Pneumatic conveying, also known as airflow conveying, is to use the energy of airflow to convey granular materials in the direction of airflow in a closed pipe, which is a specific application of fluidization technology, and the main function is to convey materials from the starting point to the end point. In the whole conveying process, the material level control is the key to ensure the conveying efficiency, but because the physical characteristics of different materials are not the same, how to choose the right level switch for the characteristics of different powders and granules in pneumatic pull cord conveyor?

We know that the physical properties of the powder are: abrasiveness, granularity, electrical conductivity, hardness, fineness, adhesion and adherence, gas permeability and retention, low melting point, flammability and explosiveness, moisture absorption and fluidity, moisture content, and friability. When choosing a level switch, the main characteristics of the powder to be considered are fineness, density, hardness, electrical conductivity, moisture absorption and fluidity, and flammability and explosiveness. From the market feedback, at present, the mainstream material level switches include resistance type material level switch, vibration type material level switch, capacitance type material level switch, RF conductivity material level switch, etc.

If the fineness and density of the material is too low, it is fatal to a rotary level switch that uses a microswitch to sense changes in the damping of the blades and output an alarm signal. The damping is too small to trigger the action of the microswitch, which can lead to product failure. The lowest measured density that can be measured by the resistance level switch on the market is 0.3g/cm³.

　　Moisture absorption and fluidity for the majority of the level switch is a hurdle that can not be bypassed, strong moisture absorption will lead to poor mobility of the powder, the existence of material wrapped probe phenomenon, so the level switch resistance to hang material strength determines the measurement of good or bad. This can be alternative solutions are RF conductivity level switch and vibrating bar level switch.

　　The hardness of different powders varies in size, and the greater the hardness, the more severe the wear on the probe in contact with the material, such as silicon powder. In addition, these harder materials generally have a higher specific gravity, and the impact on the probe is also stronger. If the side installation in the absence of a protective baffle is easy to bend, so the wear resistance, rigidity of poor capacitance level switch is not recommended.

　　Whether the material is electrically conductive is also a key consideration, and whether the dielectric constant is ≥1.6 is an important criterion for evaluation. Since RF level switches are not self-adaptive and need to be fine-tuned according to the material, a more stable range of dielectric constants is a prerequisite for the use of RF level switches.

Among the many characteristics that have the greatest impact is the adhesion, for strong adhesion of materials, contact level switches need to be used with caution.

　　In addition to the above characteristics, safety factors are also important to consider. For example, flammable and explosive substances such as flour can be extremely harmful if they explode during the conveyor belt safety devices process, so it is a basic premise to choose products with dust explosion-proof qualification; in addition, in the field of fine chemicals, many reactors are accompanied by flammable gases, such as hydrogen and chlorine gas in the synthesis of polysilicon, which are dangerous and explosive gases. Therefore, it is also necessary to pay attention to the explosion-proof of flammable and explosive gases.

　　In summary, although there are many kinds of powder level switches, in actual use, it is necessary to choose carefully according to the characteristics of different materials and products, in order to avoid unfavorable factors and defects of different products, and choose the right level switch.