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Fitting a Thermowell to Bimetal Thermometers or RTDs/Thermocouples

Thermowells | Bimetal Thermometers | thermocouple

Thermowells are essential to protecting your thermometer or temperature sensor in your high-pressure or high-temperature applications. They can protect your investment and keep your measurements accurate, no matter how extreme the environment.

One of the top questions we receive here at Ashcroft from those in the temperature measurement industry is how to properly fit a thermowell. What measurements are needed? What is a u-dimension and how does it factor in? Many factors are at play, and it can be easy to get the wrong size if you aren’t paying attention.

Some people make mistakes while they’re measuring, like forgetting to subtract the tip thickness from the overall length of the thermowell or omitting the right thread dimensions or fitting dimension. If the thermowell is too long, the thermometer or RTD/thermocouple won't thread in properly. If it’s too short, your measurement instruments won't fit in it. In either case, you will not have an accurate temperature measurement of your process. 

This article will outline the ways to measure your thermometer or RTD/thermocouple, what each measurement represents, and how to properly fit your instrument with a thermowell.

But first, let’s talk more about thermowells.

What Is a Thermowell?

Thermowells protect thermometer stems and bulbs from corrosive media, system pressure and high-velocity flow. They also allow the removal of the temperature measurement instrument without the need to shut down the process.

Industry-standard threaded thermowells, flanged thermowells, socket weld thermowells, Van Stone thermowells, weld-in thermowells and sanitary thermowells in a wide variety of lengths and materials are available to match nearly any temperature measurement device and fit any installation to meet industry standards.

How Do you Measure a Thermowell?

The first step to properly fitting a thermowell to your temperature instrument is to understand how a thermowell is supposed to fit and what parts to measure. Here are the basic parts of a thermowell, starting from the top of the instrument (also shown below in Figure 1):

  • Instrument Connection: Connection of the thermometer into the thermowell (typically ½ in. NPT)
  • Process Connection: Connection of the thermowell into the process (flange, threaded, etc.)
  • Root Diameter: Diameter of thermowell shank just below the process connection
  • Internal Bore Diameter: Typically, 0.260 in. or 0.385 in. ID
  • Shank: The portion of the thermowell below the process connection
  • Tip Diameter: Diameter of the shank at the tip of the thermowell
  • Tip wall thickness: between the end of the internal bore and outer tip of the thermowell; typically 0.250 in.

Figure 1: Parts of a Thermowell

parts of thermowell

Fitting a Thermowell to a Bimetal Thermometer

When determining the fit of your thermowell to a bimetal thermometer, you must consider the stem length (S dimension), insertion length (U dimension) and lag length (T dimension). View Figure 2 below for more.

Figure 2: Thermowell Measurements with Thermometers

thermowell and thermometer

The S dimension measures from the thermometer’s fitting to the probe. The lag accounts for the thread length and top length subtracting the tip from the stem length. Subtracting the tip is crucial to achieving an accurate measurement.

There are other factors to consider that will determine what size thermowell to use such as, does it have a lag? Is there a nozzle on the tank? Is it a regular thermowell or a customized one?

Be sure that your thermowell fits in your application first, and then find what bimetal fits in that thermowell. The tip of the probe should touch the bottom, and then you can fill the bottom with a heat-conductive paste.

Fitting a Thermowell to an RTD/Thermocouple

When fitting a thermowell to an RTD/thermocouple, you also need to consider the different measurements along the length of the temperature sensor, including stem diameter (d), lag extension length (N) and insertion length (L).

Figure 3 below illustrates the various measurements you’ll need to calculate.

Figure 3: Thermowell Measurements with RTDs/Thermocouples

thermowell and rtd                                               

L = OAL– Tip Wall Thickness

For an RTD or a thermocouple, the insertion length is referred to as the L length as is measured from the bottom threads of the lag extension. To properly size the thermowell for the RTD or thermocouple probe, you need to subtract the tip wall thickness from the overall length.

As with thermometers, be sure that your thermowell fits in your application first, and then determine the length of the probe. The probe is spring-loaded so the tip of the probe should touch the bottom of the thermowell.

We don’t like to pressure you, but we have more information.

Now that you understand the process of fitting a thermowell to your temperature instruments, you will be able to ensure that your temperature measurement equipment remains safe in your application.

To learn about calculating thermowell wake frequency, our website offers a helpful webinar explaining the calculation process.

And for more information on temperature instruments, check out our past articles:

Feel free to reach out to our temperature specialists here at Ashcroft to answer all of your temperature measurement questions!

Or view this brochure to see all of our RTD and Thermocouple temperature probe options:

About Dave Dlugos, Product Marketing Leader, Temperature Products

Dave Dlugos has a BSEE degree and 40 years of experience in the measurement industry performing design engineering and product management. He has earned 4 U.S. patents and joined Ashcroft in 2007, currently as the Product Marketing Leader for Temperature products. He is a senior member of the International Society of Automation (ISA), past ISA District 1, Vice President, ISA water and wastewater division board member and the President of CT Valley ISA Section.