Method and limits
Shedding frequency fs = St · V / B, St = 0.22 (subcritical flow)
Natural frequency fn = Hc · (1.875² / 2π L²) · √(E·I / m), cantilever on a rigid support
I = π(Dm⁴ − d⁴)/64, Dm = (A + B)/2; m = metal mass/length + fluid added mass ρf·π·Dm²/4
Screening bands fs/fn < 0.4 low risk · 0.4–0.8 in-line resonance zone · ≥ 0.8 transverse resonance risk
Steady drag stress Fd = ½·Cd·ρ·V²·Dm·L (Cd 1.4), σ = (Fd·L/2)·(A/2) / I_root
This is a simplified screening in the spirit of ASME PTC 19.3 TW. The full standard adds taper and tip corrections, Reynolds-dependent Strouhal number, Scruton number, cyclic stress and fatigue limits, and pressure rating; it is a paid standard and is not reproduced here. Use this to spot problems early, and have the final thermowell checked by the supplier's PTC 19.3 TW calculation.