You have a number of options. Extension leads may be added to element leads via soldering, brazing, or welding. Typical extension lead materials are PTFE-insulated silver or nickel-plated copper, and fiberglass-insulated nickel-plated or nickel-clad copper. I will give you a brief overview for each:
How can I attach extension lead wires to thin-film platinum temperature sensors?
Standard soldering alloys may be used. All flux residue must be removed to prevent corrosion at high temperature. The “L” series of elements have AgPd lead wires, and provide the best solderability.
Care must be taken to avoid exposing the element to temperatures exceeding the maximum temperature rating of the part. The element should be heat-sinked during the brazing operation to remove as much heat from the element body as possible. All flux residue must be removed to avoid corrosion.
Ultrasonic, resistance, or laser welding techniques may be used. Proper setup is required to avoid flattening or reducing the diameter of the element lead, which may cause lead failure over time. Laser welding provides reproducible, visually verifiable weld joints, and is highly recommended.
During the welding process, care must be taken to avoid exposing the element to temperatures exceeding the maximum temperature rating of the part. A 3 or 4 wire lead extension configuration is often used with Pt100 elements, and less frequently with Pt1000 elements. For Pt2000 elements, a 2 wire connection is usually sufficient.
For maximum accuracy, the extension lead wire should be attached at the calibration point indicated on the element data sheet. Attaching the extension lead at a different distance from the element body, or cutting the element lead wire will result in reduced accuracy.
Heraeus Sensor Technology USA has the capability to supply elements with extension wires attached via laser welding or soldering.