In an experiment, the temperature of a hot gas stream is to be measured by a thermocouple with a spherical junction. Due to the nature of this experiment, the response time of the thermocouple to register 99 percent of the initial temperature difference must be within 5 s. The properties of the thermocouple junction are k = 35 W/m·K, ρ = 8500 kg/m³, and c_p = 320 J/kg·K. If the heat transfer coefficient between the thermocouple junction and the gas is 250 W/m²·K, determine the diameter of the junction.