EN 993-15-2005 pdf download.Methods of test for dense shaped refractory products -Determination of thermal conductivity by the hot-wire (parallel) method.
A thermocouple is fitted at a specified distance from the hot wire; the thermocouple leads running parallel to the wire (see Figure 1). The increase in temperature as a function of time, measured from the moment the heating current Is switched on, is a measure of the thermal conductivity of the material from which the test pieces are made.
4 Apparatus
4.1 Furnace, electrically heated, capable of taking one or more test assembhes (see 5.1) up to a maximum temperature of 1 250 °C. The temperature at any two points in the region occupied by the test pieces shall not differ by more than 10 K. The temperature measured on the outside of the test assembly during a test (of duration about 15 mm) shall not vary by more than ± 0,5 K, and shall be known with an accuracy of ± 10 K.
4.2 Hot wire, preferably of platinum or platinum-rhodium, with a minimum length equivalent to that of the
test piece. . The voltage taps should be located in the test piece with a length between the taps of about
200 mm known to the nearest ± 0,5 mm.
Both ends of the hot wire are attached to the power source and the voltage taps to the digital multimeter (4.5). The wires to the power source may also be a continuation of the hot wire itself and shall have the same diameter as the wire within the assembly. The wires to the digital multimeter shall be of a diameter not greater than that of the hot wire when within the assembly. Leads outside the assembly shall consist of two or more tightly twisted wires of 0,5 mm diameter. The current lead connections external to the furnace shall be made with heavy gauge cable.
4.3 Power supply, to the hot wire (4.2). which shall be stabilized a.c. or d.c., but preferably a.c., and shall not vary in power by more than 2 % during the period of measurement.
A power supply to the hot wire of at least 250 W/m is required. This is equivalent to 50 W between the voltage taps for a distance of 200 mm.
4.4 Differential platinumlplatinum-rhodium thermocouple, (Type R: platinum 13 % rhodium/platinum thermocouple or Type S: platinum 10% rhodium/platinum thermocouple, see Table 1) formed from a measurement thermocouple and a reference thermocouple connected in opposition (see Figure 1). The leads of the measurement thermocouple shall run parallel to the hot wire at a distance of 15 mm ± 1 mm (see Figure 2). The output of the reference thermocouple shall be kept stable by placing it between the top outer face of the upper test piece and a cover of the same material as the test piece (see Figure 1). The diameter of the measurement thermocouple wires shall be the same as that of the hot wire and the wires of both thermocouples shall be long enough to extend outside the furnace where connections to the measuring apparatus shall be made by wire of a different type. The external connections of the thermocouple shall be isothermal.
An insulating layer may be inserted between the cover and the upper test piece. NOTE Base metal thermocouples can be used at temperatures below 1 000 °C.
5.1 Dimensions
Each test assembly shall consist of two identical test pieces, not less than 200 mm x 100 mm x 50 mm in size.
NOTE I It is recommended that the size of each lest piece be 230 mm x 114 mm x 64 mm or 230 mm x 114 mm x 76 mm Standard-size bricks can then be used as the pieces forming the test assembly, subject to the requirements of 5.2.
NOTE 2 The limits of this method are imposed by the dimensions of the test pieces. With larger test pieces, higher values of thermal conductivity can be measured. The distance between the hot wire and thennocouple should be extended to the same ratio as the test pieces. For example, with a test piece 230 mm x 180 mm x 95 mm, a thermal conducbvity of about 40 WIm.K can be measured.
5.2 Surface flatness
The surfaces of the two test pieces forming the test assembly which are in contact with each other shall, If necessary, be ground so that the deviation from flatness between two points not less than 100 mm apart Is not more than 0,2 mm.
5.3 Grooves in dense materials
In dense materials, grooves to accommodate the hot wire and the measurement thermocouple shall be machined in the lower face of the test assembly (see Figure 4). The width and depth of the grooves shall permit the arrangement shown in Figure 4 to be achieved.
The faces of the test piece shall be parallel to the nearest ± 1 mm.EN 993-15-2005 pdf download.