A flowmeter measuring chamber having a generally cylindrical outer wall, a generally cylindrical inner wall, and a piston, having a tubular wall disposed between the inner and outer walls and arranged to oscillate within the outer wall, is provided with a mechanism for controlling movement of the tubular wall. That movement is controlled so that a capillary space is maintained between the tubular wall and the inner wall and between the tubular wall and the outer wall at all positions of the piston and contact between the tubular wall and the inner and outer walls is prevented.