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Rotary paddle level sensors have been available in China for quite some time now. Their prices range from a few hundred to several thousand yuan for imported models. Essentially, they consist of three main components: the motor part. Of course, there are good and bad motors; even MOLLOET from Germany opts for motors that are not manufactured in Germany. The sealing aspect is particularly important for instruments that utilize dynamic sealing, such as anti-rotation seals. Inadequate sealing allows dust to enter the circuit components, leading to short circuits in the instrument. Currently, the most advanced technology in this regard is MOLLET’s three-layer sealing technique. Most other imported and domestic brands employ only single- or double-layer sealing. Though this may seem like a minor detail, it often has a significant impact on the lifespan of the instruments. The third section is about circuits and materials. There are significant differences in this area as well. Of course, it’s important to recommend suitable products based on the user’s budget, but it’s also necessary to inform customers that good products can help businesses reduce costs. When selecting a rotary-vane level switch, several factors must be considered: First, the suitability of the switch for the material must be considered; if it isn’t suitable, any other considerations would be pointless. From a material standpoint, ⑴ liquids cannot be used. (The liquid should be detected by a level switch; there are many ways to do this.) ⑵ Materials that are highly fluidous, such as glue or the sludge from sludge separation tanks, cannot be used with rotary-type level switches. Flour dough used in food processing plants can be used instead. A simple way to determine whether a material is suitable for use is to lift it with a stick; if drops of the material fall within 2 seconds, it is generally not suitable for use, as the fluidity of the material interferes with detection. However, slurry mixtures with heavy aggregates, such as concrete or mortar, can be used, as the resistance exerted by the aggregates on the blades is sufficient to counteract the effect of fluidity. (For materials of this type, other measurement methods can be chosen based on their characteristics; in most cases, they can be classified as liquids.) (3) Ultra-light materials cannot be measured using this method. The minimum specific gravity of materials that can be detected by rotary paddle level switches is ≥0.5 g/cm³, or 0.5 T/m³. Since the specific gravity of water is 1 T/m³, rotary paddle level switches can indeed detect materials that are lighter than water. However, this is a theoretically derived value verified through experiments; it is not real-world data. In real-world operating conditions, factors such as airflow at the feed inlet, silo vibrations, and agitation significantly degrade the detection performance. Materials that are currently known to be undetectable include down (which floats in the air) and peanut shells, etc.; however, when used in industrial dust collection hoppers together with X-shaped blades, it is feasible to detect dust in hoppers with a volume of 0.5 m3. (For such materials, a vibrating rod level switch or a diaphragm level switch can be chosen, depending mainly on the bulk density.) From the perspective of the container: (1) Containers with too small a diameter cannot be used. The minimum detection distance of the level switch is approximately 120 mm. If the blades are located at the center of a small container, the falling material can interfere with the detection by the level switch; therefore, the diameter of the container must be ≥350 mm. The container diameter mentioned here refers to the diameter of a circular container at the location where the level switch is installed, or the distance between the two container walls of a rectangular container at that location. Of course, in containers with special shapes, this value can be adjusted, primarily to ensure that the blades operate without colliding with the container walls and that the material does not impact the blades. ⑵ Containers with too low a height cannot be used. On one hand, the rotating blades require space; on the other hand, when the height is too small, the material becomes overly loose, which is unfavorable for detection. In both cases, for containers holding heavy materials (such as sand and stones), it is recommended that the height be ≥300 mm; for relatively light materials (such as corn kernels and rice grains), the recommended container height is ≥500 mm. ⑶ Materials that remain in the container for too short a time cannot be used. A customer asked me whether a vortex-type level switch could be used for his container with a capacity of 0.25 m3. His goal was to open the butterfly valve once a tank was full, so that the material could be transferred to the next container for further processing. I asked him how long it would take to fill the container until it was empty How long does it take to refill after it’s emptied? He said about 10 seconds. I judge it to be unusable; the customer said the engineer designed something like this. I bought it and installed it, but it’s useless as it gives false alarms frequently. The principle behind this is that the speed of the level switch is about 1 revolution per minute, and the mechanical response angle is 30°; therefore, 30/360*60 seconds = 5 seconds. In other words, the material needs to be in effective contact with the blade for about 5 seconds before the switch signal can be generated. 