is called the compression stroke movement from the second position to the first position is called the expansion stroke as can be seen in fig2 a the inlet 202 to the pumping chamber 208 is open when the piston is in the first position fluid may enter the pumping chamber 208 through the inlet 202 referring to fig2 b the inlet 202 is closes when the piston 205 moves in the compression stroke preferably the inlet is closed right after the compression stroke has started further movement of the piston in the compression stroke pushes the fluid towards the discharge unit 206 the fluid is compressed during the compression stroke and subsequently pushed through the discharge unit 206 when the pressure inside the pumping chamber 208 exceeds the pre set pressure of the discharge unit 206 since vapor is easily compressible the fluid displacement device 201 and the discharge unit 206 such as a spring loaded valve may be used to compress the vapor thus the discharge unit 206 ensures that the fluid such as a gas or a vapor may be compressed in order to increase the pressure of the fluid when liquid is transferred by the piston 205 towards the discharge unit 206 the force exerted by the liquid is large enough to open the discharge unit 206 and the liquid may transfer to the outlet 204 thus the fluid may be easily pumped by the fluid displacement device after having completed the compression stroke the piston starts the expansion stroke by moving the piston towards the first position the pressure inside the pumping chamber 208 will decrease as a result the discharge unit 206 is closed the pumping chamber 208 is now.