Guarded-hot-plate method

In order to guarantee that the heat flow from the metered area is one dimensional, that is, it flows strictly through the specimens and not out of the side walls of the main heater, guard heaters are used (primary guard). Air gaps between the metered area and the guard heaters form a significant thermal 

The bottom and top auxiliary heaters can be used to decrease the temperature gradient across the specimens. Since temperature measurements are taken under steady state heat 

The technique has also been demonstrated with a single specimen. In that case, one of the cold plates is removed and the auxiliary heater on that side is heated to match the temperature of the metered area. Since no temperature gradient exits in that direction, heat generated from the metered area flows uniaxially through the single specimen. 

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A guarded hot-plate method, ASTM D1518, is used to measure the rate of heat transfer over time from a warm metal plate. The fabric is placed on the constant temperature plate and covered by a second metal plate. After the temperature of the second plate has been allowed to equiUbrate, the thermal transmittance is calculated based on the temperature difference between the two plates and the energy required to maintain the temperature of the bottom plate. The units for thermal transmittance are W/m -K. Thermal resistance is the reciprocal of thermal conductivity (or transmittance). Thermal resistance is often reported as a do value, defined as the insulation required to keep a resting person comfortable at 21°C with air movement of 0.1 m/s. Thermal resistance in m -K/W can be converted to do by multiplying by 0.1548 (121). 

The guarded hot-plate method can be modified to perform dry and wet heat transfer testing (sweating skin model). Some plates contain simulated sweat glands and use a pumping mechanism to deUver water to the plate surface. Thermal comfort properties that can be deterrnined from this test are do, permeabihty index (/ ), and comfort limits. PermeabiUty index indicates moisture—heat permeabiUty through the fabric on a scale of 0 (completely impermeable) to 1 (completely permeable). This parameter indicates the effect of skin moisture on heat loss. Comfort limits are the predicted metaboHc activity levels that may be sustained while maintaining body thermal comfort in the test environment. 

A conventional steady-state guarded hot-plate method for thermal conductivity measurement was used by Cook and Leaist (1983). Their apparatus was used to perform an exploratory measurement of methane hydrate to within 12%. A sample of methane hydrate was made externally, pressed, and placed in the hot-plate cell at the Sample Disc. The lower sample heater had thermocouples contacting the top and the bottom of the sample to determine the temperature gradient. 

Figure 9.7 Guarded hot-plate method for the measurement of thermal conductivity [15]. Typical specimen dimensions are disks of 25 cm diameter and 5 cm thick [14],...

The thermal conductivity of wood-chip concrete was measured with the guarded hot plate method at JTCCM. The relationship between the packing ratio of wood-chip and the thermal conductivity is shown in Fig. 8. After the figure, it is known that the thermal conductivity decreases linearly when the packing ratio is increased, and the moisture content has a great influence on thermal conductivity of wood-chip concrete. 

Method 7A is based on the absolute guarded hot plate method described in BS 874. 1986 for determining the steady state thermal transmission properties of insulating materials in the temperature range -20 C to 100 C. 

The constant, k, is a material constant called the coefficient of thermal conductivity. It is the time rate of heat flow under steady-state conditions through a unit area and unit thickness per unit temperature gradient. Several methods for measuring thermal conductance are available, of which the guarded-hot-plate and heat-flow-meter methods are the most widely used. The most accurate method for plastics having thermal conductivities of 3.4 x 10 cal/(second)(cm)(°C) [1.4 W/m K] or less (most filled and unfilled polymers) is the guarded-hot-plate method as described in ASTM Cl77. A comparable method is described in ISO 8302. ... 

Thermal Conductivity Test. Thermal conductivity measurements were carried out on mortars containing PU waste particles by using the guarded hot plate method, according to UNI 7745. Results obtained, reported in Table 4, showed that mortars containing 10% and 30% PU waste particles are characterized by thermal conductivity values 12% and 20% lower that reference mortar respectively. On the other hand, when limestone powder was added to the mixture, due to a lower mortar porosity, thermal conductivity was similar to that of the reference mixture. 

The longitudinal heat flow (guarded hot plate) method is regarded as the most accurate and most widely used apparatus for the measurement of thermal conductivity of poor conductors of heat. This method is most suitable for dry homogeneous specimens in slab forms. The details of the technique are given by the American Society for Testing and Materials (ASTM) Standard C-177 [82]. 

THERMAL CONDUCTIVITY OF INSULATING MATERIALS BY THE GUARDED HOT PLATE METHOD. 

Researchers have different models and means of measuring the thermal conductivity, some of which are sandwiched panel design between two conductive plates (Hunt et al. 1991), guarded hot plate method (Ge et al. 2009), vibrational thermal relaxation technique developed by Bemasconi et al. (1992) and modified Tsao s model predictive theoretical technique (Cheng and Vachon 1969). 

In the guarded hot-plate method, a sheet of the material is sandwiched between a metal heater plate and a heat sink plate. The temperature differential across the thickness of the specimen is measured when the system is at equilibrium. This technique is well suited for measurements of solids, particularly in cases where orientation exists within the test specimen. It is unsuitable for measurements of molten polymers. Thermal contact resistance is a problem that must be overcome with such measurements. 

Guarded heat flow meters are variants on the guarded hot-plate technique, which utilize a heat flow meter to measure the heat flux. These techniques are comparative in nature because a reference material is needed to calibrate the heat flux meter. They utilize smaller samples and are thus able to equilibrate faster, improving the versatility of the guarded hot-plate method. 

Guarded Hot-plate Method. The guarded hot plate (Pig. 4) is symmetric with respect to the axial direction, and, ideally, equal quantities of heat flow through... 

An important refinement of the guarded hot-plate method is the inclusion of a secondary guard heater, in the form of a concentric shell, along the lateral surfaces of the sandwich from the upper heat sink to the lower. The gap between the sandwich and secondary guard heater is important and is optimized to achieve a high thermal resistance or small window between the sandwich and the outside. This design is recommended by ASTM for measurements where the working temperatures are 25°C or more above or below room temperature (21) and is especially pertinent for measurements of polymers above 50°C. 

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