Flowmeters based on this principle is used in all areas of industry and in all countries. The variety of models can be implemented even in processes involving highly corrosive abrasive slurry.
Possible limitations of pressure and temperature that may arise are mainly due to the properties of the coating material or the particular characteristics of some specific flow patterns.
Advantages:
- The measurements that are based is virtually independent of pressure, temperature and viscosity.
- Allow measurements even in the presence of solid particles (eg ore slurry).
- They have a wide range of nominal diameters: DN 2 (1 / 12 ") to 3,000 (120").
- Measuring tube longitudinal installation without moving parts.
- No load losses experienced
- Requires no bottlenecks in the cross section of the pipe (for easy cleaning, and sterilization-CIP-SIP-capable cleaning brush).
- High degree of measurement reliability and reproducibility, good stability over time.
- Minimum expenditure on maintenance and renewal.
Disadvantages:
- Only works with conductive liquids.
- The measurements are less accurate and more difficult with liquids poor conductors, for example, demineralized water.
- The depositions inside the measuring tube or the electrodes can generate
errors.
Ways to generate a magnetic field
To measure success flows with electromagnetic methods is critical as it is generated mode and controls the magnetic field in the measuring tube. Currently, two methods are used.
Generation of magnetic field pulses from continuous comment:
A proven modern method for generating magnetic field pulses is to use direct current (DC). Asylum is generated is called "pulsed DC field." The magnetic field polarity is reversed periodically so that the consecutive measuring voltages on the electrodes (U +, U-) have opposite signs (see Figure below). The induced voltage is the difference between these two measured values (Ucaudal).
This method has the advantage of eliminating interfering tensions, since they cancel in the calculation. The electronics converts the resulting voltage value is related to the average fluid velocity, the corresponding volume flow value and converts it into a standardized output signal (eg intensity between 4-20 mA).
Figure: Signal Processing and generation of magnetic field on charges of CC.
Pulsed magnetic fields CA:
This method was first developed. A sinusoidal AC power source (eg, 50 Hz network) directly connected to coils, excites and generates a magnetic field. On the one hand, the magnetic field thus obtained is intense and offers scope to distinguish what is the voltage signal that is noise. This aspect is very important in applications involving fluid problematic as sludge with solid content.
On the other hand, the measured voltage value is very low and there may be interference from AC power supply. Such interference may be the source of flow in the transformer or the same power cord, if it is close to the signal output cable. All these sources of interference are, of course, the same frequency of the line. In addition, amplified and tend to distort the measurement results.
The currents of unloaded ground along the piping system can cause similar effects. This is inevitable, especially in chemical industries. CA interfering tensions of this nature always have a negative effect on the reproducibility of the measurement. These tensions can never be completely eliminated, however effective it apantaüamiento system despite all auxiliary electrical circuits that might be available. However, the systems of magnetic field generation by newly developed AC pulses shown promise in some types of applications.
Advantages and disadvantages
In practice, the method of generating magnetic field for the flow measurement that has prevailed is that generated by pulsed-DC. This method is much more resistant to interfering voltages and their consumption is much lower. The advantages that helped keep for a while CA flowmeters - their ability to take measurements in conditions of extreme fluctuations in flow, or liquid measuring bit drivers and liquids containing solid - are available now in DC flowmeters compensatory and software models with higher excitation voltages for the magnetic field.
The lack of a defined zero point is also another drawback of the methods of generating AC magnetic field. The sinusoidal voltage is stationary, so there is no defined zero point of tension in the field generated by the coils. A magnetic field generated in this way is inherently susceptible to experience shifts of zero and in most cases you have to periodically adjust the zero point. This method does not correct the errors of zero point related to the process (zero drift).
