How to Select the Correct Thermowell for Your Application

Stainless steel thermowell

If you want to measure high temperatures in industrial processes, you need a thermocouple. In order for a thermocouple to function properly, it needs to be protected against degrading, caustic materials and fluid flows. This is the purpose of the thermowell.

A thermowell is a structure surrounding the thermocouple that protects it against the aforementioned elements. Selecting the right thermowell is crucial because it determines how your thermocouple will function.

In this article, we will discuss five factors to consider when choosing a thermowell:

  • Type of thermowell
  • Material
  • Connection type
  • Bore size
  • Insertion length

1. Type of Thermowell

Thermowells have different stem designs, so you must choose the design that best suits your application. There are three types of thermowell stems to choose from:

  • Straight thermowell: A straight thermowell has a uniform stem, offering the best protection against erosion and corrosion.
  • Stepped thermowell: A stepped thermowell is great for faster temperature responses and smoother velocities.
  • Tapered thermowell: A tapered thermowell offers the best strength and faster temperature response. 

2. Material

The material is an important factor in selecting a thermowell. For example, when dealing with conditions that involve extremely high temperatures, you need a thermowell made of a material with a high melting point to protect the temperature sensor. Thermowell materials designed for high-temperature applications include Incoloy, Hastelloy, and Haynes alloys.

If working in a highly corrosive environment, your thermowell needs to be made of a corrosion-resistant material. Stainless steels are highly resistant to corrosion from atmospheres heavy in chloride, sulphuric acids, nitric acids, and most alkalis. Incoloy handles sulphuric acid better than almost any metallic material and shows excellent resistance to phosphoric acid, oxidation, carburization, and hot corrosion.

Thermowells are also made of other materials, including titanium, Monel, molybdenum/chromium (moly/chrome) steels, carbon steels, and cobalt steels.

3. Connection Type

A thermowell must be connected to the thermocouple, thermistor, or resistance temperature detector (RTD), to protect it. There are six types of connections you can choose from: Flange, O-Ring, Socket Weld, Weld-In, Threaded, and Sanitary. The type of connection you choose must be suited for the purpose of your thermocouple and must ensure that the thermocouple wire is safe to provide an accurate temperature reading. 

4. Bore Size

Thermowells come in different bore sizes. If you are planning to use different measurement devices in the same thermowell, then you should choose one with a standard bore size, which allows for greater flexibility. A standard bore-sized thermowell can accommodate a test thermometer, bimetal thermometer, RTD, or thermocouple. 

5. Insertion Length

For your thermocouple to provide an accurate reading, the thermowell should allow it to go as far as possible into the medium in which the temperature is to be measured. The insertion length refers to the distance between where the thermowell is connected to the thermowell’s tip. A thermowell with a long insertion length will provide greater accuracy because it will allow the temperature sensor to reach the location where it is required.

Contact RAM Sensors for Expert Advice

A temperature sensing device is usually exposed to extreme conditions of pressure, heat, and material flow. This harsh environment will lead to its deterioration. As a result, the device may fail to give accurate readings over time. That’s why these devices need to be protected by thermowells to maintain the integrity of the sensor. Need expert advice in choosing a thermowell for your application? Contact RAM Sensors today!