If you're using block calibrators, furnaces, or temperature points above normal PRT (platinum resistance thermometer) temperatures (420 °C), then this Fluke Calibration 5626-20-I Secondary PRT Probe, INFO-CON for 1521 or 1522, 1/4X20 in is for you. The 5626-15-I is nominally 100W. This instrument has a temperature range of 200 °C to 661 °C. 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 5626-15-I is designed to reduce the contamination risk through the use of internal protection while not impairing performance.