A thermocouple 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 h = 400 W/m² K, and the junction thermophysical
properties are k = 20 W/mK, c = 400 J/kg K, and p = 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°C and is placed in a gas stream that is at 200°C, how long will
it take for the junction to reach 199°C?
Find:
1. Junction diameter needed for a time constant of 1 s.
2. Time required to reach 199°C in gas stream at 200°C.
Schematic:
T= 200°C
h = 400 W/m².K
Gas stream
-Leads
HD-
Thermocouple
junction
1₁= 25°C
k = 20 W/m-K
c = 400 J/kg-K
p= 8500 kg/m³