The length of an Ecklund Needle Thermocouple depends on the size of the container you will be testing.
For containers where the thermocouple will be installed into the side wall, such as cans, plastic cups, or recart packages, it should be sized to reach the middle of the container (or half of the diameter of the container).
For containers where the thermocouple will be installed into the top lid/cap or from the bottom, such as glass jars, the "cold spot" of the container must be known. The thermocouple is then sized so the tip is at the cold spot of the container. If you are not sure of the cold spot in your product, you may need multiple lengths to conduct a "cold spot study."
Note: Thermocouple lengths are measured from the tip of the needle to the end of the brass threaded fitting.
To measure your container, consider the outermost point of the container side wall, bottom, or cap, and measure to the desired location where the sensing tip will be located.
For example, if the can is a 308 diameter can, which translates to 3 - 8/16", the thermocouple length to reach the middle of the can should be one half of that dimension, or 1 - 12/16".
Or if a glass jar is 5" high, and the cold spot is 1" above the bottom of the jar, the thermocouple length would be 4".