If you're using block calibrators, furnaces, or temperature points above normal PRT (platinum resistance thermometer) temperatures (420 °C), then this Fluke Calibration 5626-12-B Secondary PRT Probe, Bare Ware, 1/4X12 in. It makes great working or check standards for calibration work up to the aluminium point. Using a regular PRT at temperatures above 500 °C exposes the platinum to contamination. If the PRT is used as a reference or calibration standard, contamination is a major problem. SPRTs (standard platinum resistance thermometers), which are more expensive and delicate, can handle the higher temperatures, but with greater risk to the instrument due to shock, contamination, or mishandling. The 5626-12-B is designed to reduce the contamination risk through the use of internal protection while not impairing performance.