A gas-lift stool is a stool or chair that incorporates a gas operated height adjustment device. This kind of chair consists of a gas filled cylinder, an internal piston, and an actuator lever which controls a bypass valve. The actuator allows the user to raise or lower the seat by controlling the amount of gas below the piston. Gas-lift stools are typically designed for the office but may also be found in the home in the form of bar or counter stools. Office gas-lift stools are typically fitted with wheels for easy movement around a desk.
Most office chairs feature seat height adjustment controls which allow the user to tailor the seating position for maximum comfort. One of the quickest and most convenient of the seat height adjustment methods is the gas-lift mechanism. This device allows a stool or chair to be lowered or raised through the entire adjustment range with a simple flick of a lever. The height adjustment mechanisms on gas-lift stools are typically located within the base and consist of a enclosed, gas filled cylinder fitted with a piston and a bypass valve. The piston is kept near the top of the cylinder by the gas pressure beneath it and is attached to a rod which supports the stool seat.
This arrangement means that the weight of the stool's occupant is effectively born by the pressure of the gas below the piston. The mechanisms on gas-lift stools are equipped with a control lever that actuates the bypass valve. This valve allows gas from the cylinder to pass into a separate reservoir or into the cylinder above the piston. Should the user want to lower the stool, the lever is depressed while still seated. This allows gas to exit the cylinder, thereby effectively lowering the pressure below the piston and allowing the seat to descend.
Once the seat is at the desired height, the lever is released and the outflow of gas stops. The seat will now remain in that position until it is adjusted again. The raising of the stool is subject to a similar process with the addition of a little design wizardry. To raise the stool, the user gets out of the chair and lifts the lever to reopen the bypass valve. Due to the fact that there is no longer any weight on the piston, the pressure of the gas below it is lower than that above it or in the reservoir, thereby causing it to flow back into the lower part of the cylinder and raising the seat as it does so.
Gas-lift stools are available in a range of designs which include low or high back and arm rest models. Gas lift mechanisms are generally found on secretary, typist, and executive office chairs although they may also be used on bar or counter stools. Gas-lift office stools are typically fitted with a set of wheels for easy movement.