Regarding spring loaded RTDs for thermowell applications, what is the difference between immersion length, bore depth and probe length?
First of all, let’s discuss the components that are used in an RTD- thermowell assembly. Starting on the process side there is the thermowell, next is the extension which connects the thermowell to the connection head. These 3 parts connected together with an RTD is considered an assembly. The thermowell alone makes up 2 of the 3 dimensions in question. The immersion length, also referred to as the “U” dimension, is the length that is actually immersed in the process. The bore depth is exactly that, the depth of the bore hole inside the thermowell. This dimension is a combination of the immersion length (minus the 1/4 inch sealed tip), and the connection end with the addition of any lag. (See illustration) The actual RTD runs the entire length of the assembly. To calculate the probe length, you must add the bore depth, the length of your extension and add the portion of the probe consumed in the connection head. Typically this dimension is 1.5 inches with a spring loaded probe.
