Why does the magnetic force of magnetic filtration products decrease with time?

The magnetic force of magnetic filtration products (such as magnetic bars) will continue to decrease during use, then what factors will affect the magnetic force? Even if we use and maintain the magnetic bar normally, the magnetic force of the magnetic bar will decrease over time, which is defined as the annual demagnetization rate.

1. The formula of annual demagnetization rate of magnetic bar

The permanent magnet bar has the phenomenon of magnetic field attenuation in use, and the size parameter that characterizes magnetic field attenuation is called the annual demagnetization rate, which is defined as follows:

Annual demagnetization rate L=(A-B)/A, of which,

A: The surface magnetic value required by the buyer when purchasing the magnet bar, for example 12,000 Gauss. Please note that this value is not the measured value at the time of the magnetic bar leaving the factory, and usually, the manufacturer will offer a higher value, such as 12300 Gauss;

B: After one year of use (the magnetic field of the magnetic bar itself will be attenuated), a new surface magnetic value obtained after measuring with a Gaussian meter

E.g.: If the required surface magnetic field is 12000 Gauss when purchasing magnetic filter products, and the surface magnetic field is 11500 Gauss measured again after one year of use, then the annual demagnetization rate:

L = (12000-11500)/12000=4.17%

2. Measuring method of surface magnetic field

To use the Gauss meter correctly, find the location of the magnetic conductive sheet between the magnetic steel, that is, the area where the highest magnetic field point locates, and slowly move the Gauss meter probe to the maximum reading (Japan KANETEC Gauss meter TM-801 or Shanghai Hengtong Gauss meter HT-208 recommended).

Actual magnetic rod test data:

The customer requested 12,000 Gauss, and the product's factory measurement reached 12,780 Gauss.

Date of measurement	Surface magnetic field value(Gs)
2021-6-1	12780
2021-7-1	12580
2021-8-2	12430
2021-9-1	12400
2021-10-5	12390
2021-11-1	12390
2021-12-1	12380
2022-1-3	12350
2022-1-28	12330
2022-3-1	12290
2022-4-1	12270
2022-5-4	12260

Statement:

1）The structure of the magnetic bar is that magnets are sealed by argon arc welding in a stainless steel tube, while the magnets are easily oxidized material. It can be seen from the above data that in the first 3 months, the magnetic bar demagnetization is larger, because when the magnets are sealed into the stainless steel tube, some air cannot be excluded, which will oxidize the magnet and cause the magnetic field decreasing. When the air is consumed, the oxidation process of the magnet slows down, thus the magnetic attenuation slows down.

2）The process of magnetic separation is essentially a magnetic bar magnetizing iron impurities with its own magnetic field. The iron impurities are magnetized from a non-magnetic state to a magnetic state, and part of the energy of the magnetic bar is transferred to the iron impurities, which also takes away the energy of the magnetic bar when the iron impurities are cleaned up, reducing the magnetism of the magnetic bar.

In summary, the magnetic field attenuation of the magnetic bar is inevitable, while the standard magnet produced by the regular factory will reduce the annual demagnetization rate. In addition, improper use and storage will also accelerate the weakening of the magnetic field, and these abnormal use conditions such as:

* Usage over the design temperature;

* Strong high-frequency vibration;

* Close to strong magnetic field sources, etc.