The present disclosure relates to a variable pressure vessel. The vessel includes a liquid chamber and a gas chamber and a moveable barrier therebetween. The vessel has a volume, a first stroke, and a second stroke. The liquid chamber and the gas chamber each have a variable volume that changes responsive to the first stroke and the second stroke. The gas chamber has an outer wall wherein at least a portion of the outer wall is thermally conductive and allows heat to transfer therethrough. Movement of the moveable barrier between the liquid chamber and the gas chamber causes the volume in the liquid chamber and the volume in the gas chamber to displace each other. The volume in the gas chamber plus the volume in the liquid chamber is generally constant and generally equals the volume in the variable pressure vessel.