Which indicator shows the temperature of the base metal during brazing?

The correct answer highlights the importance of physical changes as indicators of metal temperature during the brazing process. When base metal reaches certain temperatures, specific physical changes occur that can signal the temperature range, such as the visibility of certain colors or the melting characteristics of the filler metal.

For instance, as the base metal is heated, it may change in color, which importantly indicates the temperature but is part of a broader set of physical transformations. The melting of solder, flow characteristics of the filler metal, and changes in surface tension also illustrate physical changes that can be used to assess the effective temperature at which brazing is occurring.

Choosing this indicator underscores the importance of monitoring the actual scientific and observable changes that take place during the heating process, which are critical for effective and quality brazing. Other indicators, such as sound or duration, do not provide a direct measurement of temperature fluctuations in the base metal. Instead, they might indicate other aspects of the process but not the temperature itself, making physical changes the most relevant and scientifically supported indicator for gauging base metal temperature during brazing.