The USR-SF series is an ultra stable primary standard resistor which is an enhanced version of the USR/ASR series through the use of Bulk Metal ® Foil technology. The ultra stable resistive element utilizes new generation stress-free Bulk Metal Foil technology developed by Alpha Electronics with 37 years experience and is based on using proprietary Nickel Chrome alloy. This results in extremely low temperature coefficients as ±0.05 ppm/°C at 23°C ±5°C. This performance is unique to Alpha Electronics throughout the world. The stress-free resistance element eliminates stress factors using a special treatment process and is encapsulated in a specially-designed ceramic case to protect against humidity and oxidation. Thus, less than 0.5 ppm/year (0.2 ppm/year typical) is realized. Alpha's Bulk Metal Foil construction provides excellent AC characteristics—superior to performance of conventional wirewound standard resistors. The USR-SF, with its extreme long-term stability and low TCR, can be used in air which reduces cost and operation for maintenance of oil bath.