The nitrogen gas in present in the chair cylinder. However what keep chair at mid position, between highest and lowest position? Is there a valve that prevents chair from coming up any further from mid position. Also if it at the lowest point that then gas is fully compressed. what prevents it from pushing upward as I'm seating. Thanks in advance
The way a gas spring chair works is that the piston inside the cylinder contains a valve, which lets nitrogen gas pass from one side of the cylinder to the other (see the link below for a diagram). The compression is created because of the volume of the piston rod being introduced into the cylinder, but at all times the pressure on either side of the piston is the same. The lever on the underside of the seat opens the valve in the piston. If you sit on the chair but don't pull the lever, the gas in the piston cannot flow through the valve, so piston remains extended. When you pull that lever, the valve opens and allows gas to flow across the piston, so the piston is able to move down and the gas is compressed. When the chair is at half-height and the lever is released, the valve closes, preventing the piston from moving either way, because the pressures are equal on either side (the pressure in the cylinder is so great that the extra pressure added by sitting on the chair is tiny by comparison). When the chair is at its lowest, the compressed gas is all above the piston, holding it down. It's only when the valve is opened that gas can flow to the other side to raise it.