A number used to describe the ratio between the pressure recovery after the vena contracta and the pressure drop at the vena contracta. It is a measure of the amount of pressure recovered between the vena contracta and the valve outlet. Some manufacturers use the therm K_m to describe the pressure recovery factor. This number will be high (0. 9) for a globe style valve with a torturous follow path and lower (0. 8 to 0. 6) for a rotary style valve with a streamlined flow path. On most rotary products the F₁ factor will vary with the degree of opening of the valve closure member. Note! F₁ does not equal K_m.