Athermocouple junction, which may be approximated as a sphere, is to be used for temperature measurement in a gas stream. the convection coefficient between the junction surface and the gas is known to be h = 400 w/m2 k, and the junction thermophysical properties are k = 20 w/mk, cp = 400 j/kgk, and  = 8500 kg/m3 . determine the junction diameter needed for the thermocouple to have a time constant of 1 s. if the junction is at 25 oc and is placed in a gas stream that is at 200 oc, how long will it take for the junction to reach 199 oc?