An Electromagnetic Flow Meter is a device capable of measuring the mass flow of a fluid.
Unlike the common flow meter you can find on the market it has no moving parts, and for this reason it can be made to withstand any pressure (without leakage) and any fluid (corrosive and non corrosive).
This kind of flow meter use a magnet and two electrodes to peek the voltage that appear across the fluid moving in the magnetic field.
| How does it Work |
The Neumann Law (or Lenz Law) states that if a conductive
wire is moving at right angle through a magnetic field, a voltage E
[Volts] will appear at the end of the conductor (Fig.1):
E=B*L*V
were
B = Magnetic Induction [Weber/m2]
L = Length of the portion of the wire 'wetted' by the magnetic field [m]
V = Velocity of the wire [m/sec]
Now imagine you have a plastic tube with two electrodes on the diameter and Mercury flowing into it (fig.2). A voltage will appear on the electrodes and it will be
E=B*L*V
as in the previous example (L in this case is the inner diameter of the tube).
You can think of Mercury as tiny conductive wires next to each other : each wire, moving in the tube, will touch the two electrodes ,and thus you can measure their voltage.
An interesting fact is that if you reverse the flow, you still get a voltage but with reverse polarity (Fig.1).
Till now we have talked about a conductive fluid ,Mercury, but this stuff will also work with non conductive fluid ,provided that you use an alternating magnetic field.
Two physicists, Mittlemann and Cushing, in an unpublished work, stated that when using a non conductive fluid, if the frequency of the alternating magnetic field is v the voltage at the electrodes will be attenuated by a factor a so that:
E=B*L*V*a
were:
ï = Relative Permittivity [Adimensional]
ïo = Permittivity of Free Space [Farads / m]
s = Electrical Conductivity [MV / m]
v = Frequency [1 / sec]
ï and s depends on the substance used and can be found on a good chemistry book. ïo is a constant = 8.86E-12 [Farads / m].
If an alternating magnetic field is used, an alternating voltage will appear on the electrodes and its amplitude will be E. That is, the output is an amplitude modulated wave.
| Measuring the flow |
A perfect axisimmetric construction cannot be achieved and thus some magnetic flux lines will 'wet' the connecting wires to the electrodes. The alternating magnetic field will create an offset voltage in this wires and even if the fluid is not moving, the measured voltage will not be zero.
If you plan to use the flow meter with a volt-meter you'll need to build a 'compensator circuit' to zero the voltage when the mass flow is zero, unless you want to calculate the difference between the measured voltage and the offset voltage every time.
Fig.3 shows the typical output of an E.F.M.
If you plan to use the flow meter with a DAQ card you'll need an 'AC to DC converter'. This device will 'feel' and hold the positive peak of the alternating voltage. This is necessary because you cannot input an alternating voltage in a ADC.
| Important Characteristics |
- As soon as the fluid start to move in the tube a voltage will appear immediately at the electrodes. There is no lag, therefore you can measure instantaneous flow in pulsating system.
- When the fluid is not moving the voltage at the electrodes will be zero (if you accounted for the offset voltage).
- It has a bi-directional response therefore you will measure a voltage anytime the fluid is moving, no matter which way.
- There is no movable part and restriction in the flow pipe.
If the fluid is moving with a certain speed V1 a voltage E1 will appear at the electrodes. If the fluid is moving with a speed V2 two times V1, a voltage E2 two times E1 will appear on the electrodes. That is, an E.F.M. has a linear calibration curve. (fig.4)
If you are not sure about the strength of the magnetic field (and thus
you don't know what voltage will appear at the electrodes), just let a
constant fluid flow Q1 pass through the flow meter and read the voltage
E1 with an oscilloscope.
The calibration curve will pass from 0 Volts when there is no flow ,and the point E1, Q1 (fig.5).
Remember that if you are using a constant magnetic field the voltage will reverse polarity if you reverse the flow.
