An infusion device, comprising: a controller operably coupled to a gas reservoir, the controller configured to control an infusion of fluid from a fluid source to a liquid sink by: a) exerting a negative pressure on the gas reservoir in fluid communication with a gas-side chamber separated from a fluid-side chamber by a flexible membrane to draw liquid from the fluid source into the fluid-side chamber through a one-way inlet valve until the fluid-side chamber fills with fluid; b) exerting a positive pressure on the gas reservoir in fluid communication with the gas-side chamber to deliver liquid in the fluid-side chamber to the liquid sink through a one-way outlet valve, wherein the one-way inlet valve and the one-way outlet valve each are passively operated and only open when a pressure differential between an upstream fluid and a downstream fluid reaches a predetermined cracking pressure; c) monitoring pressure in the gas reservoir during the steps of exerting the negative pressure and exerting the positive pressure; and d) determining volumes of fluid in the fluid-side chamber from incremental changes in volume of the gas reservoir and the gas-side chamber and any connecting dead space by an ideal gas law relationship, wherein P 1 V 1 = P 2 V 2, wherein P 1 and P 2 are pressures measured at two times before and after volume changes and V 1 and V 2 are volumes at the two times. a controller operably coupled to a gas reservoir, the controller configured to control an infusion of fluid from a fluid source to a liquid sink by: a) exerting a negative pressure on the gas reservoir in fluid communication with a gas-side chamber separated from a fluid-side chamber by a flexible membrane to draw liquid from the fluid source into the fluid-side chamber through a one-way inlet valve until the fluid-side chamber fills with fluid; b) exerting a positive pressure on the gas reservoir in fluid communication with the gas-side chamber to deliver liquid in the fluid-side chamber to the liquid sink through a one-way outlet valve, wherein the one-way inlet valve and the one-way outlet valve each are passively operated and only open when a pressure differential between an upstream fluid and a downstream fluid reaches a predetermined cracking pressure; c) monitoring pressure in the gas reservoir during the steps of exerting the negative pressure and exerting the positive pressure; and d) determining volumes of fluid in the fluid-side chamber from incremental changes in volume of the gas reservoir and the gas-side chamber and any connecting dead space by an ideal gas law relationship, wherein P 1 V 1 = P 2 V 2, wherein a) exerting a negative pressure on the gas reservoir in fluid communication with a gas-side chamber separated from a fluid-side chamber by a flexible membrane to draw liquid from the fluid source into the fluid-side chamber through a one-way inlet valve until the fluid-side chamber fills with fluid; b) exerting a positive pressure on the gas reservoir in fluid communication with the gas-side chamber to deliver liquid in the fluid-side chamber to the liquid sink through a one-way outlet valve, wherein the one-way inlet valve and the one-way outlet valve each are passively operated and only open when a pressure differential between an upstream fluid and a downstream fluid reaches a predetermined cracking pressure; c) monitoring pressure in the gas reservoir during the steps of exerting the negative pressure and exerting the positive pressure; and d) determining volumes of fluid in the fluid-side chamber from incremental changes in volume of the gas reservoir and the gas-side chamber and any connecting dead space by an ideal gas law relationship, wherein P 1 V 1 = P 2 V 2, wherein P 1 and P 2 are pressures measured at two times before and after volume changes and V 1 and V 2 are volumes at the two times.