5 seconds is half of the time required to fill the container, so to control the upper limit, it is only possible to install it at half the height of the storage bin. We are all aware that the level of material at the feed inlet is higher, while it is lower on the sides; even if it is installed at half height, the feed inlet will actually be overflowing by the time a signal is sent. Later, I suggested that he conduct 20 rounds of time measurement and then install a timer switch to control it, and the result was quite good. Therefore, we recommend that the container be filled with material for at least 1 minute, and at least 30 seconds is required before a anti-rotation level switch can be installed. If the time available is too short and a rotary-type level switch must still be used, then we need to be commissioned to help you design a level switch with rapid response. ⑷ It cannot be used when the temperature inside the chamber is >400°C. Currently, the maximum temperature inside the tank for conventional anti-rotation level switches is 85°C; for some manufacturers whose components are of lower quality, this limit is 70°C. This calculated value is based on the fact that the electrical components inside can withstand temperatures of 85°C. Since the casing is made of cast aluminum, which is an excellent heat-dissipating material, when the temperature inside the compartment reaches 85°C, heat conduction and dissipation ensure that the internal temperature of the unit generally does not exceed 50°C, which is within a safe range. The high-temperature resistant type can handle temperatures of 200°C in the warehouse, while the ultra-high temperature resistant type can withstand temperatures up to 400°C. Why 400°C? Since the melting point of aluminum is 660°C, it begins to soften at temperatures above 400°C; the mounting area softens and deforms, which in turn causes the internal bearings to get stuck. Therefore, 400°C is the current limit. Of course, we could use materials with a higher melting point and better heat dissipation capabilities to make heat sinks, but the cost is too high, making widespread use of them impractical. II. Based on what we have learned from the previous content, we can determine whether it is possible to install a level switch at our site. Next, we will discuss how to make the selection. The selection process can basically be carried out following the steps described in this article. Step 1: Voltage selection. There are two main factors to consider when choosing the power supply voltage: one is the voltage level of the power source available at the site, and the other is the voltage required by the equipment controlled by the level switch. Generally, we mainly consider the voltage of the device being controlled, as the power supply for that device can also be used directly by the level switch. Level switches are used to control alarms, AC contactors, elevators, PLCs, etc.; therefore, we need to consider what voltage is required by these devices, in other words, the switch should be selected based on the voltage level it needs to control. The RS-10 anti-rotation level switch series offers various voltage options including 220V AC, 24V AC, 110V AC, and 24V DC; the voltages mentioned here refer to the power supply voltage for the level switch. The supply voltage of the level switch should be as consistent as possible with the voltage of the switch circuit it controls; this not only facilitates on-site installation but also helps to prevent electrical interference. Some customers asked if there were rotary level switches available at 380V; the answer was no, but those operating at 220V can be used. These kinds of questions are basically asked by dealers. 380V is three-phase power, and the power consumption of a level switch is only 4W; it’s therefore impossible to use three-phase power for it. However, anyone with basic electrical knowledge knows that by using two of the three phases, an alternating current of 220V can be obtained. So feel free to purchase it; just hand it over to an installer with an electrician’s license. Step 2: Temperature selection. The temperature referred to in rotary magnetometric level switches is the temperature inside the silo, and it is divided into three temperature ranges: normal temperature (≤85°C), high temperature (≤200°C), and ultra-high temperature (≤400°C). You just need to determine it based on the actual situation. It is important to note that when the temperature inside the warehouse is near the critical value of the gradient range, the selection should be increased by one range. For example, if the temperature in the chamber is 180°C, it is advisable to use ultra-high temperature materials (≤400°C). It’s a simple principle: on one hand, fault in the temperature control system must be taken into account, as just one such fault can cause fatal damage to the level switch. On the other hand, this is caused by the complexity of the environment and the time-related effects of heat conduction; if the distance between the level switch and the heat source is less than that between the temperature sensor or temperature switch and the heat source, then by the time the temperature detection device detects a temperature