I was uncertain wether to put this in the general physics forum, but I decided against it. How do length adjustable gas springs(like for example in office chairs)work ? I know how gas springs work and i even have an idea how length adjustable ones could work. Since you hear a "gas flowing sound" when when you pull the lever on the chair and since the whole apperatus is pressurized and so an airflow out of the spring would be irreversible, I assume there is no air exchange between the inside and outside. It has to work as a closed system. My Idea: If you could close the piston orifice in the upper picture, the gas spring would stop having a roughly constant force to extension profile and would behave like a normal mechanical spring(following an approximately linear curve with its equillibrium position on the locking position) The gas springs could additionally apply forces much higher than in their "free" states. That behaviour is necessary in chairs.You want the chair to be able to hold you up in its locked position but move down with you when you try to adjust the height. There are however a few problems with this. The spring hardness would be higher if you "locked" the spring in one of the more extreme (very short or long) positions due to the smaller volumes that get encompassed on one of the sides.