Using liquids in Airpot Air Dashpots

Occasionally we get asked if liquids can be used in place of air in our dashpots. Let’s talk about some of the issues.

Because liquids are essentially incompressible, a liquid-filled dashpot wouldn’t suffer from the “air spring” effect that all air-filled dashpots have. This “air spring” is usually undesirable as it reduces the damping at the start of the stroke when the air dashpot is compressing air and building pressure.  A liquid-filled dashpot would generate that same pressure (and therefore damping force) right at the start of the stroke giving a more nearly constant damping coefficient (“c”). Well, theoretically, that is. There is always some non-zero looseness in the joints of the connecting rod and some air bubbles in the liquid that reduce the actual rate of force rise in the dashpot.