If you reverse the flow in an alternated field you'll get the same AM wave ,and if you use the AC to DC converter you'll measure the same output voltage because it will 'feel' and hold the positive peak of the alternating voltage.
Upaya awal untuk menciptakan flow meter elektromagnetik menggunakan medan magnet konstan memiliki beberapa keberhasilan, tapi itu tidak lama sebelum disadari bahwa pengukuran yang dihasilkan melayang dengan waktu.
Butuh beberapa saat untuk bekerja di luar fisika mengapa ini harus. Apa yang sebenarnya terjadi adalah bahwa ion positif biaya dalam aliran air dalam pipa dengan elektroda di dinding pipa, tertarik pada elektroda negatif dan ion negatif ke elektroda positif. Hal ini menciptakan sebuah rezim aliran laminar bercabang dan ini mempengaruhi akurasi pengukuran.
Solusinya sederhana tapi itu harus menunggu untuk kemajuan dalam elektronika praktis untuk membuat berdenyut membalikkan medan magnet mungkin. Disadari juga bahwa konduktivitas sebenarnya cairan yang diukur bukan faktor asalkan konduktivitas listriknya adalah antara 5 sampai 20 micro-siemens/cm sebagai minimum. Dengan kedatangan flowmeter magnetik praktis (atau magmeter karena mereka kadang-kadang disebut), kehandalan pengukuran larutan untuk kedua arus kecil di 50mm (2 ") pipa diameter dalam proses tanaman dan arus yang sangat besar di 1,8 (72" ) diameter drainase tanah dan pipa bantuan banjir, menjadi legendaris.
Mag meter tersebut memiliki aplikasi luas,
bahkan di mana saja di mana ada cairan konduktif yang akan diukur. Aplikasi ini meliputi pendinginan tanaman proses, sistem bantuan banjir, pengolahan air limbah, pengolahan air, pengolahan makanan dan banyak pabrik pengolahan kimia.
Tapi mengapa menggunakan flowmeter elektromagnetik di tempat mengatakan sebuah flowmeter ultrasonik yang memanfaatkan efek Doppler nomor pemancar / sensor pasang ditetapkan siram di dinding pipa bukan elektroda? Jawabannya adalah bahwa keduanya dapat digunakan untuk cairan konduktif tapi skor flowmeter ultrasonik dalam cairan non-konduktif seperti kebanyakan produk minyak bumi dan bahan kimia. Fitur besar dari kedua jenis meteran adalah pipa jelas melahirkan melalui flow meter yang kemudian tidak akan membatasi atau menyebabkan masalah penyumbatan jenis flow meter yang menyusup ke aliran atau memodifikasi aliran seperti flow meter lubang atau meteran baling-baling.
Ada banyak jenis flowmeter yang telah dikembangkan untuk pabrik pengolahan yang sangat kompleks dalam industri dan yang ada di pasar. Ada tumpang tindih besar pada kesesuaian mereka untuk persyaratan khusus oleh perancang pabrik. Pada akhir hari, pengukuran aliran metode seleksi datang ke: cairan yang akan diukur, kisaran arus, suhu operasi dan tekanan, konduktivitas listrik tentu saja, jenis cairan, pembatasan pengaturan pipa tumbuhan dan biaya.
Akhirnya kembali ke mag meter, ada dua tipe dasar instalasi magflow seperti yang tersedia, jenis inline dan jenis penyisipan. Jenis Inline flowmeter elektromagnetik adalah yang paling akurat, itu yang terkandung dalam unit flensa dengan diameter yang sama dengan pipa. Ukuran adalah dari diameter 6mm sampai diameter 2000mm untuk pertimbangan berat praktis. Penyisipan mag meter terpasang di salah satu sisi pipa biasanya dalam tee cabang kecil. Unit terintegrasi berisi kedua elektro-magnet dan elektroda. Aplikasi berasal dari diameter kecil sampai diameter besar.
