RTD Length

I am assuming that your RTD is used in junction with a thermowell, i.e. the application is neither surface or skin temperature (of a vessel or pipe etc.) nor is it an application for turbo-machinery monitoring (i.e. for bearing temperatures etc.).

The length of your element (RTD) is governed by the length or depth of your thermowell.

For an application in a pipe with fluid flow the appropriate thermowell length is determined from the wake frequency calculations (See the ASME PTC 19.3), i.e. considerations and analysis of induced vibration due to fluid flow (Von Karman effect), material resonant frequency amongst other factors.

For an application in a vessel a thermowell length can only be selected based on knowledge of the application (filling patterns, outflows etc.) and the physical details of the vessel design (to identify the most suitable location for the thermowell), other than that good common sense should be employed.