The SF-DF and SF-DFR models are manufactured by Lesjofors, and are extension spring that are made up of stainless steel which meet the standards of EN 10270-1-SM and EN 10270-3-1.4310. It can operate at a maximum temperature of 120 °C and 250 °C, which complies with the directives of EN 10270-1 and EN 10270-3-1.4310 respectively.
There is a loop that can be cut utilizing pliers or a hacksaw to form a normal hook, which is utilized to measure the spring force at a certain length.