In a previous Understanding the Basics blog, I provided an overview of temperature sensors, including RTDs and thermocouples. That article explained how they are used to measure the temperature of original equipment manufacturing (OEM) systems, light industrial processes or other complex applications. Since that piece was published, we have received more questions about RTDs and Thermocouples and the lag options you have when ordering these instruments.
This article will recap definitions of RTDs and thermocouples, explain why choosing the right lag extension for your instrument is important and help you determine which lag type is best for your application. When you are done reading, I will also provide links to additional resources to help you deepen your knowledge on these topics.
So, let's get started.
What is an RTD and Thermocouple?
Resistance temperature detectors, or RTDs, and thermocouples are both types of temperature sensors designed to detect temperature changes in industrial and OEM processes and convert them to readable output signals. These signals are transmitted to a control system that operators can view to monitor any occurring changes.
Thermocouples are known for their wide temperature ranges (over 2000 °C or 3632 °F) and fast response times.
Figure 1. Ashcroft® S80 Thermocouple.
Resistance Temperature Detectors (RTDs) are known for providing high accuracy and stability, They are often used in industrial applications.
Figure 2. Ashcroft® S81 RTD.
What is a lag extension for an RTD or Thermocouple?
The lag extension of an RTD or thermocouple provides a method of connecting the electrical enclosure (head) of the probe to the thermowell (a device designed to prevent your temperature instrumentation from getting damaged). If the temperature probe is used without a thermowell, then the lag extension connects to the process.
Many types of lag extensions are commonly used. The lag extension type and length are usually part of the application requirements. See Figure 3 to see how the lag extension length N is defined.
Figure 3. Lag extension length N.
Common lag extension types.
There are many lag extension types for RTDs and thermocouples to accommodate the different requirements of industrial applications. Choosing the right one ensures optimal performance and protection of the temperature sensors. Here are the most common options:
1. Pipe nipples. This type allows the head to be directly threaded into the thermowell. They can be of various lengths, typically ½ NPT x ½ NPT.
Figure 4. Pipe nipple example.
2. Welded pipe nipples. They can be of various lengths but typically they are very short. They are used for threading the head directly into a process without the use of a thermowell. The nipple is welded directly to the probe sheath which provides the process seal between the probe and the head.
Figure 5. Welded pipe nipples example.
3. Plug without lag extension. The plug encloses the head around the probe sheath. It however is not sealed. This is typically used for air temperature measurement. To connect the probe to a process or thermowell, a compression-style fitting is used to provide the seal.
Figure 6. Plug without lag extension example.
4. Compression fitting. These are used when it is necessary to seal the head from the environment. A second compression fitting could also be used to connect the probe to a process or thermowell.
Figure 7. Compression fitting example.
5. Nipple union. A nipple of various lengths is supplied with a union fitting. The union typically has ½ NPT female threads so the user will need to add their own nipple to thread the probe into a thermowell.
Figure 8. Nipple union example.
6. Nipple union nipple. This is the most common lag extension type with common lag lengths of 4 or 6 inches. Custom lag lengths can also be ordered. The union allows the head of the probe to be rotated into the desired location easily.
Figure 9. Nipple union nipple example.
7. Spring-loaded nipple. The spring-loaded nipple is used in place of the DIN plate to hold the probe sheath and to make sure the probe is held in contact with the thermowell tip for best heat transfer and vibration resistance. The spring-loaded nipple is often used with a union and another nipple.
Figure 10. Spring-loaded nipple example.
Ready to learn more?
I hope this information sheds some light on the topic of RTD and thermocouple lag extensions. If you still have questions, please reach out to our product experts. Until then, take a look at some other articles that may be of interest to you:
Or, download our guide to learn about RTD and Thermocouple Temperature Probes.
