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Case Study #004: Problems with J-Type Thermocouple Reading

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Home > Blog Archive > Category: Case Studies > Case Study #004: Problems with J-Type Thermocouple Reading

Case Study #004

A customer called in about having problems with their thermocouple readings. The customer was attempting to measure the surface temperature of a barrel two separate ways, using two different types of thermocouples. The first measurement was done with a Fluke handheld surface probe thermocouple. It read 646°F. The second measurement was taken using a J-type thermocouple probe with an attached bayonet adapter. This time it measured 450°F. After changing out the J-type thermocouple several times and getting about the same readings each time, the customer gave us a call.

The customer believed that, based on the way the plastic was reacting to the heat, the Fluke thermocouple was giving him the correct reading. The first step I took in this case was drawing out how and where the readings were being made. I came up with the following diagram:

diagram showing how the thermocouple reading was being made

Why Are the Thermocouple Readings Different?

Both thermocouples, the Fluke and the J-type, were reading correctly. So, why are the readings different? The reason is that the Fluke surface probe thermocouple, as shown in the above diagram, is reading the true temperature of the surface. The J-type thermocouple is NOT a surface probe thermocouple, therefore, it is not reading the actual surface temperature. The probe tip is 450°F whereas the surface itself is 646°F. This particular probe had a drill-shaped tip, designed to fit a hole that has been drilled into a barrel, not a flat surface (see image below).

diagram showing the tip of the j-type thermocouple probe touching the flat surface of the barrel

How Is This Happening?

The heat being transferred from the barrel through a point is a very small amount of heat. This small amount of heat is being removed by conduction to the back of the probe and through the wire. This pulls the temperature down at the junction, giving a true reading of 450°F.

The Solution to the J-Type Thermocouple Reading Issue

Drill a shallow hole into the barrel and insert the J thermocouple down into the hole. It will now read the correct surface temperature of the barrel.

Have a Question About a Heating Application?

Contact the Thermal Corporation engineers for assistance! We will work with you on any heating application struggle you may be encountering. Contact us today at engineering@thermalcorp.com or by phone at (800) 633-2962 x152.

Written by Jim Dixon and Shelby Reece
Edited by Kyle Otte
Date Published: 09.09.2019
Last Updated: 09.09.2019

bayonet probe thermocouple diagram

How to Determine Probe Length of a Fixed Bayonet Probe Thermocouple

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Home > Blog Archive > Category: Industrial Heating Technology > How to Determine Probe Length of a Fixed Bayonet Probe Thermocouple

Thermal Corporation bayonet probe thermocouple image

Using the Fixed Bayonet Probe Thermocouple Table to Determine the Probe Length

We recently received a question about how to determine what the probe length is of a fixed bayonet probe thermocouple. To do this, take a look at the table located on our Thermocouple Accessories page of the Thermal Corporation website. This chart can also be found in our product catalog in the Thermocouples & RTDs section!

For Example…

You have a 1/2″ hole depth and would like to use a 7/8″ long bayonet adapter. What would the probe length be?

chart used to determine probe length of a Thermal Corporation fixed bayonet probe thermocouple

Thermal Corporation has an easy-to-follow chart that you can use to find out what the probe length should be. Take a look at the chart displayed above. According to the above chart you would, first, look at the “C” Hole Depth column. Find the row that has the corresponding hole depth. This would be the 1/4″-1/2″ row. Then, follow that row over to the right until you get to the 7/8″ Long Adapter column. In that box, you can see that the probe length for those measurements is 1-3/4″.

We hope this helps those of you who may be a little lost on figuring out what the probe length should be for your fixed bayonet probe thermocouple! If you are still a little confused, feel free to contact Thermal Corporation customer service with your concerns and our representatives will be happy to help you out! Contact information for Customer Service is below!

Do you have a question that could possibly be featured on our blog?

Contact Thermal Corporation customer service for basic questions by email at sales@thermalcorp.com, by phone at (800) 633-2962, or by using the chat feature located at the bottom of each page of our website! For any technical questions, contact our engineering department at engineering@thermalcorp.com.

Written by Shelby Reece
Date Published: 11.20.2018
Last Updated: 09.06.2019