Frequently Asked Questions:
Thermowells

See How to Select Thermowells by viewing our Thermowell brochure.

Considerations when selecting a thermowell for your instrument:

• Material – must withstand process corrosion and velocity (Ex: 304SS or 316SS is very common)
• Process connection needed for installation (Ex: NPT size, or flange size/rating)
• Insertion length – the length of thermowell measured from below process connection (threads or flange) to tip of thermowell.
• Bore Size to suit your instrument – Ex: .250” probe diameter will have a .260” Bore thermowell.
• For other options, please contact Tel-Tru directly on the contact us page or at customerservice@teltru.com.

A thermowell is a protective barrier that shields the thermometer or RTD from the pressure, velocity, and corrosive nature of the process fluid. It is required in any pressurized system or when corrosive fluids are present, as it allows for instrument removal without stopping the flow or compromising the process containment.

Material selection is vital and depends on the specific process media and temperature. 316 Stainless Steel (316SS) is typically preferred over 304SS due to its superior resistance to pitting and corrosion, making it the standard choice for most industrial applications, including those requiring NACE documentation.

NACE (National Association of Corrosion Engineers) standards specify material requirements to prevent stress corrosion cracking in sour gas environments (H₂S). Tel-Tru produces thermowells that can be treated and documented to meet these NACE standards for critical Oil & Gas or chemical processing applications.

Proper sizing dictates that the sensor tip of your instrument must be positioned within the final third of the thermowell’s insertion depth for optimal heat transfer and fastest response. Tel-Tru thermowells are designed and manufactured to ensure perfect alignment with the thermometer stem, guaranteeing high performance.

Thermowells are machined from solid bar stock, providing exceptional strength to withstand extreme pressure and fluid velocity. For high-velocity steam or gas applications, specialized tapered thermowells or wake frequency calculations can be used to prevent fatigue failure and maximize safety.