There are two families of current sensors provided by Valeport, and they differ in the way that the velocity is measured. Other methods also exist.
Sensors that rely on the timing and rotation of an impeller to provide a speed are typically known as mechanical current meters. A particular impeller will have a known rate of rotation in different flow speeds. Mechanical systems are robust, easily maintained and very widely used around the world by industry and academia alike, and typically give a reading every second (1 Hz) during which counts of impeller rotation are made and a speed derived.
Electromagnetic (EM) current meters have the benefits of higher levels of precision than mechanical systems and a user-selected measurement rate (for example between 1 and 8Hz), or readings taken in bursts with a “sleep” period between allowing for long subsea deployments – this approach could be used to measure the peak flows in a tidal cycle only, rather than low flows that may be of little interest to the study at hand. A magnetic field is generated in the flow, and measurements of electrical potential are made between electrodes, using the water as a conductor. By comparing the variations between different electrodes, the velocity of the flow can be determined.
Both mechanical and EM current meters use fluxgate compasses to determine their orientation to magnetic North and so allow the real-world direction of the velocity component to be calculated. The housing of a mechanical current meter is designed to align themselves with the dominant current flow and so can be seen to swivel on their mountings during a tidal cycle whereas this is not required for EM systems.
As with many other Valeport instruments, current meters may also be fitted with additional sensors to provide information on depth, temperature, or many other parameters, and can be provided with remote/onboard power, and onboard storage or telemetered data.