A fluid meter comprising: a chamber having an internal diameter, closed at a first end with a first structure and closed at a second end with a second structure which can be part of the chamber, and the chamber having a hub situated on the second structure, that is concentric with the chamber; and a piston that orbits in a path constrained by an external diameter of the piston against the internal diameter of the chamber, the piston having a peg against a center shaft of the chamber, the piston having an orbit guided by a divider wall of the chamber, and the piston having a nose gap that can run along a length or part of the length of the divider wall, and between the internal diameter of the chamber and the external diameter of the piston is an X gap; and wherein: the piston has a density of less than 1.0 g/cm3; X is equal to or greater than zero; a fluid can enter the chamber through an input port in the first or second structure or side faces of the chamber, and exit the chamber through an output port in the first or second structure or the side faces of the chamber; for one rotation of the piston in the chamber caused by a flow of the fluid into the inlet port, with a flow of the fluid out of the outlet port, there is a fixed-volume discrete parcel of the fluid for each rotation of the piston, characterized in that the piston comprises a material to which additives are added to change a density of the material and consequently of the piston, and the piston has a buoyance that is adjusted by adding glass bubbles or subtracting the glass bubbles to or from, respectively, a base material of the piston. a chamber having an internal diameter, closed at a first end with a first structure and closed at a second end with a second structure which can be part of the chamber, and the chamber having a hub situated on the second structure, that is concentric with the chamber; and a piston that orbits in a path constrained by an external diameter of the piston against the internal diameter of the chamber, the piston having a peg against a center shaft of the chamber, the piston having an orbit guided by a divider wall of the chamber, and the piston having a nose gap that can run along a length or part of the length of the divider wall, and between the internal diameter of the chamber and the external diameter of the piston is an X gap; and wherein: the piston has a density of less than 1.0 g/cm3; X is equal to or greater than zero; a fluid can enter the chamber through an input port in the first or second structure or side faces of the chamber, and exit the chamber through an output port in the first or second structure or the side faces of the chamber; for one rotation of the piston in the chamber caused by a flow of the fluid into the inlet port, with a flow of the fluid out of the outlet port, there is a fixed-volume discrete parcel of the fluid for each rotation of the piston, the piston has a density of less than 1.0 g/cm3; X is equal to or greater than zero; a fluid can enter the chamber through an input port in the first or second structure or side faces of the chamber, and exit the chamber through an output port in the first or second structure or the side faces of the chamber; for one rotation of the piston in the chamber caused by a flow of the fluid into the inlet port, with a flow of the fluid out of the outlet port, there is a fixed-volume discrete parcel of the fluid for each rotation of the piston, the piston comprises a material to which additives are added to change a density of the material and consequently of the piston, and the piston has a buoyance that is adjusted by adding glass bubbles or subtracting the glass bubbles to or from, respectively, a base material of the piston.