What must be done to ensure that panels used for testing are effective?

The requirement that panels used for testing must have dimensions of at least 18 inches by 18 inches by 4 inches is vital for ensuring the effectiveness of the panels in refractory testing. These dimensions provide a sufficient volume and surface area to accurately observe and measure the properties of the refractory material being tested. Large enough dimensions facilitate the uniform distribution of heat and allow for better control of variables during the testing process.

Using panels that meet this size requirement ensures that any thermal or mechanical properties being assessed are representative of how the material would perform in real-world applications. Smaller dimensions may lead to inaccurate results due to edge effects or inadequate sampling of the material's properties. Therefore, adhering to the specified dimensions is essential for reliable and valid test outcomes in the context of refractory materials.