Electromagnetic flow measuring principle

Electromagnetic flowmeters, also known as magmeters or magnetic flowmeters, have been around since 1939. Swiss cleric and inventor Father Bonaventura Thürlemann (1909 – 1997) pioneered the industrial utilization of this measuring principle.

The physical phenomenon on which the technique is based, however, has been known for a great deal longer. The English physicist Michael Faraday (1791 – 1867) realized that electric charges are induced in a conductive metal rod of length (L) moved at velocity (v) through a magnetic field (B) and, consequently, a voltage (U_e) of a few millivolts is generated between the ends of the rod. Faraday also discovered that the magnitude of the voltage induced in this way is directly proportional to the velocity (v) of movement and the strength (B) of the magnetic field:

U_e = B ⋅ L ⋅ v 

U_e: Induced voltage
B: Strength of magnetic field
L: Length of the electric conductor (corresponds to the electrode spacing in the measuring tube)
v: Velocity of conductor movement (corresponds to the fluid velocity in the measuring tube)

In an electromagnetic flowmeter (Fig. 1), the conductive fluid flowing inside the measuring tube corresponds to the metal rod in Faraday’s experiment. The magnetic field of constant strength is generated by two field coils, one on each side of the measuring tube. Two electrodes on the inside wall of the pipe detect the voltage generated as the fluid flows through this field. The measuring tube is electrically insulated from the fluid and the electrode with a non-conductive lining (e.g., polyurethane, hard rubber, PTFE, PFA polyamide, etc.). Given a magnetic field of constant strength (B), the equation U_e = B ⋅ L ⋅ v shows that the induced measuring voltage (U_e) is directly proportional to the flow velocity (v). Pipe cross section (A) is known, so volumetric flow (Q_V) is easily calculated:

Q_v​: Volumetric flow
v: Flow velocity
A: Pipe cross section
U_e​: Induced voltage
B: Strength of magnetic field
L: Length of the electric conductor

The major advantage of this measuring principle is that it is unaffected by pressure, temperature and viscosity. The flow profile has a minimal effect on the results of measurement. These are properties that render electromagnetic flowmeters extremely attractive for a very wide range of industrial metering applications.

Measuring tube (a):
In physical terms, it is important that the measuring tube should neither obstruct nor distort the magnetic field, therefore it is made from stainless steel.

Lining (b):
The lining is the necessary insulator between the electrodes and the measuring tube, keeping the induced voltage from discharging to the pipe. Physical and chemical resistance to the fluid are also important properties for the liner. Polyurethane, hard rubber and PFA/PTFE are among the most commonly used materials.

Coil system (c):
The magnetic field is generated by two copper-wire coils with magnetic cores mounted outside the measuring tube.

Electrodes (d₁–d₃):

• Measuring electrodes (d₁) for detecting the induced voltage. Process conditions dictate the electrode material, which might be stainless steel, hastelloy, tantalum or platinum.
• Reference or ground electrode (d₂) for potential equalization between meter and fluid. Separate grounding disks (rings) can be installed for the same purpose.
• Empty-pipe detection electrode (d₃) for detecting partly filled or empty measuring tubes. The transmitter triggers an alarm if the fluid ceases to wet this electrode.

Using a pulsed direct current (DC) is a proven modern way of generating the magnetic field. It generates what is known as a “pulsed DC field”. The polarity of the magnetic field is periodically reversed, with the result that consecutive measuring voltages (U₊, U_–) at the electrodes have opposing signs (see Fig. 4). The difference obtained from the two measured values corresponds to the induced voltage (U_flow):

U_flow: Induced voltage
U₊: Positive measuring voltage
U_–: Negative measuring voltage

In this way, interference voltages are eliminated from the calculation. The resulting measured voltage, which corresponds to the average flow velocity, is converted into a volumetric flow signal by the electronics and made available as standardized output signals (e.g., 4–20 mA current output).