The temperature of a hot gas stream is measured by a thermocouple with a spherical junction that must register 99 % of the initial temperature difference within 5 s. k = 35 W/m·K, ρ = 8500 kg/m^{3}, c_{p} = 320 J/kg·K, h = 250 W/m^{2}·K. Determine the junction diameter. (answer in mm)