When to Use Straight, Tapered or Stepped Thermowells

You should use straight, tapered, or stepped thermowells based on how much flow velocity, vibration and temperature responsiveness your application can tolerate.

Thermowells play a critical role in protecting temperature sensors from corrosive media, high pressure and process velocity, while still allowing accurate and repeatable temperature measurement. Selecting the wrong shank geometry can lead to excessive vibration, slow response times, or premature mechanical failure.

As a long-standing manufacturer of temperature instrumentation, Ashcroft works closely with engineers to help ensure thermowells are selected and validated for real-world operating conditions, including compliance with recognized mechanical design standards.

In this article, you will learn how straight, tapered and stepped thermowell shanks differ, how each design affects strength, vibration resistance and response time, and how wake frequency calculations factor into proper thermowell selection.

What are the different types of thermowell shank designs?

Thermowell shanks are available in three primary shapes: straight, tapered and stepped. Each type offers distinct mechanical and measurement performance characteristics.

While material selection and process connection are important, the geometry of the thermowell shank plays a major role in how the thermowell responds to flow-induced forces, temperature changes, and vibration. The shank shape directly influences stiffness, natural frequency, and thermal mass, all of which impact long-term reliability.

Selecting the correct shank design helps balance sensor protection, mechanical durability, and measurement performance for the intended application.

Figure 1. Three types of thermowell shanks

The shape of the shank directly affects how well the thermowell withstands flow-induced stress, how quickly it responds to temperature changes and how well it integrates with your temperature instrument.

Here is a closer look at the key features, benefits and use cases for each shank design.

When Should a Straight Shank Thermowell Be Used?

Straight shank thermowells should be used in static or low-velocity applications where vibration and dynamic loading are minimal. They have a constant outer diameter from the process connection to the tip. Their uniform geometry provides predictable mechanical behavior and consistent wall thickness, which can be beneficial in certain environments.

Key benefits of straight shank thermowells

• Consistent wall thickness that supports structural integrity in corrosive or erosive services
• Stable mass distribution that provides predictable heat transfer in non-turbulent conditions

Typical applications

• Well-suited for static or low-velocity conditions, such as tank measurements or systems with minimal flow turbulence.
• Often selected for low-pressure gas, light liquid service, or applications where dynamic loading is negligible.

Limitations to consider

• Lower natural frequency compared to tapered designs
• Greater susceptibility to resonance and vibration fatigue in high-velocity or pulsating flow
• Not well suited for compressible fluids or fast-flowing processes without mechanical validation

When is a tapered shank thermowell the best choice?

Tapered shank thermowells are the preferred choice for most flow applications where vibration resistance and mechanical reliability are required. Their design gradually reduces in diameter from the mounting point toward the tip, increasing stiffness while improving flow dynamics. This geometry makes them the most commonly used and broadly recommended shank design.

Key benefits of tapered shank thermowells 

• Superior vibration resistance compared to straight or stepped shanks. This helps prevent failure due to cyclic stresses.
• Optimized wake frequency performance due to reduced cross-section and increased stiffness.
• Improved sensor response time over straight shanks by minimizing thermal mass near the tip.
• Meets ASME PTC 19.3 TW-2016 recommendations in more high-flow applications than straight shanks.

Common applications

• Preferred in high-velocity liquid, gas, or steam pipelines where mechanical vibration is a concern.
• Widely used in refineries, chemical plants, and power generation systems, especially when high process reliability is required.
• A great choice for engineers performing wake frequency analysis who need to meet vibration criteria without changing process parameters.

Limitations to consider

• Slightly higher cost due to machining complexity.
• Still requires wake frequency evaluation for critical or high-energy services.

When should a stepped shank thermowell be used?

Stepped shank thermowells should be used when fast temperature response is critical and flow-induced vibration is low. They feature an abrupt reduction in diameter near the tip, minimizing thermal mass and allowing the sensor to respond more quickly to temperature changes. This design prioritizes responsiveness over mechanical robustness.

Key benefits of stepped shank thermowells

• Fastest thermal response of the three shank types
• Improved sensitivity for precision control loops
• Reduced insertion force, which can simplify installation in tight spaces or smaller bores

Typical applications

• Frequently used in laboratories, pilot plants and controlled systems where real-time temperature measurement is critical.
• Ideal for controlled liquid systems with stable flow and minimal vibration.
• Suitable for batch processes or systems requiring short cycling times and high measurement sensitivity.

Limitations to consider

• The abrupt diameter change introduces stress concentration points, which can reduce mechanical durability under vibration.
• Not recommended in high-velocity or pulsating flow systems unless supported by vibration analysis or dampening mechanisms.

Figure 2. Thermowell shank comparison

What is a thermowell wake?

A wake is the region of recirculating flow that forms immediately downstream of a thermowell or other solid body as the process fluid moves past it. As the media flows across the thermowell shank, alternating vortices are shed, creating turbulence that can cause the thermowell to resonate or oscillate. This phenomenon is known as vortex shedding and the resulting pattern is referred to as a Von Kármán vortex street.

The shedding rate is calculated and compared to the natural frequency of the thermowell. If the frequency of the wake is too close to the natural frequency of the thermowell then the vortex shedding can destroy the thermowell, resulting in possible damage to the temperature instrument and your process as the broken shank flows through your system.

Figure 3. Wake turbulence illustration

Why are wake frequency calculations important for thermowell selection?

Wake frequency calculations are critical because they determine whether a thermowell can safely withstand flow-induced forces over its service life.

Even properly installed thermowells can fail if wake frequency analysis is not performed in high-flow or high-energy systems. These calculations evaluate the relationship between vortex shedding frequency and the thermowell’s natural frequency.

Important considerations include:

• Flow velocity and fluid density
• Insertion length and support conditions
• Mounting method and process connection

The ASME PTC 19.3 TW standard is widely recognized as the industry benchmark for evaluating thermowell strength, vibration limits, pressure capability, and fatigue life relative to intended service duration.

How are thermowell designs evolving for high-velocity applications?

Recent advances in thermowell engineering have introduced innovative shank designs with helical or vortex geometries, commonly referred to as vortex thermowells. These advanced designs modify the wake pattern around the shank to reduce vortex shedding forces and better withstand intense flow-induced stresses. As a result, they are increasingly applied in extreme velocity or high-density services where conventional straight, tapered, or stepped thermowells cannot safely meet vibration and fatigue requirements.Recent advances in thermowell engineering have introduced innovative shank designs with helical or vortex geometries, commonly referred to as vortex thermowells. These advanced designs modify the wake pattern around the shank to reduce vortex shedding forces and better withstand intense flow-induced stresses. As a result, they are increasingly applied in extreme velocity or high-density services where conventional straight, tapered, or stepped thermowells cannot safely meet vibration and fatigue requirements.

Ready to learn more?

You now have a more in-depth understanding of the different types of thermowell shank types and how they each offer a distinct combination of mechanical performance, thermal responsiveness and suitability for different process conditions.  For additional insights on thermowells and other temperature instruments, see the added resources below. 

Contact us anytime to talk to one of our product experts if you have additional questions. In the meantime, feel free to download our product guide to learn about other temperature solutions.