A gas lift springs consist of a precision rod attached to a piston moving within a sealed cylinder containing nitrogen at high pressure.Output forces are the result of the differential between the pressure in the cylinder and atmospheric pressure outside the cylinder acting on the cross-section of the rod.As the piston rod is introduced into the cylinder (compression stroke), the internal pressure increases according to the volume of gas displaced by the rod.This increase in Force or K-Factor is normally 5% - 50%.In comparison to mechanical springs, gas springs are almost constant force.