Abstract: A high accuracy flowmeter for testing and calibrating flowmeters and the like, and more particularly a positive displacement flowmeter which may include a cylinder (12), a piston (10) within the cylinder (12) which makes a clearance seal with the inside of the cylinder, an encoder (40) associated with the piston, and a control valve (V) for controlling the supply of a fluid flow (A1) to be measured. The flowmeter may have an arrangement for receiving differently-sized interchangeable modules for measurement of different flow rate ranges, each module having for example a compatible piston and cylinder of a predetermined size. The flowmeter includes a channel for receiving a fluid flow so as to move the piston (10) within its cylinder (12); and a device (36, 38) including the encoder (40) for detecting movement of the piston and generating electrical signals representative of the fluid flow to be measured.