Fluid exchange devices and related controls, systems, and methods

A method of measuring a value related to movement of a piston comprising:

• passing a piston through at least one sensor;
• inducing an electrical property in the at least one sensor with the piston;
• measuring the electrical property in the at least one sensor;
• and
• determining if the value related to the movement of the piston based on the measured electrical property of the piston meets a threshold value.

A system for exchanging pressure between at least two fluid streams comprising:

• a pressure exchanging device, comprising:
• a first chamber and a first piston configured to travel in the first chamber;
• and
• a second chamber and a second piston configured to travel in the second chamber, wherein the first piston and the second piston are configured to exchange pressure between fluids in the first chamber and the second chamber;
• one or more sensors for detecting the first piston or the second piston;
• and
• a control system configured to control movement of the first piston and the second piston by selectively directing flow of a fluid into one or more of the first chamber and the second chamber based on signals received from the one or more sensors, the control system configured to:
• determine a passing of the first piston or the second piston through and beyond the one or more sensors;
• inducing an electrical property in the one or more sensors with the first piston or the second piston;
• and
• measuring the electrical property in the one or more sensors.