Cross Array Hot Wire Method

This method, which uses a cross-wire welded at the center, is given in ISO 8894-1 1987. One of the two wires is cormected to a power source used as a heating element. The other wire is a thermocouple wire. This cross-wire is sandwiched between two blocks of the refractory material. Power is fed into the heating element for a short time. The temperature rise in the blocks is measured. This temperature rise is related to the thermal conductivity of the material. Thermal conductivity values up to 2 W/mK can be measured by this method. 

In this method, which is illustrated in the standard ISO 8894-2 1990, the heating element and the thermocouple wires are arranged in a parallel manner. This type of arrangement is useful in measuring the thermal conductivity up to 25 W/mK. 

Eschner et al. [10] suggested that the differences in the transient and steady-state methods were about 10%-15%. The transient methods give higher values. Later investigations [11] suggested that if sufficient care was taken with methods and specimen preparation, then only a scatter of maximum 5% may occur. Possible reasons for the differing results are as follows  

As temperature increases, thermal conductivity decreases. This is because the vibrations of phonons become greater as the temperature increases. This makes the heat transfer more random rather directional. Hence, heat transfer in any direction decreases. This is true in dense refractories, where the 

Variation of thermal conductivity with temperature for two dense refractories (CERAM data). 

---

Cross array hot wire method (BS EN 993-14 1998 and ISO 8894-1 1987) — In this method, two wires are welded together and sandwiched between the blocks of the test material. One is a heater, and the other is a thermocouple, and they lie in as cross arrangement. Hence the name. Power is fed to the hot wire for a short time, and the temperature rise measured. The temperature rise is related to the thermal conductivity of the material. This arrangement is more suited to measurement of materials with conductivities up to 2 W/mK. 

---