Enter the temperature associated with the external design pressure. The software automatically updates materials properties for external pressure calculations when you change the design temperature. The design external pressure at this temperature is a completely different design case than the internal pressure case. Therefore, this temperature may be different than the temperature for internal pressure.
Many external pressure charts have both lower and upper limits on temperature. If your design temperature is below the lower limit, use the lower limit. If your temperature is above the upper limit the component may not be designed for vacuum conditions.