Guarded hot-plate

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. 

In the guarded hot plate, the heat source is surrounded by a guard heater with an independent power supply. The guard heater is kept at the same temperature as the heat source so that all the heat from the heat source flows normally through the test pieces and the losses from the edges come only from the guard heater. 

The thermal conductivity of solid rubbers is of the order of 1-2 x l10 W/mk which is in the region of fairly low conductivity where experimental errors due to heat loss will be greatest. A heated disc procedure or unguarded hot plate is satisfactory for some purposes, particularly if thin test pieces can be used. However, for the lowest conductivity materials a guarded hot plate is really necessary to give precise results. 

A comparison of guarded hot plate, transient plane source and modified hot wire methods has been made54 using polyurethane foam, and the strengths and weaknesses of the techniques discussed,... 

ISO 8302, 1991. Thermal insulation - Determination of steady state thermal resistance and related properties - Guarded hot plate apparatus. 

BS EN 12664, 2001. Thermal performance of building materials and products. Determination of thermal resistance by means of a guarded hot plate and heat flow meter methods. Dry and moist products of medium and low thermal resistance,... 

ASTM Cl77, 2004, Standard test method for steady state heat flux measurements and thermal transmission properties by means of the guarded hot plate apparatus. 

ASTM C1044, 2003. Standard practice for using a guarded hot plate apparatus or thin heater apparatus in the single sided mode. 

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 guarded hot plate is a standard instrument for measuring the relative thermal resistance of textiles as heat flows from a heated plate in contact with the textile and dissipates into still air at a lower ambient temperature via radiation, conduction, and convection. By design, it minimizes errors due to edge heat losses and validates the total quantity of heat flowing through the specimens. Convection and surface radiation can be controlled by use of a hood (2j+). Simpler devices such as the Reeves warmth tester and a chamois-covered copper cylinder also measure thermal... 

Guarded hot plate Reeves warmth tester Chamois-covered copper cylinder Togmeter Equilibrium temp. Equilibrium temp. Equilibrium temp. Heat flow disks... 

ASTM C 177-85 Standard Test Methods for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of die Guarded-Hot-Plate Apparatus, 16 pp (DOD Adopted) (Area PACK) (AS) (Comm C-16)... 

D Buttner, J. Fricke, R. Krapf, H. Reiss, "Measurement of the thermal conductivity of evacuated load-bearing, high-temperature powder and glass board insulations with a 700x700 mm guarded hot-plate device", in Proc. 8th Eur. Conf. Thermophys. Prop, Baden-Baden, 1982. 

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. 

The thermal conductivity was measured using an absolute, steady-state technique that is similar to the ASTM-177 guarded-hot-plate specification [4]. The primary difference between the technique used here and a typical guarded-hot-plate is that one specimen... 

The thermal resistance between substrate and coating is probably negligibly small since the coating adheres well to the substrate, particularly for the few thermal cycles tested over here. For the thermal conductivity contribution of the 403 stainless steel substrate, we use data obtained previously on 410 stainless steel [8], Figure 5 shows thermal conductivity as a function of temperature for 410 stainless steel as measured in the guarded hot plate. 

B.J. Filla, Design and Fabrication of a Miniature High-Temperature Guarded-Hot-Plate Apparatus, Thermal Conductivity, 21, Plenum Press, pp.67-74 (1990). 

ASTM C177-93, "Steady-State Thermal Transmission Properties by Means of the Guarded Hot Plate," Philadelphia (1993). 

De Ponte, E., and Di Pilippo, P, Design Criteria for Guarded Hot Plate Apparatus, Heat Transmission Measurements in Thermal Insulations, ASTM STP 544, Am. Soc. Testing Mats., Philadelphia (1974). 

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. 

In order to reduce the heat lost IVom the exposed edges, the samples and the heat source are made as thin as po.ssiblc, and the apparatus is surrounded with a material of low thermal conductivity such as vermiculite. However, the side losses may still be a significant fraction of the total heat input, and this must be allowed for in the calculations. The heat loss may be evaluated by using a pair of samples of know n conductivity the conductivity of the calibration material would have been measured in an absolute instrument such as a guarded hot plate (sec below). Ideally the conductivity of the calibration samples should be similar to that of the test samples. 

An improvement on the unguarded hot plate is the guarded hot plate. This is the most accurate method available for solid materials (including foams) and is recommended by... 

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