Ensuring accurate and reliable temperature readings in harsh industrial applications is vital. The challenge is protecting your temperature sensor from the damaging effects of corrosive media, high pressure and velocity in industrial applications. That's where thermowells can help. However, with so many types of thermowells and styles of thermowell shanks, it may not be immediately clear how to select the correct one for your application.
As a leading manufacturer of temperature instruments, Ashcroft offers multiple styles of thermowells, including threaded, flanged, socket-weld, weld-in and sanitary. Each style can be manufactured with one of three shank types: straight, tapered or stepped. Depending on the shank, there are implications for strength, sensor response time and vibration resistance. The type you choose will ultimately depend on the demands of the application.
Read this article to learn the differences between thermowell shank types, their main benefits and applications, and to understand how factors like wake frequency calculations can help you make the most informed and efficient decisions.
Three Types of Thermowell Shanks
Thermowells are critical components in temperature measurement systems, protecting sensors while ensuring accurate readings in harsh or dynamic process environments. But their ability to do their job depends on more than their material or process connection. It also depends on the geometry of the shank.
Thermowell shanks come in three primary shapes—straight, tapered, and stepped. Each type offers distinct mechanical and measurement performance characteristics. Selecting the right shank type is critical to ensuring sensor accuracy, mechanical durability, and compliance with safety standards in your application.
Figure 1. Three thermowell shank types
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.
1. Straight Shank Thermowells
Straight shank thermowells are the most basic of the three types, featuring a uniform outer diameter from the process connection to the tip. Their simple design and structural consistency make them easy to produce, but not always the best choice for corrosive and static flow applications.
Benefits of the straight shank:
- Consistent wall thickness helps maintain structural integrity in high-corrosion or erosive environments, especially when used with corrosion-resistant alloys.
- Stable mass distribution offers predictable heat transfer rates in non-turbulent environments.
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:
- Straight geometry results in a lower natural frequency, making it more vulnerable to resonance and vibration-induced fatigue in fast-flowing processes.
- Not ideal for compressible fluids at high velocities or pulsating flow systems without mechanical validation.
2. Tapered Shank Thermowells
Tapered thermowells feature a gradual reduction in diameter from the process connection toward the tip, improving flow dynamics and resistance to mechanical stress. This makes them the most used and recommended shank design.
Benefits of the tapered shank:
- 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.
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:
- Slightly more expensive due to machining complexity.
- Still requires detailed wake frequency evaluation for critical installations.
3. Stepped Shank Thermowells
Stepped thermowells transition abruptly from a larger diameter near the mounting point to a smaller diameter shaft closer to the tip. This design is engineered to maximize sensor responsiveness, making it ideal for applications where fast reaction to temperature changes is crucial.
Benefits:
- Fastest thermal response among the three shank types, due to the lower tip mass.
- Improved sensitivity and quicker readout times, making them valuable for precision control loops.
- Reduced insertion force, making installation easier in tight spaces or smaller bores.
Applications:
- Frequently used in laboratories, pilot plants, and controlled systems where real-time temperature measurement is critical.
- Ideal for liquid systems with stable flow and minimal vibration.
- Suitable for batch processes or systems requiring short cycling times and high measurement sensitivity.
Limitations:
- 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
Understanding Wake Frequency Calculations
In a previous article, we described how a wake forms behind the thermowell as fluid flows past it, creating alternating swirling patterns, known as vortex shedding or a Von Kármán trail, that can cause the thermowell to vibrate or oscillate.
Figure 2. 
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.
Why wake frequency calculations are important
The combination of flow rate, fluid density, and thermowell geometry determines how likely a thermowell will vibrate in response to flow-induced forces. Inadequate wake frequency evaluation in the field has led to thermowell failures. It is important to note that:
- Even properly installed thermowells must undergo wake frequency analysis for high-flow systems.
- Calculations must account for flow velocity, insertion length, support location, and mounting method.
- The ASME PTC 19.3 TW code is the gold standard for evaluating the natural frequency of the thermowell, stress under vortex shedding, pressure limit and fatigue life relative to the intended service duration.
Emerging trends for thermowell design
Recent advances in thermowell engineering have introduced innovative shank designs featuring helical or vortex shapes, commonly referred to as vortex thermowells. This leading-edge solution addresses the limitations of traditional designs in demanding, high-velocity process environments where conventional thermowells are unable to withstand intense flow-induced stresses.
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, these articles may interest you:
- What is a Wake Frequency Application
- When to Use a Vortex Thermowell in High Velocity Applications
- How to Calculate Thermowell Stem Length for Temperature Instruments
- How Much Do Thermowells Cost? Five Factors to Consider.
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.
