The error sources or problems in temperature measurement increase roughly as the difference increases between the measured temperature and room temperature. Above about 500°C, significant problems arise because metallurgical and chemical reactions affect the property used to measure temperature. For example, recrystallization, grain growth, and chemical contamination change the electrical resistivity, the thermoelectric power, or the emissivity of metals used in temperature sensors or standards. Similarly, such reactions can affect the mechanical properties of the materials used to construct these instruments. Consequently, devices and apparatus used at high temperatures tend to become inaccurate and unreliable during long-term, high-temperature service. The degree to which these reactions can be controlled or avoided directly affects the reliability of temperature measurements.