RTD length and sheath diameter

[helpyou]

HI

what is the engineering meaning of RTD length and its sheath diameter in the temperature measurement? for example what does it mean when sheath diameter is 4 mm and not 6 mm ? same with length, long and short RTDs? and why thermocouples usually come with small sheath diameter than RTDs?

Thank you

 

[danw2]

RTD sensor element is encased in mineral insulation (MgO, a white powder) inside a stainless steel sheath. Sheath diameter varies. The larger the diameter, the 'stronger' or more robust the sheath, but a longer the thermal lag time period ,for heat transfer (not as responsive). Thermocouples are just wires welded at the junction end, so they can fit into a smaller OD sheath. The larger the diameter, the more difficult it is to bend the sheath without crimping it.

Google RTD or thermocouple images and you'll see that they come in all lengths depending on where the insertion point is versus where the measurement point is, at the end of the sheath.

Either RTD or T/C are ordered with a custom part number for length, sheath OD, T/C type, connection as just lead wire, or a head with a terminal block, or lots of other choices. Google images will give you some idea of the variations.