Thermowell Calculator

As fluid flows past a thermowell, it sheds vortices behind the rod at a frequency that depends on velocity, tip diameter and the fluid's Reynolds number. If that shedding frequency gets close to the thermowell's own natural (mechanical) frequency, the thermowell resonates and can fail by fatigue. The wake frequency calculation compares the two frequencies to flag that risk before installation.

ASME PTC 19.3 TW is the industry standard for thermowell mechanical design, covering wake-frequency resonance, static and dynamic stress, and pressure rating. This calculator performs the first-pass wake frequency screen from that standard; a full PTC 19.3 TW calculation also checks Scruton number, static stress, and stepped/tapered geometry, which this simplified tool does not model.

A shedding-to-natural frequency ratio below 0.6 is generally considered acceptable. A ratio between 0.6 and 0.8 is marginal and warrants a full ASME PTC 19.3 TW calculation before finalizing the design. A ratio of 0.8 or above indicates high resonance risk and typically requires a shorter insertion length, larger tip OD, or reduced process velocity.

This calculator assumes a solid, uniform 316 stainless steel rod (E = 193 GPa, density 7950 kg/m³) with a constant diameter over the full insertion length. Real thermowells are often stepped or tapered, which raises the natural frequency compared with this simplified straight-rod model — treat results here as a conservative first-pass screen, not a final design sign-off.

If you don't enter a density, the calculator defaults to 1000 kg/m³ for liquids (water) or 10 kg/m³ for gases as a rough placeholder. Actual gas density depends heavily on pressure and temperature, and liquid hydrocarbons or oils differ substantially from water — for an accurate result, select "Custom" and enter your process fluid's actual density.