of 180°C, the location where the level switch is situated may already be above 200°C℃ ； Temperature detection devices can only sense the surface temperature, but the interior of the material often has a higher temperature due to chemical reactions and physical heat accumulation. Therefore, we offer a more reasonable solution for reference: when the temperature in the warehouse is greater than 60°C, it is recommended to use a high-temperature type; when the temperature exceeds 160°C, an ultra-high-temperature type should be chosen; and when the temperature is above 360°C, if other level switches are available, it is best to avoid using rotary-type level switches. Step 3: Installation method selection. The installation methods for anti-rotation level switches mainly include five options: installation using standard nuts, installation using 75px-long carbon steel (stainless steel) welded nuts, installation using 150px-long carbon steel welded nuts, flange installation, and double flange installation. Standard nut installation: This is the most basic method of installation. This installation method requires that the conditions for installing fixing nuts inside the silo be met. Make a hole in the silo wall with a diameter greater than 32 mm and less than 36 mm; insert the threaded part of the level switch into this hole, then screw on the mounting nut inside the silo, and finally install the blade. This method is the simplest, but it has one drawback: when the level switch is installed on the side of the storage bin, it is often difficult to meet the requirement that the front end of the level switch be inclined downward, which affects its service life. Installation of 75px long carbon steel (stainless steel) welding nuts: This method is the most commonly used and reliable approach. Make holes in the wall of the silo with a diameter greater than 38 mm, then insert welding nuts; at this point, it is possible to adjust so that the inner end of the nut is lower than its outer end. Next, weld the nut to the silo wall (the material of the nut should be the same as that of the silo wall). After that, simply screw in the level switch. If C-type or D-type blades are used, since these blades can pass through the nuts, there is no need to install them inside the silo; however, other types of blades need to be installed inside the silo. The greatest advantage of this installation method is that it allows the blade section to tilt downward when installing the side level switch. Installation of 150px long carbon steel welding nuts: This installation method is not commonly used and is not highly recommended. It has three advantages: first, it allows the angle of downward inclination to be adjusted; second, it acts as a kind of shaft protection sleeve, helping to reduce the impact of materials on the shaft; third, it enables slight adjustment of the detection length. The downside is that no blade can pass directly through the nut, including C-type and D-type blades. Flange installation: This method is generally used when installation inside the tank is not possible, or when the equipment already has flange mounting points. When using flange mounting, it is essential to verify with the seller the diameter and spacing of the flange mounting holes. Because the specification standards for flanges vary, and different manufacturers use different standards. Double-flange installation: This installation method addresses the shortcomings of single-flange installation in terms of checking flange spacing and hole diameter, and is suitable for users who are installing new systems independently. It should be emphasized here that, starting in 2018, the double-flange design used in the RS-10 series manufactured by Wood Instruments features a cross-slot in the female flange that is welded to the tank wall; this slot is intended for inserting the blades directly into the tank from outside. Step 4: Blade selection. For anti-rotation level switches, the choice of blade is crucial. Of course, you can develop blades suitable for your specific needs by conducting multiple tests based on your actual operating conditions. For the RS-10 series, the blades available in the standard configuration are of types A, B, C, D, and X; each of them has its own characteristics, so please choose carefully. Type A blade: The Type A blade is also known as a shovel-shaped blade; the two 45° chamfers on its inner side ensure that it does not scrape against the curved side walls when operating in containers with a small diameter. Its shape is close to square, and its moderate size indicates that it has strong impact resistance. Therefore, heavier materials are suitable for Type A blades, such as sand, stones, cement, etc ； Containers with a smaller diameter are suitable for using Type A blades, such as those with a diameter of 400 mm or 500 mm. Type B blades: Also known as straight-blade types, these blades have a width that is roughly twice that of Type A blades, and the torque generated by the reaction forces is also about twice as large. Therefore, the materials that Type B blades can handle have a wider range of compatibility than those handled by Type A blades; for example, various grains, most chemical raw materials, plastic pellets, and so on. For cylindrical containers, it is recommended that the diameter be over 400 mm; a too-small diameter can easily lead to scraping against the walls of