How we combine cutting-edge 21st century technology with established 19th century physics principles
The principle of electromagnetic induction
In 1831, Michael Faraday discovered the principle of electromagnetic induction.
He found that if he took a coil of wire and moved a permanent magnet into it, he could measure an electromotive force (voltage) across the wire.
With this discovery, Faraday had proven a fundamental relationship between electric and magnetic fields, which has since become one of the cornerstones of modern physics.
In 1895, the Dutch physicist Hendrik Antoon Lorentz, succeeded in deriving a formula for the forces acting on a charge carrier in a magnetic field.
This formula states that the force acting on the charge is determined by the value of the charge (q), the strength of the magnetic field (B) and the speed (v) at which the charge is travelling.
F = B*q*v
q = charge of object
v = velocity of object
B = magnetic field strength
F = resultant force
As water contains charge carriers, the above-mentioned principle can be exploited by our velocity meter technology.
All of our probes contain a coil which creates a magnetic field of a fixed strength.
This magnetic field influences the charge in the water and a force, which is dependent on the water velocity (as shown in equation 1) acts upon them.
This force pushes the positive and negative charges in the water apart and creates an electric field.
E = F/q
E = electric field strength
F = force acting on the charge
q = charge
U = measured voltage
E = electric field strength
x = distance between electrodes
In order to measure this electric field, we must first know in which direction it is created.
Since the electric field is generated by the motion, we can use the right hand rule to determine the direction.
In the adjacent figure , “Motion” stands for the water flow direction, “Field” stands for the magnetic field direction and “Current” stands for the direction in which the electric field is created.
We can apply this rule to our probe.
The magnetic field generated by the coil is approximately pointing upwards (as shown previously).
This makes us find that the electric field must be pointing in the direction indicated by the green arrow.
Therefore our probe contains two electrodes aligned in this direction, measuring the voltage caused by the electric field.
By substituting equations 1 and 2 into equation 3, we get equation 4.
Since both the magnetic field strength and the distance between the electrodes are constant, we can deduce the water flow velocity from the voltage measurement.
Although the physics and mathematics in this explanation have been simplified, this should give you a good general understanding of how our measurement devices function.