A metering pump system includes a variable displacement pump having an inlet and an outlet, a flow sensing valve fluidically connected to the outlet of the variable displacement pump, and an actuator mechanically coupled to a displacement mechanism of the variable displacement pump. The metering pump system also includes an electronic engine controller in communication with the flow sensing valve. An electrohydraulic servo valve is electrically connected to the electronic engine controller and hydraulically connected to the actuator. The electrohydraulic servo valve is configured to hydraulically drive the actuator to move the displacement mechanism to change displacement of the variable displacement pump.

The invention relates to a positive displacement pump 1 for conveying medical fluids, which pump has a pumping chamber 5 and a positive displacement element 15, and to a blood treatment apparatus comprising a positive displacement pump 1. In addition, the invention relates to a method for controlling a positive displacement pump 1 for conveying medical fluids. The positive displacement pump 1 according to the invention has an actuation member 20 in operative connection with the positive displacement element 20 for displacing or deforming the positive displacement element in order to convey the fluid into the pumping chamber or out of the pumping chamber, and a drive device 21 for displacing the actuation member 20. A control unit 14 is provided for to controlling the drive device 21 and an inlet valve 12 and outlet valve 13. The actuation member 20 is in operative connection with the positive displacement element 15 via a working chamber 19 that is filled with gas and has a sealed volume. In order to achieve a high conveying precision, the pressure in the working chamber 5 is kept constant when the positive displacement element 15 is moved.

The invention relates to a positive displacement pump 1 for conveying medical fluids, which pump has a pumping chamber 5 and a positive displacement element 15, and to a blood treatment apparatus comprising a positive displacement pump 1. In addition, the invention relates to a method for controlling a positive displacement pump 1 for conveying medical fluids. The positive displacement pump 1 according to the invention has an actuation member 20 in operative connection with the positive displacement element 20 for displacing or deforming the positive displacement element in order to convey the fluid into the pumping chamber or out of the pumping chamber, and a drive device 21 for displacing the actuation member 20. A control unit 14 is provided for to controlling the drive device 21 and an inlet valve 12 and outlet valve 13. The actuation member 20 is in operative connection with the positive displacement element 15 via a working chamber 19 that is filled with gas and has a sealed volume. In order to achieve a high conveying precision, the pressure in the working chamber 5 is kept constant when the positive displacement element 15 is moved.

A linear compressor may include a shell, a cylinder provided in the shell to define a compression space for a refrigerant, a frame to fix the cylinder to the shell, a piston reciprocated within the cylinder, a discharge valve disposed on or at one side of the cylinder, a discharge cover coupled to the frame and having at least one chamber to reduce pulsation of the refrigerant discharged through the discharge valve, and a valve spring disposed on the discharge cover to provide a restoring force to the discharge valve. The discharge cover may include a cover body having a discharge hole, through which the refrigerant discharged through the discharge valve may be discharged outside of the discharge cover, and a guide passage defined in the cover body to guide at least a portion of the refrigerant discharged through the discharge valve into the at least one chamber.