the silo. C-type blade: The C-type blade, also known as the sickle-shaped blade, is a new type of widely used blade. It has many advantages, such as: it can pass directly through 75px welding nuts, enabling installation and maintenance outside the compartment ； With a working diameter of 130 mm, it features high reaction torque and a wide detection range, enabling it to handle the widest range of materials. However, it also has weaknesses. First, its detection length is relatively large, at around 220 mm, so it cannot be used in silos with too small a diameter ； Second, the blades are narrow and long, with relatively low rigidity, making them unsuitable for silos subjected to high impact forces. Therefore, C-type blades are widely used in top mounting, in conjunction with 75px welding nuts, and in side mounting with no significant impact forces. If there is significant impact force, it is essential to install an arc-shaped baffle more than 200 mm above the location where the level switch is installed in order to prevent impacts. D-type blade: The D-type blade is also known as the half-blade type, and it represents a new type of widely used blade. Its semi-hemispherical design allows it to come to a stop in a vertically downward position during the upward movement of the material, as further rotation requires overcoming both the resistance of the material and the force of gravity. In this state, when the bin discharges material, a downward flow of the material occurs, and this flow has almost no effect on it. Therefore, the D-type blade is the most suitable blade for a level switch installed at the lower limit of the silo. X-type blades: X-type blades are also known as cross-shaped blades. The X-type blade has the largest contact area with the material among the five types of blades, so it experiences the greatest reaction force. It is suitable for detecting lighter materials, such as PP powder, polyester pellets, grain bran, wood chips, and particleboard shavings. However, it has certain requirements regarding the diameter and height of the container; the diameter must be over 400 mm. If your material is a conventional type, the container diameter is greater than 600 mm and the container height is greater than 1.5 meters, and it is not possible to determine the type of blade to use, then it is generally advisable to use a combination of C-type blades at the upper limit and D-type blades at the lower limit. If the blade you choose after reading this article is not suitable, you can discuss it with me. As a manufacturing company, we hope to research and develop new product applications in more various application environments. Step 5: Length selection. We recommend using the standard products whenever possible. If, due to the actual conditions on site, it is necessary to use an extended version, please contact us and we will customize the length according to your specific requirements. The detection length of a rotary type level switch refers to the distance from the bottom of the mounting thread to the top of the blade; therefore, it includes the blade, the extended shaft, the connecting shaft, and the mounting thread – in other words, it is the distance from the outside of the tank wall to the top of the detection element. A simpler way to put it is to specify by how much it needs to be extended, that is, the additional length added to the original detection length. For example, if the detection length of a C-type blade is 220 mm and it is extended by 500 mm, then the resulting detection length will be 720 mm. The maximum extended length of the anti-rotation level switch is 1.5 meters; we recommend keeping the extended length within 1 meter. When the product is mounted on the side, we do not recommend lengthening it, as the longer the shaft, the greater the bending moment generated at the root of the shaft due to the forces acting on the blades, which makes it easier for the shaft to deform. Additionally, the load on the bearing supports also increases. Step 6: Selection of shaft protection – When the level switch is installed on the side and an extension is required, a shaft protection type must be chosen. There are two main reasons for choosing the shaft-protection type. The first is that the flowability of the material is too poor, resulting in a high level of material below the feed inlet and a lower level near the walls of the silo; therefore, it is necessary to probe a certain distance deep into the silo. In the second case, there are hard deposits or adherences of material on the warehouse walls; to prevent these deposits from preventing the rotation of the level switch shaft, a shaft-protection type is also required. (Regarding this, one can refer to the Jieke Shi LJ25 enhanced version of rotary paddle level switch; pictures of it are easily found online.) Additionally, it’s important to note that for solid materials containing moisture, rotary paddle level switches must not be used (the internal motor in these switches rotates continuously, making it difficult to achieve a proper dynamic seal). Many users in the metallurgy and coke industries have reported that within a month of using such switches, water and dust managed to get into the circuit board, resulting in its damage. For this operating condition, the following can be chosen: a passive diaphragm level switch, or a diaphragm switch (refer to the MJB diaphragm level switch)