In low density gases with a Scruton Number (Nsc) of >2.5 Reynolds Number <105, the in-line resonance is suppressed and therefore the acceptable ratio will be:
fs<0.8fnc
If a thermowell passes cyclic stress conditions for operation at the in-line resonance condition, the acceptable ratio will be:
fs<0.8fnc
If a thermowell fails the cyclic stress condition for operation at the in-line resonance condition, the thermowell natural frequency will be high enough to limit excitation of the in-line resonance. Therefore, the acceptable ratio will be:
fs<0.8fnc