When the pneumatic cylinder on your office or computer chair goes kaput, as they are wont to do, often after just a few years, you may find that even the manufacturer doesn't have any suggestions on how to fix it. Or you might want to buy a new gas cylinder, but these are costly and can go bad, as well. You're in luck -- folks reluctant to consign an otherwise perfectly fine, comfortable office chair to the landfill can make a PVC sleeve to hold the chair permanently at your preferred height.
PVC or Vacuum Extension Wand
Lift the seat manually to the height you find most comfortable. This may be its fully extended position.
Lift the chair and its seat post out from the base and its casters. Lay the chair horizontally on the floor of your work space.
Push off or cut off with a dremel tool any decorative or nonfunctioning collars around the seat post. Place the base back on the seat post temporarily so it seats fully, and mark the top of the base on the seat post. Set the base aside and measure from the mark to the base of the seat.
Cut to fit the length between your marks a piece of white PVC pipe or a similarly shaped tube, such as a spare wet-dry vacuum extension wand, which has the advantage of being black and less noticeable.
Place the cylinder on the seat post and slide the seat post back onto the base.
You can also buy ready-made kits to fix your office chair. To install the plastic, C-shaped spacers, raise the chair to its highest position and raise the cylinder skirt, a moveble outer ring, if present.
Snap on one spacer at a time and press each up the cylinder rod, the shiny metal post, until the spacers are holding the seat at a height equal to your knee level when standing. Sit down on the seat to activate the bond between the spacers.