/min); Tsi = skin temperature (°C); Ta = ambient temperature (°C); Di = saturated absolute humidity (g/m3); Ai = the segment area (m2). The sweat rate can be altered by regulating the air flow rate. However, moisture vapor may not penetrate the garment to the ambient environment but rather escape from the opening of garment, especially in the case of increasing air flow rate.

Swiss sweating thermal manikin “SAM”: Richards and Mattle [444] introduced the Swiss “Sweating Agile Manikin,” or “SAM,” which has 26 individually controlled body parts with temperature sensors attached. Each body part can be heated separately. One hundred twenty-five artificial sweat glands are distributed all over the manikin’s body, which can simulate perspiration. The sweat rate of this manikin is 0-41 g/m2/h. The manikin can walk at the speed of 3 km/h in order to simulate realistic human motion, and the wrists and ankles of this manikin are connected through an external drive assembly. Due to this, this manikin can move through various curves under computer control.

Hong Kong sweating thermal manikin “Walter” : The previous three manikins (Coppelius, Taro, SAM) were developed towards the end of the last century; however, their usability was very limited due to high cost. At this point, Fan [445] successfully developed a prototype of alow-cost fabric manikin in 1989 at the University of Leeds. However, the manikin developed by Fan was nonperspiring because the skin of the manikin was made up of a nonbreathable neoprene coated waterproof fabric. This low-cost fabric manikin concept was further improved at Hong Kong Polytechnic University by Fan and Chen to develop the perspiring fabric thermal manikin called “Walter” [306]. Walter has a breathable fabric skin made of polytetrafluoroethylene (PTFE) Goretex membrane. The pores in this fabric are too small to allow water molecules to pass through but large enough to allow the passage of molecules of water vapor [375]. Sweat glands present inside the manikin control the sweat. This manikin can maintain a constant body temperature by circulating water from its center to its extremities, and can move at a speed of 2.48 km/h to simulate actual human movement. As noted by the researchers, the strength of the membrane and seams used to construct the manikin must possess high strength in order to make the manikin durable in the moving condition [306].

The Walter manikin is also hung in a controlled chamber similar to Coppelius. However, the technique used to measure evaporative resistance in the Walter manikin is different from the technique used with the Coppelius manikin. As previously explained, in the case of Coppelius, evaporative resistance is evaluated indirectly by calculating the difference between the weight of water supplied to the manikin and the weight increase of the clothed manikin. However, in the case of Walter, evaporative resistance is measured directly. In this case, water is supplied to the Walter manikin as per the rate of sweating. During sweating, the water in the manikin reduces. As a consequence, water automatically flows to the manikin from a water reservoir through siphon action at the same atmospheric pressure. Therefore, the amount of water reduction in the water reservoir is proportional to the sweat rate of the manikin or the evaporative resistance of the clothing. It seems that the sweat rate in the Walter manikin is dependent on the type of clothing being tested. In real life, the perspiration of human beings is also dependent on their worn clothing; thus, Walter can simulate the real life perspiration of human beings. However, Walter cannot simulate real life situations in all contexts. It has been found that the sweating rate in different parts of a real human body is not equal. For example, the sweat rate for foreheads and underarms is much higher than other body parts. However, the Walter manikin cannot generate different sweat rates at its different parts. Furthermore, liquid sweats are formed at the skin surface of a real human body, whereas the Walter manikin can only allow the water vapor to pass through the skin. In real life, most sweat is also initially moved by liquid transport, then evaporates, transmits through moisture vapor, and at is last absorbed or condensed within the clothing worn by human beings; liquid transport further takes place when the condensed water exceeds a certain amount, and so on. This situation is somewhat different in the perspiring Walter manikin. Here, the moisture vapor from the skin surface of the manikin is initially transmitted through moisture vapor, and then is absorbed by or condensed on the tested garment; liquid transport further takes place, and so on. It seems that there is a difference in the initial liquid transport (immediately after sweat generation) between human beings and the manikin.

United States sweating thermal manikin “ADAM”: The United States sweating thermal manikin is called ADvanced Automotive Manikin (“ADAM”) and was developed by Measurement Technology Northwest for the National Department of Renewable Energy (NDRE) [446]. ADAM has a 175 cm height and 61 kg weight, which is designed to match the 50th percentile of American males. ADAM has completely human-like geometry and weight with prosthetic joints to simulate the human range of motion. The manikin is equipped with sophisticated surface sensors that interact with its ambient environments. Not only does it respond to thermal inputs such as radiation and convection, but it also is affected by the environmental flow field and temperature field. There are 126 individually controlled segments in this manikin, each with a typical area of 120 cm2. Each segment is a stand-alone device with integrated heating, a temperature sensor, sweat distribution and dispensing, a heat flux gauge, and a local controller to manage the closed loop operation of the segment. Here, distributed resistance wires provide uniform heating across each segment surface; the temperature of each segment is determined by an array of thermistors, typically four, on each segment; the sweat rate of each segment is controlled by a fluid control valve; and the heat flux gauge measures the heat loss in the manikin interior from each segment. The sweating surface is an all-metal construction, optimized for thermal uniformity and temperature response speed (temperature response time that approximates human skin); and the variable porosity within the surface provides lateral sweat distribution and flow regulation across the segment. A breathing system was installed in this manikin in 2004 to permit inhalation and exhalation at a rate of 5 L/min; the breathing system can also permit a continuous high level of exhalation at 15 L/min. This manikin can generate realistic and uniform sweating as well, as it is rugged, durable, and requires low levels of maintenance. Most importantly, this manikin is self-contained, with enough battery power, wireless data transfer capability, and internal sweat reservoirs for at least 2 h of use with no external connections. Presently, this manikin with different postures is mainly used in transient and nonuniform thermal environments of automobiles (vehicles, aircrafts, etc.) to evaluate the thermal and evaporative resistance of clothing. This manikin can proficiently evaluate the thermoregulatory response of a person who is wearing (1) a moisture impermeable suit used by first-responder personnel such as Hazmat, (2) a flight suit, (3) a battle dress suit, and (4) others [extra vehicular activity (EVA) suit, and personal cooling systems used with such suits].

United States sweating thermal manikin “Newton”: The thermal manikin “Newton” was produced by the Measurement Technology Northwest Company in the United States. This manikin is constructed of a thermally conductive aluminum-filled carbon-epoxy shell with embedded heating and sensor wire elements. The Newton body form (height 175 cm, surface area 1.8 m2, weight 30 kg) is available as either a 50th percentile western or Asian male, in either dry or sweating format. At present, this manikin body can be divided into 20 segments, 26 segments, or 34 segments. This manikin is attached to a breathing machine with anose/mouth manifold or filter. Newton was developed using advanced CAD digital modeling to ensure repeatability in manufacturing. This manikin is fully jointed, and can perform motorized walking motion at ankles, elbows, knees, and hips to allow virtually any possible body pose. The sweat rate of this manikin can be controlled manually.

Japanese sweating thermal manikin “KEM”: According to Fukazawa et al. [434], the manikin “KEM” was developed by the Kyoto Electronic Manufacturing of Japan. This manikin has 17 segments with movable joints and 17 sweat glands under the water vapor permeable skin which distribute the water. The sweat rate produced by this manikin is 0-1500 g/m2/h, and the sweat rate of each gland can be individually controlled using pistons. The sweating mechanism of KEM is not a new technique; it is similar to that of the Finnish sweating thermal manikin Coppelius, developed in the 1980s, in terms of sweating sources and water vapor permeable skin.

Although the previously mentioned sweating thermal manikins (Coppelius, Taro, SAM, Walter, Newton, ADAM, KEM) are very popular for evaluating the thermal and evaporative resistance of regular clothing under normal ambient environments, these manikins have rarely been used to evaluate the thermal and evaporative resistance of thermal protective clothing [444]. This may be due to the unavailability of the manikins to researchers who usually focus on the resistance provided by thermal protective clothing. However, although most of the manikins are used to assess the thermal and evaporative resistance of regular clothing, the findings from these studies can be partially applied to thermal protective clothing. Furthermore, the sweat generation rates of these thermal manikins are also very limited compared to the sweat generated by firefighters in actual fire hazard scenarios. Thus, the evaluation of evaporative resistance of thermal protective clothing using sweating thermal manikins may not be accurate for real situations. Additionally, these sweating manikins are expensive and can only be operated by a skilled and trained operator. As a consequence, these manikins are exclusively used for high-end research and development purposes [445].

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